text:
Agnosticism

Agnosticism is the view or belief that the existence of God, the divine, or the supernatural is either unknowable in principle or currently unknown in fact. It can also mean an apathy towards such religious belief and refer to personal limitations rather than a worldview. Another definition is the view that "human reason is incapable of providing sufficient rational grounds to justify either the belief that God exists or the belief that God does not exist."

The English biologist Thomas Henry Huxley coined the word agnostic in 1869, and said that he originally coined the word Agnostic "to denote people who, like [himself], confess themselves to be hopelessly ignorant concerning a variety of matters [including of course the matter of God's existence], about which metaphysicians and theologians, both orthodox and heterodox, dogmatise with the utmost confidence." Earlier thinkers had written works that promoted agnostic points of view, such as Sanjaya Belatthiputta, a 5th-century BCE Indian philosopher who expressed agnosticism about any afterlife; and Protagoras, a 5th-century BCE Greek philosopher who expressed agnosticism about the existence of "the gods".

Defining agnosticism
"[The agnostic] principle may be stated in various ways, but they all amount to this: that it is wrong for a man to say that he is certain of the objective truth of any proposition unless he can produce evidence which logically justifies that certainty. This is what Agnosticism asserts; and, in my opinion, it is all that is essential to Agnosticism."

"Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle ... Positively the principle may be expressed: In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration. And negatively: In matters of the intellect do not pretend that conclusions are certain which are not demonstrated or demonstrable."

"That which Agnostics deny and repudiate, as immoral, is the contrary doctrine, that there are propositions which men ought to believe, without logically satisfactory evidence; and that reprobation ought to attach to the profession of disbelief in such inadequately supported propositions."

"Consequently, agnosticism puts aside not only the greater part of popular theology, but also the greater part of anti-theology. On the whole, the 'bosh' of heterodoxy is more offensive to me than that of orthodoxy, because heterodoxy professes to be guided by reason and science, and orthodoxy does not."

Being a scientist, above all else, Huxley presented agnosticism as a form of demarcation. A hypothesis with no supporting, objective, testable evidence is not an objective, scientific claim. As such, there would be no way to test said hypotheses, leaving the results inconclusive. His agnosticism was not compatible with forming a belief as to the truth, or falsehood, of the claim at hand. Karl Popper would also describe himself as an agnostic. According to philosopher William L. Rowe, in this strict sense, agnosticism is the view that human reason is incapable of providing sufficient rational grounds to justify either the belief that God exists or the belief that God does not exist.

George H. Smith, while admitting that the narrow definition of atheist was the common usage definition of that word, and admitting that the broad definition of agnostic was the common usage definition of that word, promoted broadening the definition of atheist and narrowing the definition of agnostic. Smith rejects agnosticism as a third alternative to theism and atheism and promotes terms such as agnostic atheism (the view of those who do not hold a belief in the existence of any deity but claim that the existence of a deity is unknown or inherently unknowable) and agnostic theism (the view of those who believe in the existence of a deity(s) but claim that the existence of a deity is unknown or inherently unknowable).

Etymology
Agnostic was used by Thomas Henry Huxley in a speech at a meeting of the Metaphysical Society in 1869 to describe his philosophy, which rejects all claims of spiritual or mystical knowledge.

Early Christian church leaders used the Greek word gnosis (knowledge) to describe "spiritual knowledge". Agnosticism is not to be confused with religious views opposing the ancient religious movement of Gnosticism in particular; Huxley used the term in a broader, more abstract sense. Huxley identified agnosticism not as a creed but rather as a method of skeptical, evidence-based inquiry.

The term agnostic is also cognate with the Sanskrit word ajñasi, which translates literally to "not knowable", and relates to the ancient Indian philosophical school of Ajñana, which proposes that it is impossible to obtain knowledge of metaphysical nature or ascertain the truth value of philosophical propositions; and even if knowledge was possible, it is useless and disadvantageous for final salvation.

In recent years, scientific literature dealing with neuroscience and psychology has used the word to mean "not knowable". In technical and marketing literature, "agnostic" can also mean independence from some parameters—for example, "platform agnostic" (referring to cross-platform software), or "hardware-agnostic".

Qualifying agnosticism
Scottish Enlightenment philosopher David Hume contended that meaningful statements about the universe are always qualified by some degree of doubt. He asserted that the fallibility of human beings means that they cannot obtain absolute certainty except in trivial cases where a statement is true by definition (e.g. tautologies such as "all bachelors are unmarried" or "all triangles have three corners").

Types

 * Strong agnosticism (also called "hard", "closed", "strict", or "permanent agnosticism"): The view that the question of the existence or nonexistence of a deity or deities, and the nature of ultimate reality is unknowable by reason of our natural inability to verify any experience with anything but another subjective experience. A strong agnostic would say, "I cannot know whether a deity exists or not, and neither can you."
 * Weak agnosticism (also called "soft", "open", "empirical", "hopeful", or "temporal agnosticism"): The view that the existence or nonexistence of any deities is currently unknown but is not necessarily unknowable; therefore, one will withhold judgment until evidence, if any, becomes available. A weak agnostic would say, "I don't know whether any deities exist or not, but maybe one day, if there is evidence, we can find something out."
 * Apathetic agnosticism: The view that no amount of debate can prove or disprove the existence of one or more deities, and if one or more deities exist, they do not appear to be concerned about the fate of humans. Therefore, their existence has little to no impact on personal human affairs and should be of little interest. An apathetic agnostic would say, "I don't know whether any deity exists or not, and I don't care if any deity exists or not."

Hindu philosophy
Throughout the history of Hinduism there has been a strong tradition of philosophic speculation and skepticism.

The Rig Veda takes an agnostic view on the fundamental question of how the universe and the gods were created. Nasadiya Sukta (Creation Hymn) in the tenth chapter of the Rig Veda says: "But, after all, who knows, and who can say Whence it all came, and how creation happened? The gods themselves are later than creation, so who knows truly whence it has arisen?

Whence all creation had its origin, He, whether he fashioned it or whether he did not, He, who surveys it all from highest heaven, He knows – or maybe even he does not know."

Hume, Kant, and Kierkegaard
Aristotle, Anselm, Aquinas, Descartes, and Gödel presented arguments attempting to rationally prove the existence of God. The skeptical empiricism of David Hume, the antinomies of Immanuel Kant, and the existential philosophy of Søren Kierkegaard convinced many later philosophers to abandon these attempts, regarding it impossible to construct any unassailable proof for the existence or non-existence of God.

In his 1844 book, Philosophical Fragments, Kierkegaard writes: "Let us call this unknown something: God. It is nothing more than a name we assign to it. The idea of demonstrating that this unknown something (God) exists, could scarcely suggest itself to Reason. For if God does not exist it would of course be impossible to prove it; and if he does exist it would be folly to attempt it. For at the very outset, in beginning my proof, I would have presupposed it, not as doubtful but as certain (a presupposition is never doubtful, for the very reason that it is a presupposition), since otherwise I would not begin, readily understanding that the whole would be impossible if he did not exist. But if when I speak of proving God's existence I mean that I propose to prove that the Unknown, which exists, is God, then I express myself unfortunately. For in that case I do not prove anything, least of all an existence, but merely develop the content of a conception."

Hume was Huxley's favourite philosopher, calling him "the Prince of Agnostics". Diderot wrote to his mistress, telling of a visit by Hume to the Baron D'Holbach, and describing how a word for the position that Huxley would later describe as agnosticism did not seem to exist, or at least was not common knowledge, at the time.

"The first time that M. Hume found himself at the table of the Baron, he was seated beside him. I don't know for what purpose the English philosopher took it into his head to remark to the Baron that he did not believe in atheists, that he had never seen any. The Baron said to him: 'Count how many we are here.' We are eighteen. The Baron added: 'It isn't too bad a showing to be able to point out to you fifteen at once: the three others haven't made up their minds.'"

Charles Darwin
Raised in a religious environment, Charles Darwin (1809–1882) studied to be an Anglican clergyman. While eventually doubting parts of his faith, Darwin continued to help in church affairs, even while avoiding church attendance. Darwin stated that it would be "absurd to doubt that a man might be an ardent theist and an evolutionist". Although reticent about his religious views, in 1879 he wrote that "I have never been an atheist in the sense of denying the existence of a God. – I think that generally ... an agnostic would be the most correct description of my state of mind."

Thomas Henry Huxley

Agnostic views are as old as philosophical skepticism, but the terms agnostic and agnosticism were created by Huxley (1825–1895) to sum up his thoughts on contemporary developments of metaphysics about the "unconditioned" (William Hamilton) and the "unknowable" (Herbert Spencer). Though Huxley began to use the term agnostic in 1869, his opinions had taken shape some time before that date. In a letter of September 23, 1860, to Charles Kingsley, Huxley discussed his views extensively: "I neither affirm nor deny the immortality of man. I see no reason for believing it, but, on the other hand, I have no means of disproving it. I have no a priori objections to the doctrine. No man who has to deal daily and hourly with nature can trouble himself about a priori difficulties. Give me such evidence as would justify me in believing in anything else, and I will believe that. Why should I not? It is not half so wonderful as the conservation of force or the indestructibility of matter ...

It is no use to talk to me of analogies and probabilities. I know what I mean when I say I believe in the law of the inverse squares, and I will not rest my life and my hopes upon weaker convictions ...

That my personality is the surest thing I know may be true. But the attempt to conceive what it is leads me into mere verbal subtleties. I have champed up all that chaff about the ego and the non-ego, noumena and phenomena, and all the rest of it, too often not to know that in attempting even to think of these questions, the human intellect flounders at once out of its depth."

And again, to the same correspondent, May 6, 1863: "I have never had the least sympathy with the a priori reasons against orthodoxy, and I have by nature and disposition the greatest possible antipathy to all the atheistic and infidel school. Nevertheless I know that I am, in spite of myself, exactly what the Christian would call, and, so far as I can see, is justified in calling, atheist and infidel. I cannot see one shadow or tittle of evidence that the great unknown underlying the phenomenon of the universe stands to us in the relation of a Father [who] loves us and cares for us as Christianity asserts. So with regard to the other great Christian dogmas, immortality of soul and future state of rewards and punishments, what possible objection can I—who am compelled perforce to believe in the immortality of what we call Matter and Force, and in a very unmistakable present state of rewards and punishments for our deeds—have to these doctrines? Give me a scintilla of evidence, and I am ready to jump at them."

Of the origin of the name agnostic to describe this attitude, Huxley gave the following account: "When I reached intellectual maturity and began to ask myself whether I was an atheist, a theist, or a pantheist; a materialist or an idealist; Christian or a freethinker; I found that the more I learned and reflected, the less ready was the answer; until, at last, I came to the conclusion that I had neither art nor part with any of these denominations, except the last. The one thing in which most of these good people were agreed was the one thing in which I differed from them. They were quite sure they had attained a certain 'gnosis'—had, more or less successfully, solved the problem of existence; while I was quite sure I had not, and had a pretty strong conviction that the problem was insoluble. And, with Hume and Kant on my side, I could not think myself presumptuous in holding fast by that opinion ...

So I took thought, and invented what I conceived to be the appropriate title of 'agnostic'. It came into my head as suggestively antithetic to the 'gnostic' of Church history, who professed to know so much about the very things of which I was ignorant. ... To my great satisfaction the term took."

In 1889, Huxley wrote:"Therefore, although it be, as I believe, demonstrable that we have no real knowledge of the authorship, or of the date of composition of the Gospels, as they have come down to us, and that nothing better than more or less probable guesses can be arrived at on that subject."

William Stewart Ross
William Stewart Ross (1844–1906) wrote under the name of Saladin. He was associated with Victorian Freethinkers and the organization the British Secular Union. He edited the Secular Review from 1882; it was renamed Agnostic Journal and Eclectic Review and closed in 1907. Ross championed agnosticism in opposition to the atheism of Charles Bradlaugh as an open-ended spiritual exploration.

In Why I am an Agnostic (c. 1889) he claims that agnosticism is "the very reverse of atheism".

Bertrand Russell
Bertrand Russell (1872–1970) declared Why I Am Not a Christian in 1927, a classic statement of agnosticism. He calls upon his readers to "stand on their own two feet and look fair and square at the world with a fearless attitude and a free intelligence".

In 1939, Russell gave a lecture on The existence and nature of God, in which he characterized himself as an atheist. He said: "The existence and nature of God is a subject of which I can discuss only half. If one arrives at a negative conclusion concerning the first part of the question, the second part of the question does not arise; and my position, as you may have gathered, is a negative one on this matter."

However, later in the same lecture, discussing modern non-anthropomorphic concepts of God, Russell states: "That sort of God is, I think, not one that can actually be disproved, as I think the omnipotent and benevolent creator can."

In Russell's 1947 pamphlet, Am I An Atheist or an Agnostic? (subtitled A Plea For Tolerance in the Face of New Dogmas), he ruminates on the problem of what to call himself: "As a philosopher, if I were speaking to a purely philosophic audience I should say that I ought to describe myself as an Agnostic, because I do not think that there is a conclusive argument by which one can prove that there is not a God. On the other hand, if I am to convey the right impression to the ordinary man in the street I think I ought to say that I am an Atheist, because when I say that I cannot prove that there is not a God, I ought to add equally that I cannot prove that there are not the Homeric gods."

In his 1953 essay, What Is An Agnostic? Russell states: "An agnostic thinks it impossible to know the truth in matters such as God and the future life with which Christianity and other religions are concerned. Or, if not impossible, at least impossible at the present time.

Are Agnostics Atheists?

No. An atheist, like a Christian, holds that we can know whether or not there is a God. The Christian holds that we can know there is a God; the atheist, that we can know there is not. The Agnostic suspends judgment, saying that there are not sufficient grounds either for affirmation or for denial."

Later in the essay, Russell adds: "I think that if I heard a voice from the sky predicting all that was going to happen to me during the next twenty-four hours, including events that would have seemed highly improbable, and if all these events then produced to happen, I might perhaps be convinced at least of the existence of some superhuman intelligence."

Leslie Weatherhead
In 1965, Christian theologian Leslie Weatherhead (1893–1976) published The Christian Agnostic, in which he argues: "... many professing agnostics are nearer belief in the true God than are many conventional church-goers who believe in a body that does not exist whom they miscall God."

Although radical and unpalatable to conventional theologians, Weatherhead's agnosticism falls far short of Huxley's, and short even of weak agnosticism: "Of course, the human soul will always have the power to reject God, for choice is essential to its nature, but I cannot believe that anyone will finally do this."

Robert G. Ingersoll
Robert G. Ingersoll (1833–1899), an Illinois lawyer and politician who evolved into a well-known and sought-after orator in 19th-century America, has been referred to as the "Great Agnostic".

In an 1896 lecture titled Why I Am An Agnostic, Ingersoll related why he was an agnostic: "Is there a supernatural power—an arbitrary mind—an enthroned God—a supreme will that sways the tides and currents of the world—to which all causes bow? I do not deny. I do not know—but I do not believe. I believe that the natural is supreme—that from the infinite chain no link can be lost or broken—that there is no supernatural power that can answer prayer—no power that worship can persuade or change—no power that cares for man.

I believe that with infinite arms Nature embraces the all—that there is no interference—no chance—that behind every event are the necessary and countless causes, and that beyond every event will be and must be the necessary and countless effects.

Is there a God? I do not know. Is man immortal? I do not know. One thing I do know, and that is, that neither hope, nor fear, belief, nor denial, can change the fact. It is as it is, and it will be as it must be."

In the conclusion of the speech he simply sums up the agnostic position as: "We can be as honest as we are ignorant. If we are, when asked what is beyond the horizon of the known, we must say that we do not know."

In 1885, Ingersoll explained his comparative view of agnosticism and atheism as follows:

"The Agnostic is an Atheist. The Atheist is an Agnostic. The Agnostic says, 'I do not know, but I do not believe there is any God.' The Atheist says the same."

Bernard Iddings Bell
Canon Bernard Iddings Bell (1886–1958), a popular cultural commentator, Episcopal priest, and author, lauded the necessity of agnosticism in Beyond Agnosticism: A Book for Tired Mechanists, calling it the foundation of "all intelligent Christianity". Agnosticism was a temporary mindset in which one rigorously questioned the truths of the age, including the way in which one believed God. His view of Robert Ingersoll and Thomas Paine was that they were not denouncing true Christianity but rather "a gross perversion of it". Part of the misunderstanding stemmed from ignorance of the concepts of God and religion. Historically, a god was any real, perceivable force that ruled the lives of humans and inspired admiration, love, fear, and homage; religion was the practice of it. Ancient peoples worshiped gods with real counterparts, such as Mammon (money and material things), Nabu (rationality), or Ba'al (violent weather); Bell argued that modern peoples were still paying homage—with their lives and their children's lives—to these old gods of wealth, physical appetites, and self-deification. Thus, if one attempted to be agnostic passively, he or she would incidentally join the worship of the world's gods.

In Unfashionable Convictions (1931), he criticized the Enlightenment's complete faith in human sensory perception, augmented by scientific instruments, as a means of accurately grasping Reality. Firstly, it was fairly new, an innovation of the Western World, which Aristotle invented and Thomas Aquinas revived among the scientific community. Secondly, the divorce of "pure" science from human experience, as manifested in American Industrialization, had completely altered the environment, often disfiguring it, so as to suggest its insufficiency to human needs. Thirdly, because scientists were constantly producing more data—to the point where no single human could grasp it all at once—it followed that human intelligence was incapable of attaining a complete understanding of universe; therefore, to admit the mysteries of the unobserved universe was to be actually scientific.

Bell believed that there were two other ways that humans could perceive and interact with the world. Artistic experience was how one expressed meaning through speaking, writing, painting, gesturing—any sort of communication which shared insight into a human's inner reality. Mystical experience was how one could "read" people and harmonize with them, being what we commonly call love. In summary, man was a scientist, artist, and lover. Without exercising all three, a person became "lopsided".

Bell considered a humanist to be a person who cannot rightly ignore the other ways of knowing. However, humanism, like agnosticism, was also temporal, and would eventually lead to either scientific materialism or theism. He lays out the following thesis:

 * 1) Truth cannot be discovered by reasoning on the evidence of scientific data alone. Modern peoples' dissatisfaction with life is the result of depending on such incomplete data. Our ability to reason is not a way to discover Truth but rather a way to organize our knowledge and experiences somewhat sensibly. Without a full, human perception of the world, one's reason tends to lead them in the wrong direction.
 * 2) Beyond what can be measured with scientific tools, there are other types of perception, such as one's ability know another human through loving. One's loves cannot be dissected and logged in a scientific journal, but we know them far better than we know the surface of the sun. They show us an indefinable reality that is nevertheless intimate and personal, and they reveal qualities lovelier and truer than detached facts can provide.
 * 3) To be religious, in the Christian sense, is to live for the Whole of Reality (God) rather than for a small part (gods). Only by treating this Whole of Reality as a person—good and true and perfect—rather than an impersonal force, can we come closer to the Truth. An ultimate Person can be loved, but a cosmic force cannot. A scientist can only discover peripheral truths, but a lover is able to get at the Truth.
 * 4) There are many reasons to believe in God but they are not sufficient for an agnostic to become a theist. It is not enough to believe in an ancient holy book, even though when it is accurately analyzed without bias, it proves to be more trustworthy and admirable than what we are taught in school. Neither is it enough to realize how probable it is that a personal God would have to show human beings how to live, considering they have so much trouble on their own. Nor is it enough to believe for the reason that, throughout history, millions of people have arrived at this Wholeness of Reality only through religious experience. The aforementioned reasons may warm one toward religion, but they fall short of convincing. However, if one presupposes that God is in fact a knowable, loving person, as an experiment, and then lives according that religion, he or she will suddenly come face to face with experiences previously unknown. One's life becomes full, meaningful, and fearless in the face of death. It does not defy reason but exceeds it.
 * 5) Because God has been experienced through love, the orders of prayer, fellowship, and devotion now matter. They create order within one's life, continually renewing the "missing piece" that had previously felt lost. They empower one to be compassionate and humble, not small-minded or arrogant.
 * 6) No truth should be denied outright, but all should be questioned. Science reveals an ever-growing vision of our universe that should not be discounted due to bias toward older understandings. Reason is to be trusted and cultivated. To believe in God is not to forego reason or to deny scientific facts, but to step into the unknown and discover the fullness of life.

Demographics

Demographic research services normally do not differentiate between various types of non-religious respondents, so agnostics are often classified in the same category as atheists or other non-religious people.

A 2010 survey published in Encyclopædia Britannica found that the non-religious people or the agnostics made up about 9.6% of the world's population. A November–December 2006 poll published in the Financial Times gives rates for the United States and five European countries. The rates of agnosticism in the United States were at 14%, while the rates of agnosticism in the European countries surveyed were considerably higher: Italy (20%), Spain (30%), Great Britain (35%), Germany (25%), and France (32%).

A study conducted by the Pew Research Center found that about 16% of the world's people, the third largest group after Christianity and Islam, have no religious affiliation. According to a 2012 report by the Pew Research Center, agnostics made up 3.3% of the US adult population. In the U.S. Religious Landscape Survey, conducted by the Pew Research Center, 55% of agnostic respondents expressed "a belief in God or a universal spirit", whereas 41% stated that they thought that they felt a tension "being non-religious in a society where most people are religious".

According to the 2021 Australian Bureau of Statistics, 38.9% of Australians have "no religion", a category that includes agnostics. Between 64% and 65% of Japanese, and up to 81% of Vietnamese, are atheists, agnostics, or do not believe in a god. An official European Union survey reported that 3% of the EU population is unsure about their belief in a god or spirit.

Criticism
Agnosticism is criticized from a variety of standpoints. Some atheists criticize the use of the term agnosticism as functionally indistinguishable from atheism; this results in frequent criticisms of those who adopt the term as avoiding the atheist label.

Theistic
Theistic critics claim that agnosticism is impossible in practice, since a person can live only either as if God did not exist (etsi deus non-daretur), or as if God did exist (etsi deus daretur).

Christian
According to Pope Benedict XVI, strong agnosticism in particular contradicts itself in affirming the power of reason to know scientific truth. He blames the exclusion of reasoning from religion and ethics for dangerous pathologies such as crimes against humanity and ecological disasters. "Agnosticism", said Benedict, "is always the fruit of a refusal of that knowledge which is in fact offered to man ... The knowledge of God has always existed". He asserted that agnosticism is a choice of comfort, pride, dominion, and utility over truth, and is opposed by the following attitudes: the keenest self-criticism, humble listening to the whole of existence, the persistent patience and self-correction of the scientific method, a readiness to be purified by the truth.

The Catholic Church sees merit in examining what it calls "partial agnosticism", specifically those systems that "do not aim at constructing a complete philosophy of the unknowable, but at excluding special kinds of truth, notably religious, from the domain of knowledge". However, the Church is historically opposed to a full denial of the capacity of human reason to know God. The Council of the Vatican declares, "God, the beginning and end of all, can, by the natural light of human reason, be known with certainty from the works of creation".

Blaise Pascal argued that even if there were truly no evidence for God, agnostics should consider what is now known as Pascal's Wager: the infinite expected value of acknowledging God is always greater than the finite expected value of not acknowledging his existence, and thus it is a safer "bet" to choose God.

Atheistic
According to Richard Dawkins, a distinction between agnosticism and atheism is unwieldy and depends on how close to zero a person is willing to rate the probability of existence for any given god-like entity. About himself, Dawkins continues, "I am agnostic only to the extent that I am agnostic about fairies at the bottom of the garden." Dawkins also identifies two categories of agnostics; "Temporary Agnostics in Practice" (TAPs), and "Permanent Agnostics in Principle" (PAPs). He states that "agnosticism about the existence of God belongs firmly in the temporary or TAP category. Either he exists or he doesn't. It is a scientific question; one day we may know the answer, and meanwhile we can say something pretty strong about the probability", and considers PAP a "deeply inescapable kind of fence-sitting".

Ignosticism
A related concept is ignosticism, the view that a coherent definition of a deity must be put forward before the question of the existence of a deity can be meaningfully discussed. If the chosen definition is not coherent, the ignostic holds the noncognitivist view that the existence of a deity is meaningless or empirically untestable. A. J. Ayer, Theodore Drange, and other philosophers see both atheism and agnosticism as incompatible with ignosticism on the grounds that atheism and agnosticism accept the statement "a deity exists" as a meaningful proposition that can be argued for or against.

title: Agnosticism


text:
Argon

Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide (400 ppmv), and more than 500 times as abundant as neon (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Nearly all argon in Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas.

The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning 'lazy' or 'inactive', as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.

Argon is extracted industrially by the fractional distillation of liquid air. It is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. It is also used in incandescent, fluorescent lighting, and other gas-discharge tubes. It makes a distinctive blue-green gas laser. It is also used in fluorescent glow starters.

Characteristics

Argon has approximately the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon is colorless, odorless, nonflammable and nontoxic as a solid, liquid or gas. Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperature.

Although argon is a noble gas, it can form some compounds under various extreme conditions. Argon fluorohydride (HArF), a compound of argon with fluorine and hydrogen that is stable below 17 K, has been demonstrated. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of argon are trapped in a lattice of water molecules. Ions, such as, and excited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several more argon compounds that should be stable but have not yet been synthesized.

History

Argon (Greek ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive") is named in reference to its chemical inactivity. This chemical property of this first noble gas to be discovered impressed the namers. An unreactive gas was suspected to be a component of air by Henry Cavendish in 1785.

Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide, water, and nitrogen from a sample of clean air. They first accomplished this by replicating an experiment of Henry Cavendish's. They trapped a mixture of atmospheric air with additional oxygen in a test-tube (A) upside-down over a large quantity of dilute alkali solution (B), which in Cavendish's original experiment was potassium hydroxide, and conveyed a current through wires insulated by U-shaped glass tubes (CC) which sealed around the platinum wire electrodes, leaving the ends of the wires (DD) exposed to the gas and insulated from the alkali solution. The arc was powered by a battery of five Grove cells and a Ruhmkorff coil of medium size. The alkali absorbed the oxides of nitrogen produced by the arc and also carbon dioxide. They operated the arc until no more reduction of volume of the gas could be seen for at least an hour or two and the spectral lines of nitrogen disappeared when the gas was examined. The remaining oxygen was reacted with alkaline pyrogallate to leave behind an apparently non-reactive gas which they called argon.

Before isolating the gas, they had determined that nitrogen produced from chemical compounds was 0.5% lighter than nitrogen from the atmosphere. The difference was slight, but it was important enough to attract their attention for many months. They concluded that there was another gas in the air mixed in with the nitrogen. Argon was also encountered in 1882 through independent research of H. F. Newall and W. N. Hartley. Each observed new lines in the emission spectrum of air that did not match known elements.

Until 1957, the symbol for argon was "A", but now it is "Ar".

Occurrence
Argon constitutes 0.934% by volume and 1.288% by mass of Earth's atmosphere. Air is the primary industrial source of purified argon products. Argon is isolated from air by fractionation, most commonly by cryogenic fractional distillation, a process that also produces purified nitrogen, oxygen, neon, krypton and xenon. Earth's crust and seawater contain 1.2 ppm and 0.45 ppm of argon, respectively.

Isotopes
The main isotopes of argon found on Earth are (99.6%),  (0.34%), and  (0.06%). Naturally occurring, with a half-life of 1.25 years, decays to stable (11.2%) by electron capture or positron emission, and also to stable  (88.8%) by beta decay. These properties and ratios are used to determine the age of rocks by K–Ar dating.

In Earth's atmosphere, is made by cosmic ray activity, primarily by neutron capture of  followed by two-neutron emission. In the subsurface environment, it is also produced through neutron capture by, followed by proton emission. is created from the neutron capture by followed by an alpha particle emission as a result of subsurface nuclear explosions. It has a half-life of 35 days.

Between locations in the Solar System, the isotopic composition of argon varies greatly. Where the major source of argon is the decay of potassium-40 in rocks, will be the dominant isotope, as it is on Earth. Argon produced directly by stellar nucleosynthesis is dominated by the alpha-process nuclide. Correspondingly, solar argon contains 84.6% (according to solar wind measurements), and the ratio of the three isotopes 36Ar : 38Ar : 40Ar in the atmospheres of the outer planets is 8400 : 1600 : 1. This contrasts with the low abundance of primordial in Earth's atmosphere, which is only 31.5 ppmv (= 9340 ppmv × 0.337%), comparable with that of neon (18.18 ppmv) on Earth and with interplanetary gasses, measured by probes.

The atmospheres of Mars, Mercury and Titan (the largest moon of Saturn) contain argon, predominantly as.

The predominance of radiogenic is the reason the standard atomic weight of terrestrial argon is greater than that of the next element, potassium, a fact that was puzzling when argon was discovered. Mendeleev positioned the elements on his periodic table in order of atomic weight, but the inertness of argon suggested a placement before the reactive alkali metal. Henry Moseley later solved this problem by showing that the periodic table is actually arranged in order of atomic number (see History of the periodic table).

Compounds
Argon's complete octet of electrons indicates full s and p subshells. This full valence shell makes argon very stable and extremely resistant to bonding with other elements. Before 1962, argon and the other noble gases were considered to be chemically inert and unable to form compounds; however, compounds of the heavier noble gases have since been synthesized. The first argon compound with tungsten pentacarbonyl, W(CO)5Ar, was isolated in 1975. However, it was not widely recognised at that time. In August 2000, another argon compound, argon fluorohydride (HArF), was formed by researchers at the University of Helsinki, by shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride with caesium iodide. This discovery caused the recognition that argon could form weakly bound compounds, even though it was not the first. It is stable up to 17 kelvins (−256 °C). The metastable dication, which is valence-isoelectronic with carbonyl fluoride and phosgene, was observed in 2010. Argon-36, in the form of argon hydride (argonium) ions, has been detected in interstellar medium associated with the Crab Nebula supernova; this was the first noble-gas molecule detected in outer space.

Solid argon hydride (Ar(H2)2) has the same crystal structure as the MgZn2 Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the H2 molecules in Ar(H2)2 dissociate above 175 GPa.

Production
Argon is extracted industrially by the fractional distillation of liquid air in a cryogenic air separation unit; a process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K, and liquid oxygen, which boils at 90.2 K. About 700,000 tonnes of argon are produced worldwide every year.

Applications

Argon has several desirable properties:
 * Argon is a chemically inert gas.
 * Argon is the cheapest alternative when nitrogen is not sufficiently inert.
 * Argon has low thermal conductivity.
 * Argon has electronic properties (ionization and/or the emission spectrum) desirable for some applications.

Other noble gases would be equally suitable for most of these applications, but argon is by far the cheapest. It is inexpensive, since it occurs naturally in air and is readily obtained as a byproduct of cryogenic air separation in the production of liquid oxygen and liquid nitrogen: the primary constituents of air are used on a large industrial scale. The other noble gases (except helium) are produced this way as well, but argon is the most plentiful by far. The bulk of its applications arise simply because it is inert and relatively cheap.

Industrial processes
Argon is used in some high-temperature industrial processes where ordinarily non-reactive substances become reactive. For example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning.

For some of these processes, the presence of nitrogen or oxygen gases might cause defects within the material. Argon is used in some types of arc welding such as gas metal arc welding and gas tungsten arc welding, as well as in the processing of titanium and other reactive elements. An argon atmosphere is also used for growing crystals of silicon and germanium.

Argon is used in the poultry industry to asphyxiate birds, either for mass culling following disease outbreaks, or as a means of slaughter more humane than electric stunning. Argon is denser than air and displaces oxygen close to the ground during inert gas asphyxiation. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen within the dead bird, argon also enhances shelf life.

Argon is sometimes used for extinguishing fires where valuable equipment may be damaged by water or foam.

Scientific research
Liquid argon is used as the target for neutrino experiments and direct dark matter searches. The interaction between the hypothetical WIMPs and an argon nucleus produces scintillation light that is detected by photomultiplier tubes. Two-phase detectors containing argon gas are used to detect the ionized electrons produced during the WIMP–nucleus scattering. As with most other liquefied noble gases, argon has a high scintillation light yield (about 51 photons/keV ), is transparent to its own scintillation light, and is relatively easy to purify. Compared to xenon, argon is cheaper and has a distinct scintillation time profile, which allows the separation of electronic recoils from nuclear recoils. On the other hand, its intrinsic beta-ray background is larger due to contamination, unless one uses argon from underground sources, which has much less  contamination. Most of the argon in Earth's atmosphere was produced by electron capture of long-lived ( + e− →  + ν) present in natural potassium within Earth. The activity in the atmosphere is maintained by cosmogenic production through the knockout reaction (n,2n) and similar reactions. The half-life of is only 269 years. As a result, the underground Ar, shielded by rock and water, has much less contamination. Dark-matter detectors currently operating with liquid argon include DarkSide, WArP, ArDM, microCLEAN and DEAP. Neutrino experiments include ICARUS and MicroBooNE, both of which use high-purity liquid argon in a time projection chamber for fine grained three-dimensional imaging of neutrino interactions.

At Linköping University, Sweden, the inert gas is being utilized in a vacuum chamber in which plasma is introduced to ionize metallic films. This process results in a film usable for manufacturing computer processors. The new process would eliminate the need for chemical baths and use of expensive, dangerous and rare materials.

Preservative
Argon is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents (argon has the European food additive code E938). Aerial oxidation, hydrolysis, and other chemical reactions that degrade the products are retarded or prevented entirely. High-purity chemicals and pharmaceuticals are sometimes packed and sealed in argon.

In winemaking, argon is used in a variety of activities to provide a barrier against oxygen at the liquid surface, which can spoil wine by fueling both microbial metabolism (as with acetic acid bacteria) and standard redox chemistry.

Argon is sometimes used as the propellant in aerosol cans.

Argon is also used as a preservative for such products as varnish, polyurethane, and paint, by displacing air to prepare a container for storage.

Since 2002, the American National Archives stores important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to inhibit their degradation. Argon is preferable to the helium that had been used in the preceding five decades, because helium gas escapes through the intermolecular pores in most containers and must be regularly replaced.

Laboratory equipment

Argon may be used as the inert gas within Schlenk lines and gloveboxes. Argon is preferred to less expensive nitrogen in cases where nitrogen may react with the reagents or apparatus.

Argon may be used as the carrier gas in gas chromatography and in electrospray ionization mass spectrometry; it is the gas of choice for the plasma used in ICP spectroscopy. Argon is preferred for the sputter coating of specimens for scanning electron microscopy. Argon gas is also commonly used for sputter deposition of thin films as in microelectronics and for wafer cleaning in microfabrication.

Medical use
Cryosurgery procedures such as cryoablation use liquid argon to destroy tissue such as cancer cells. It is used in a procedure called "argon-enhanced coagulation", a form of argon plasma beam electrosurgery. The procedure carries a risk of producing gas embolism and has resulted in the death of at least one patient.

Blue argon lasers are used in surgery to weld arteries, destroy tumors, and correct eye defects.

Argon has also been used experimentally to replace nitrogen in the breathing or decompression mix known as Argox, to speed the elimination of dissolved nitrogen from the blood.

Lighting

Incandescent lights are filled with argon, to preserve the filaments at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as in plasma globes and calorimetry in experimental particle physics. Gas-discharge lamps filled with pure argon provide lilac/violet light; with argon and some mercury, blue light. Argon is also used for blue and green argon-ion lasers.

Miscellaneous uses
Argon is used for thermal insulation in energy-efficient windows. Argon is also used in technical scuba diving to inflate a dry suit because it is inert and has low thermal conductivity.

Argon is used as a propellant in the development of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). Compressed argon gas is allowed to expand, to cool the seeker heads of some versions of the AIM-9 Sidewinder missile and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure.

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily ice core and ground water dating. Also, potassium–argon dating and related argon-argon dating are used to date sedimentary, metamorphic, and igneous rocks.

Argon has been used by athletes as a doping agent to simulate hypoxic conditions. In 2014, the World Anti-Doping Agency (WADA) added argon and xenon to the list of prohibited substances and methods, although at this time there is no reliable test for abuse.

Safety
Although argon is non-toxic, it is 38% more dense than air and therefore considered a dangerous asphyxiant in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless. A 1994 incident, in which a man was asphyxiated after entering an argon-filled section of oil pipe under construction in Alaska, highlights the dangers of argon tank leakage in confined spaces and emphasizes the need for proper use, storage and handling.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Argon


text:
Arsenic

Arsenic is a chemical element with the symbol As and the atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is a notoriously toxic heavy metal. It occurs naturally in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. It has various allotropes, but only the grey form, which has a metallic appearance, is important to industry.

The primary use of arsenic is in alloys of lead (for example, in car batteries and ammunition). Arsenic is a common n-type dopant in semiconductor electronic devices. It is also a component of the III–V compound semiconductor gallium arsenide. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides, treated wood products, herbicides, and insecticides. These applications are declining with the increasing recognition of the toxicity of arsenic and its compounds.

A few species of bacteria are able to use arsenic compounds as respiratory metabolites. Trace quantities of arsenic may be an essential dietary element in rats, hamsters, goats, chickens, and presumably other species. A role in human metabolism is not known. However, arsenic poisoning occurs in multicellular life if quantities are larger than needed. Arsenic contamination of groundwater is a problem that affects millions of people across the world.

The United States' Environmental Protection Agency states that all forms of arsenic are a serious risk to human health. The United States' Agency for Toxic Substances and Disease Registry ranked arsenic as number 1 in its 2001 Priority List of Hazardous Substances at Superfund sites. Arsenic is classified as a Group-A carcinogen.

Physical characteristics

The three most common arsenic allotropes are grey, yellow, and black arsenic, with grey being the most common. Grey arsenic (α-As, space group R$\overline{3}$m No. 166) adopts a double-layered structure consisting of many interlocked, ruffled, six-membered rings. Because of weak bonding between the layers, grey arsenic is brittle and has a relatively low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next. This relatively close packing leads to a high density of 5.73 g/cm3. Grey arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized. Grey arsenic is also the most stable form. Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus (P4). Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecular, is the most volatile, least dense, and most toxic. Solid yellow arsenic is produced by rapid cooling of arsenic vapor, As4. It is rapidly transformed into grey arsenic by light. The yellow form has a density of 1.97 g/cm3. Black arsenic is similar in structure to black phosphorus. Black arsenic can also be formed by cooling vapor at around 100–220 °C and by crystallization of amorphous arsenic in the presence of mercury vapors. It is glassy and brittle. Black arsenic is also a poor electrical conductor.

Arsenic sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at 887 K. The triple point is at 3.63 MPa and 1090 K.

Isotopes
Arsenic occurs in nature as one stable isotope, 75As, and is therefore called a monoisotopic element. As of 2024, at least 32 radioisotopes have also been synthesized, ranging in atomic mass from 64 to 95. The most stable of these is 73As with a half-life of 80.30 days. All other isotopes have half-lives of under one day, with the exception of 71As (t1/2=65.30 hours), 72As (t1/2=26.0 hours), 74As (t1/2=17.77 days), 76As (t1/2=26.26 hours), and 77As (t1/2=38.83 hours). Isotopes that are lighter than the stable 75As tend to decay by β+ decay, and those that are heavier tend to decay by β− decay, with some exceptions.

At least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most stable of arsenic's isomers is 68mAs with a half-life of 111 seconds.

Chemistry
Arsenic has a similar electronegativity and ionization energies to its lighter pnictogen congener phosphorus and therefore readily forms covalent molecules with most of the nonmetals. Though stable in dry air, arsenic forms a golden-bronze tarnish upon exposure to humidity which eventually becomes a black surface layer. When heated in air, arsenic oxidizes to arsenic trioxide; the fumes from this reaction have an odor resembling garlic. This odor can be detected on striking arsenide minerals such as arsenopyrite with a hammer. It burns in oxygen to form arsenic trioxide and arsenic pentoxide, which have the same structure as the more well-known phosphorus compounds, and in fluorine to give arsenic pentafluoride. Arsenic makes arsenic acid with concentrated nitric acid, arsenous acid with dilute nitric acid, and arsenic trioxide with concentrated sulfuric acid; however, it does not react with water, alkalis, or non-oxidising acids. Arsenic reacts with metals to form arsenides, though these are not ionic compounds containing the As3− ion as the formation of such an anion would be highly endothermic and even the group 1 arsenides have properties of intermetallic compounds. Like germanium, selenium, and bromine, which like arsenic succeed the 3d transition series, arsenic is much less stable in the +5 oxidation state than its vertical neighbors phosphorus and antimony, and hence arsenic pentoxide and arsenic acid are potent oxidizers.

Compounds
Compounds of arsenic resemble in some respects those of phosphorus which occupies the same group (column) of the periodic table. The most common oxidation states for arsenic are: −3 in the arsenides, which are alloy-like intermetallic compounds, +3 in the arsenites, and +5 in the arsenates and most organoarsenic compounds. Arsenic also bonds readily to itself as seen in the square As4(3-) ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal owing to the influence of the lone pair of electrons.

Inorganic compounds
One of the simplest arsenic compounds is the trihydride, the highly toxic, flammable, pyrophoric arsine (AsH3). This compound is generally regarded as stable, since at room temperature it decomposes only slowly. At temperatures of 250–300 °C decomposition to arsenic and hydrogen is rapid. Several factors, such as humidity, presence of light and certain catalysts (namely aluminium) facilitate the rate of decomposition. It oxidises readily in air to form arsenic trioxide and water, and analogous reactions take place with sulfur and selenium instead of oxygen.

Arsenic forms colorless, odorless, crystalline oxides As2O3 ("white arsenic") and As2O5 which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic(V) acid is a weak acid and its salts, known as arsenates, are a major source of arsenic contamination of groundwater in regions with high levels of naturally-occurring arsenic minerals. Synthetic arsenates include Scheele's Green (cupric hydrogen arsenate, acidic copper arsenate), calcium arsenate, and lead hydrogen arsenate. These three have been used as agricultural insecticides and poisons.

The protonation steps between the arsenate and arsenic acid are similar to those between phosphate and phosphoric acid. Unlike phosphorous acid, arsenous acid is genuinely tribasic, with the formula As(OH)3.

A broad variety of sulfur compounds of arsenic are known. Orpiment (As2S3) and realgar (As4S4) are somewhat abundant and were formerly used as painting pigments. In As4S10, arsenic has a formal oxidation state of +2 in As4S4 which features As-As bonds so that the total covalency of As is still 3. Both orpiment and realgar, as well as As4S3, have selenium analogs; the analogous As2Te3 is known as the mineral kalgoorlieite, and the anion As2Te− is known as a ligand in cobalt complexes.

All trihalides of arsenic(III) are well known except the astatide, which is unknown. Arsenic pentafluoride (AsF5) is the only important pentahalide, reflecting the lower stability of the +5 oxidation state; even so, it is a very strong fluorinating and oxidizing agent. (The pentachloride is stable only below −50 °C, at which temperature it decomposes to the trichloride, releasing chlorine gas. )

Alloys
Arsenic is used as the group 5 element in the III-V semiconductors gallium arsenide, indium arsenide, and aluminium arsenide. The valence electron count of GaAs is the same as a pair of Si atoms, but the band structure is completely different which results in distinct bulk properties. Other arsenic alloys include the II-V semiconductor cadmium arsenide.

Organoarsenic compounds

A large variety of organoarsenic compounds are known. Several were developed as chemical warfare agents during World War I, including vesicants such as lewisite and vomiting agents such as adamsite. Cacodylic acid, which is of historic and practical interest, arises from the methylation of arsenic trioxide, a reaction that has no analogy in phosphorus chemistry. Cacodyl was the first organometallic compound known (even though arsenic is not a true metal) and was named from the Greek κακωδία "stink" for its offensive, garlic-like odor; it is very toxic.

Occurrence and production

Arsenic is the 53rd most abundant element in the Earth's crust, comprising about 1.5 parts per million (0.00015%). Typical background concentrations of arsenic do not exceed 3 ng/m3 in the atmosphere; 100 mg/kg in soil; 400 μg/kg in vegetation; 10 μg/L in freshwater and 1.5 μg/L in seawater. Arsenic is the 22nd most abundant element in seawater and ranks 41st in abundance in the universe.

Minerals with the formula MAsS and MAs2 (M = Fe, Ni, Co) are the dominant commercial sources of arsenic, together with realgar (an arsenic sulfide mineral) and native (elemental) arsenic. An illustrative mineral is arsenopyrite (FeAsS), which is structurally related to iron pyrite. Many minor As-containing minerals are known. Arsenic also occurs in various organic forms in the environment.

In 2014, China was the top producer of white arsenic with almost 70% world share, followed by Morocco, Russia, and Belgium, according to the British Geological Survey and the United States Geological Survey. Most arsenic refinement operations in the US and Europe have closed over environmental concerns. Arsenic is found in the smelter dust from copper, gold, and lead smelters, and is recovered primarily from copper refinement dust.

On roasting arsenopyrite in air, arsenic sublimes as arsenic(III) oxide leaving iron oxides, while roasting without air results in the production of gray arsenic. Further purification from sulfur and other chalcogens is achieved by sublimation in vacuum, in a hydrogen atmosphere, or by distillation from molten lead-arsenic mixture.

History

The word arsenic has its origin in the Syriac word ܙܪܢܝܟܐ zarnika, from Arabic al-zarnīḵ الزرنيخ 'the orpiment', based on Persian zar ("gold") from the word زرنيخ zarnikh, meaning "yellow" (literally "gold-colored") and hence "(yellow) orpiment". It was adopted into Greek (using folk etymology) as arsenikon (ἀρσενικόν) – a neuter form of the Greek adjective arsenikos (ἀρσενικός), meaning "male", "virile".

Latin-speakers adopted the Greek term as arsenicum, which in French ultimately became arsenic, whence the English word "arsenic". Arsenic sulfides (orpiment, realgar) and oxides have been known and used since ancient times. Zosimos (c. 300 AD) describes roasting sandarach (realgar) to obtain cloud of arsenic (arsenic trioxide), which he then reduces to gray arsenic. As the symptoms of arsenic poisoning are not very specific, the substance was frequently used for murder until the advent in the 1830s of the Marsh test, a sensitive chemical test for its presence. (Another less sensitive but more general test is the Reinsch test.) Owing to its use by the ruling class to murder one another and its potency and discreetness, arsenic has been called the "poison of kings" and the "king of poisons". Arsenic became known as "the inheritance powder" due to its use in killing family members in the Renaissance era.

During the Bronze Age, arsenic was melted with copper to make arsenical bronze. Jabir ibn Hayyan described the isolation of arsenic before 815 AD. Albertus Magnus (Albert the Great, 1193–1280) later isolated the element from a compound in 1250, by heating soap together with arsenic trisulfide. In 1649, Johann Schröder published two ways of preparing arsenic. Crystals of elemental (native) arsenic are found in nature, although rarely.

Cadet's fuming liquid (impure cacodyl), often claimed as the first synthetic organometallic compound, was synthesized in 1760 by Louis Claude Cadet de Gassicourt through the reaction of potassium acetate with arsenic trioxide.

In the Victorian era, women would eat "arsenic" ("white arsenic" or arsenic trioxide) mixed with vinegar and chalk to improve the complexion of their faces, making their skin paler (to show they did not work in the fields). The accidental use of arsenic in the adulteration of foodstuffs led to the Bradford sweet poisoning in 1858, which resulted in 21 deaths. From the late-18th century wallpaper production began to use dyes made from arsenic, which was thought to increase the pigment's brightness. One account of the illness and 1821 death of Napoleon I implicates arsenic poisoning involving wallpaper.

Two arsenic pigments have been widely used since their discovery – Paris Green in 1814 and Scheele's Green in 1775. After the toxicity of arsenic became widely known, these chemicals were used less often as pigments and more often as insecticides. In the 1860s, an arsenic byproduct of dye production, London Purple, was widely used. This was a solid mixture of arsenic trioxide, aniline, lime, and ferrous oxide, insoluble in water and very toxic by inhalation or ingestion But it was later replaced with Paris Green, another arsenic-based dye. With better understanding of the toxicology mechanism, two other compounds were used starting in the 1890s. Arsenite of lime and arsenate of lead were used widely as insecticides until the discovery of DDT in 1942.

In small doses, soluble arsenic compounds act as stimulants, and were once popular as medicine by people in the mid-18th to 19th centuries;  this use was especially prevalent for sport animals such as race horses or work dogs and continued into the 20th century. A 2006 study of the remains of the Australian racehorse Phar Lap determined that its 1932 death was caused by a massive overdose of arsenic. Sydney veterinarian Percy Sykes stated, "In those days, arsenic was quite a common tonic, usually given in the form of a solution (Fowler's Solution) ... It was so common that I'd reckon 90 per cent of the horses had arsenic in their system."

Agricultural

The toxicity of arsenic to insects, bacteria, and fungi led to its use as a wood preservative. In the 1930s, a process of treating wood with chromated copper arsenate (also known as CCA or Tanalith) was invented, and for decades, this treatment was the most extensive industrial use of arsenic. An increased appreciation of the toxicity of arsenic led to a ban of CCA in consumer products in 2004, initiated by the European Union and United States. However, CCA remains in heavy use in other countries (such as on Malaysian rubber plantations).

Arsenic was also used in various agricultural insecticides and poisons. For example, lead hydrogen arsenate was a common insecticide on fruit trees, but contact with the compound sometimes resulted in brain damage among those working the sprayers. In the second half of the 20th century, monosodium methyl arsenate (MSMA) and disodium methyl arsenate (DSMA) – less toxic organic forms of arsenic – replaced lead arsenate in agriculture. These organic arsenicals were in turn phased out in the United States by 2013 in all agricultural activities except cotton farming.

The biogeochemistry of arsenic is complex and includes various adsorption and desorption processes. The toxicity of arsenic is connected to its solubility and is affected by pH. Arsenite (AsO3(3-)) is more soluble than arsenate (AsO4(3-)) and is more toxic; however, at a lower pH, arsenate becomes more mobile and toxic. It was found that addition of sulfur, phosphorus, and iron oxides to high-arsenite soils greatly reduces arsenic phytotoxicity.

Arsenic is used as a feed additive in poultry and swine production, in particular it was used in the U.S. until 2015 to increase weight gain, improve feed efficiency, and prevent disease. An example is roxarsone, which had been used as a broiler starter by about 70% of U.S. broiler growers. In 2011, Alpharma, a subsidiary of Pfizer Inc., which produces roxarsone, voluntarily suspended sales of the drug in response to studies showing elevated levels of inorganic arsenic, a carcinogen, in treated chickens. A successor to Alpharma, Zoetis, continued to sell nitarsone until 2015, primarily for use in turkeys.

Medical use
During the 17th, 18th, and 19th centuries, a number of arsenic compounds were used as medicines, including arsphenamine (by Paul Ehrlich) and arsenic trioxide (by Thomas Fowler), for treating diseases such as cancer or psoriasis. Arsphenamine, as well as neosalvarsan, was indicated for syphilis, but has been superseded by modern antibiotics. However, arsenicals such as melarsoprol are still used for the treatment of trypanosomiasis, since although these drugs have the disadvantage of severe toxicity, the disease is almost uniformly fatal if untreated. In 2000 the US Food and Drug Administration approved arsenic trioxide for the treatment of patients with acute promyelocytic leukemia that is resistant to all-trans retinoic acid.

A 2008 paper reports success in locating tumors using arsenic-74 (a positron emitter). This isotope produces clearer PET scan images than the previous radioactive agent, iodine-124, because the body tends to transport iodine to the thyroid gland producing signal noise. Nanoparticles of arsenic have shown ability to kill cancer cells with lesser cytotoxicity than other arsenic formulations.

Alloys
The main use of arsenic is in alloying with lead. Lead components in car batteries are strengthened by the presence of a very small percentage of arsenic. Dezincification of brass (a copper-zinc alloy) is greatly reduced by the addition of arsenic. "Phosphorus Deoxidized Arsenical Copper" with an arsenic content of 0.3% has an increased corrosion stability in certain environments. Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made from GaAs are much faster (but also much more expensive) than those made from silicon. Unlike silicon, GaAs has a direct bandgap, and can be used in laser diodes and LEDs to convert electrical energy directly into light.

Military
After World War I, the United States built a stockpile of 20,000 tons of weaponized lewisite (ClCH=CHAsCl2), an organoarsenic vesicant (blister agent) and lung irritant. The stockpile was neutralized with bleach and dumped into the Gulf of Mexico in the 1950s. During the Vietnam War, the United States used Agent Blue, a mixture of sodium cacodylate and its acid form, as one of the rainbow herbicides to deprive North Vietnamese soldiers of foliage cover and rice.

Other uses

 * Copper acetoarsenite was used as a green pigment known under many names, including Paris Green and Emerald Green. It caused numerous arsenic poisonings. Scheele's Green, a copper arsenate, was used in the 19th century as a coloring agent in sweets.
 * Arsenic is used in bronzing and pyrotechnics.
 * As much as 2% of produced arsenic is used in lead alloys for lead shot and bullets.
 * Arsenic is added in small quantities to alpha-brass to make it dezincification-resistant. This grade of brass is used in plumbing fittings and other wet environments.
 * Arsenic is also used for taxonomic sample preservation. It was also used in embalming fluids historically.
 * Arsenic was used in the taxidermy process up until the 1980s.
 * Arsenic was used as an opacifier in ceramics, creating white glazes.
 * Until recently, arsenic was used in optical glass. Modern glass manufacturers have ceased using both arsenic and lead.

Bacteria
Some species of bacteria obtain their energy in the absence of oxygen by oxidizing various fuels while reducing arsenate to arsenite. Under oxidative environmental conditions some bacteria use arsenite as fuel, which they oxidize to arsenate. The enzymes involved are known as arsenate reductases (Arr).

In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just as ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that, over the course of history, these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive. One strain, PHS-1, has been isolated and is related to the gammaproteobacterium Ectothiorhodospira shaposhnikovii. The mechanism is unknown, but an encoded Arr enzyme may function in reverse to its known homologues.

In 2011, it was postulated that the Halomonadaceae strain GFAJ-1 could be grown in the absence of phosphorus if that element were substituted with arsenic, exploiting the fact that the arsenate and phosphate anions are similar structurally. The study was widely criticised and subsequently refuted by independent researcher groups.

Essential trace element in higher animals
Arsenic may be an essential trace mineral in birds, involved in the synthesis of methionine metabolites, with feeding recommendations being between 0.012 and 0.050 mg/kg.

Some evidence indicates that arsenic is an essential trace mineral in mammals. However, the biological function is not known.

Heredity
Arsenic has been linked to epigenetic changes, heritable changes in gene expression that occur without changes in DNA sequence. These include DNA methylation, histone modification, and RNA interference. Toxic levels of arsenic cause significant DNA hypermethylation of tumor suppressor genes p16 and p53, thus increasing risk of carcinogenesis. These epigenetic events have been studied in vitro using human kidney cells and in vivo using rat liver cells and peripheral blood leukocytes in humans. Inductively coupled plasma mass spectrometry (ICP-MS) is used to detect precise levels of intracellular arsenic and other arsenic bases involved in epigenetic modification of DNA. Studies investigating arsenic as an epigenetic factor can be used to develop precise biomarkers of exposure and susceptibility.

The Chinese brake fern (Pteris vittata) hyperaccumulates arsenic from the soil into its leaves and has a proposed use in phytoremediation.

Biomethylation

Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolized through a process of methylation. For example, the mold Scopulariopsis brevicaulis produces trimethylarsine if inorganic arsenic is present. The organic compound arsenobetaine is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. The average person's intake is about 10–50 μg/day. Values about 1000 μg are not unusual following consumption of fish or mushrooms, but there is little danger in eating fish because this arsenic compound is nearly non-toxic.

Exposure
Naturally occurring sources of human exposure include volcanic ash, weathering of minerals and ores, and mineralized groundwater. Arsenic is also found in food, water, soil, and air. Arsenic is absorbed by all plants, but is more concentrated in leafy vegetables, rice, apple and grape juice, and seafood. An additional route of exposure is inhalation of atmospheric gases and dusts. During the Victorian era, arsenic was widely used in home decor, especially wallpapers. In Europe, an analysis based on 20,000 soil samples across all 28 countries show that 98% of sampled soils have concentrations less than 20 mg kg-1. In addition, the As hotspots are related to frequent fertilization and close distance to mining activities.

Occurrence in drinking water
Extensive arsenic contamination of groundwater has led to widespread arsenic poisoning in Bangladesh and neighboring countries. It is estimated that approximately 57 million people in the Bengal basin are drinking groundwater with arsenic concentrations elevated above the World Health Organization's standard of 10 parts per billion (ppb). However, a study of cancer rates in Taiwan suggested that significant increases in cancer mortality appear only at levels above 150 ppb. The arsenic in the groundwater is of natural origin, and is released from the sediment into the groundwater, caused by the anoxic conditions of the subsurface. This groundwater was used after local and western NGOs and the Bangladeshi government undertook a massive shallow tube well drinking-water program in the late twentieth century. This program was designed to prevent drinking of bacteria-contaminated surface waters, but failed to test for arsenic in the groundwater. Many other countries and districts in Southeast Asia, such as Vietnam and Cambodia, have geological environments that produce groundwater with a high arsenic content. Arsenicosis was reported in Nakhon Si Thammarat, Thailand, in 1987, and the Chao Phraya River probably contains high levels of naturally occurring dissolved arsenic without being a public health problem because much of the public uses bottled water. In Pakistan, more than 60 million people are exposed to arsenic polluted drinking water indicated by a 2017 report in Science. Podgorski's team investigated more than 1200 samples and more than 66% exceeded the WHO minimum contamination level.

Since the 1980s, residents of the Ba Men region of Inner Mongolia, China have been chronically exposed to arsenic through drinking water from contaminated wells. A 2009 research study observed an elevated presence of skin lesions among residents with well water arsenic concentrations between 5 and 10 μg/L, suggesting that arsenic induced toxicity may occur at relatively low concentrations with chronic exposure. Overall, 20 of China's 34 provinces have high arsenic concentrations in the groundwater supply, potentially exposing 19 million people to hazardous drinking water.

A study by IIT Kharagpur found high levels of Arsenic in groundwater of 20% of India's land, exposing more than 250 million people. States such as Punjab, Bihar, West Bengal, Assam, Haryana, Uttar Pradesh, and Gujarat have highest land area exposed to arsenic.

In the United States, arsenic is most commonly found in the ground waters of the southwest. Parts of New England, Michigan, Wisconsin, Minnesota and the Dakotas are also known to have significant concentrations of arsenic in ground water. Increased levels of skin cancer have been associated with arsenic exposure in Wisconsin, even at levels below the 10 ppb drinking water standard. According to a recent film funded by the US Superfund, millions of private wells have unknown arsenic levels, and in some areas of the US, more than 20% of the wells may contain levels that exceed established limits.

Low-level exposure to arsenic at concentrations of 100 ppb (i.e., above the 10 ppb drinking water standard) compromises the initial immune response to H1N1 or swine flu infection according to NIEHS-supported scientists. The study, conducted in laboratory mice, suggests that people exposed to arsenic in their drinking water may be at increased risk for more serious illness or death from the virus.

Some Canadians are drinking water that contains inorganic arsenic. Private-dug–well waters are most at risk for containing inorganic arsenic. Preliminary well water analysis typically does not test for arsenic. Researchers at the Geological Survey of Canada have modeled relative variation in natural arsenic hazard potential for the province of New Brunswick. This study has important implications for potable water and health concerns relating to inorganic arsenic.

Epidemiological evidence from Chile shows a dose-dependent connection between chronic arsenic exposure and various forms of cancer, in particular when other risk factors, such as cigarette smoking, are present. These effects have been demonstrated at contaminations less than 50 ppb. Arsenic is itself a constituent of tobacco smoke.

Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable increase in risk for bladder cancer at 10 ppb. According to Peter Ravenscroft of the Department of Geography at the University of Cambridge, roughly 80 million people worldwide consume between 10 and 50 ppb arsenic in their drinking water. If they all consumed exactly 10 ppb arsenic in their drinking water, the previously cited multiple epidemiological study analysis would predict an additional 2,000 cases of bladder cancer alone. This represents a clear underestimate of the overall impact, since it does not include lung or skin cancer, and explicitly underestimates the exposure. Those exposed to levels of arsenic above the current WHO standard should weigh the costs and benefits of arsenic remediation.

Early (1973) evaluations of the processes for removing dissolved arsenic from drinking water demonstrated the efficacy of co-precipitation with either iron or aluminium oxides. In particular, iron as a coagulant was found to remove arsenic with an efficacy exceeding 90%. Several adsorptive media systems have been approved for use at point-of-service in a study funded by the United States Environmental Protection Agency (US EPA) and the National Science Foundation (NSF). A team of European and Indian scientists and engineers have set up six arsenic treatment plants in West Bengal based on in-situ remediation method (SAR Technology). This technology does not use any chemicals and arsenic is left in an insoluble form (+5 state) in the subterranean zone by recharging aerated water into the aquifer and developing an oxidation zone that supports arsenic oxidizing micro-organisms. This process does not produce any waste stream or sludge and is relatively cheap.

Another effective and inexpensive method to avoid arsenic contamination is to sink wells 500 feet or deeper to reach purer waters. A recent 2011 study funded by the US National Institute of Environmental Health Sciences' Superfund Research Program shows that deep sediments can remove arsenic and take it out of circulation. In this process, called adsorption, arsenic sticks to the surfaces of deep sediment particles and is naturally removed from the ground water.

Magnetic separations of arsenic at very low magnetic field gradients with high-surface-area and monodisperse magnetite (Fe3O4) nanocrystals have been demonstrated in point-of-use water purification. Using the high specific surface area of Fe3O4 nanocrystals, the mass of waste associated with arsenic removal from water has been dramatically reduced.

Epidemiological studies have suggested a correlation between chronic consumption of drinking water contaminated with arsenic and the incidence of all leading causes of mortality. The literature indicates that arsenic exposure is causative in the pathogenesis of diabetes.

Chaff-based filters have recently been shown to reduce the arsenic content of water to 3 μg/L. This may find applications in areas where the potable water is extracted from underground aquifers.

San Pedro de Atacama
For several centuries, the people of San Pedro de Atacama in Chile have been drinking water that is contaminated with arsenic, and some evidence suggests they have developed some immunity.

Hazard maps for contaminated groundwater
Around one-third of the world's population drinks water from groundwater resources. Of this, about 10 percent, approximately 300 million people, obtains water from groundwater resources that are contaminated with unhealthy levels of arsenic or fluoride. These trace elements derive mainly from minerals and ions in the ground.

Redox transformation of arsenic in natural waters
Arsenic is unique among the trace metalloids and oxyanion-forming trace metals (e.g. As, Se, Sb, Mo, V, Cr, U, Re). It is sensitive to mobilization at pH values typical of natural waters (pH 6.5–8.5) under both oxidizing and reducing conditions. Arsenic can occur in the environment in several oxidation states (−3, 0, +3 and +5), but in natural waters it is mostly found in inorganic forms as oxyanions of trivalent arsenite [As(III)] or pentavalent arsenate [As(V)]. Organic forms of arsenic are produced by biological activity, mostly in surface waters, but are rarely quantitatively important. Organic arsenic compounds may, however, occur where waters are significantly impacted by industrial pollution.

Arsenic may be solubilized by various processes. When pH is high, arsenic may be released from surface binding sites that lose their positive charge. When water level drops and sulfide minerals are exposed to air, arsenic trapped in sulfide minerals can be released into water. When organic carbon is present in water, bacteria are fed by directly reducing As(V) to As(III) or by reducing the element at the binding site, releasing inorganic arsenic.

The aquatic transformations of arsenic are affected by pH, reduction-oxidation potential, organic matter concentration and the concentrations and forms of other elements, especially iron and manganese. The main factors are pH and the redox potential. Generally, the main forms of arsenic under oxic conditions are H3AsO4, H2AsO4-, HAsO4(2-), and AsO4(3-) at pH 2, 2–7, 7–11 and 11, respectively. Under reducing conditions, H3AsO4 is predominant at pH 2–9.

Oxidation and reduction affects the migration of arsenic in subsurface environments. Arsenite is the most stable soluble form of arsenic in reducing environments and arsenate, which is less mobile than arsenite, is dominant in oxidizing environments at neutral pH. Therefore, arsenic may be more mobile under reducing conditions. The reducing environment is also rich in organic matter which may enhance the solubility of arsenic compounds. As a result, the adsorption of arsenic is reduced and dissolved arsenic accumulates in groundwater. That is why the arsenic content is higher in reducing environments than in oxidizing environments.

The presence of sulfur is another factor that affects the transformation of arsenic in natural water. Arsenic can precipitate when metal sulfides form. In this way, arsenic is removed from the water and its mobility decreases. When oxygen is present, bacteria oxidize reduced sulfur to generate energy, potentially releasing bound arsenic.

Redox reactions involving Fe also appear to be essential factors in the fate of arsenic in aquatic systems. The reduction of iron oxyhydroxides plays a key role in the release of arsenic to water. So arsenic can be enriched in water with elevated Fe concentrations. Under oxidizing conditions, arsenic can be mobilized from pyrite or iron oxides especially at elevated pH. Under reducing conditions, arsenic can be mobilized by reductive desorption or dissolution when associated with iron oxides. The reductive desorption occurs under two circumstances. One is when arsenate is reduced to arsenite which adsorbs to iron oxides less strongly. The other results from a change in the charge on the mineral surface which leads to the desorption of bound arsenic.

Some species of bacteria catalyze redox transformations of arsenic. Dissimilatory arsenate-respiring prokaryotes (DARP) speed up the reduction of As(V) to As(III). DARP use As(V) as the electron acceptor of anaerobic respiration and obtain energy to survive. Other organic and inorganic substances can be oxidized in this process. Chemoautotrophic arsenite oxidizers (CAO) and heterotrophic arsenite oxidizers (HAO) convert As(III) into As(V). CAO combine the oxidation of As(III) with the reduction of oxygen or nitrate. They use obtained energy to fix produce organic carbon from CO2. HAO cannot obtain energy from As(III) oxidation. This process may be an arsenic detoxification mechanism for the bacteria.

Equilibrium thermodynamic calculations predict that As(V) concentrations should be greater than As(III) concentrations in all but strongly reducing conditions, i.e. where sulfate reduction is occurring. However, abiotic redox reactions of arsenic are slow. Oxidation of As(III) by dissolved O2 is a particularly slow reaction. For example, Johnson and Pilson (1975) gave half-lives for the oxygenation of As(III) in seawater ranging from several months to a year. In other studies, As(V)/As(III) ratios were stable over periods of days or weeks during water sampling when no particular care was taken to prevent oxidation, again suggesting relatively slow oxidation rates. Cherry found from experimental studies that the As(V)/As(III) ratios were stable in anoxic solutions for up to 3 weeks but that gradual changes occurred over longer timescales. Sterile water samples have been observed to be less susceptible to speciation changes than non-sterile samples. Oremland found that the reduction of As(V) to As(III) in Mono Lake was rapidly catalyzed by bacteria with rate constants ranging from 0.02 to 0.3-day−1.

Wood preservation in the US
As of 2002, US-based industries consumed 19,600 metric tons of arsenic. Ninety percent of this was used for treatment of wood with chromated copper arsenate (CCA). In 2007, 50% of the 5,280 metric tons of consumption was still used for this purpose. In the United States, the voluntary phasing-out of arsenic in production of consumer products and residential and general consumer construction products began on 31 December 2003, and alternative chemicals are now used, such as Alkaline Copper Quaternary, borates, copper azole, cyproconazole, and propiconazole.

Although discontinued, this application is also one of the most concerning to the general public. The vast majority of older pressure-treated wood was treated with CCA. CCA lumber is still in widespread use in many countries, and was heavily used during the latter half of the 20th century as a structural and outdoor building material. Although the use of CCA lumber was banned in many areas after studies showed that arsenic could leach out of the wood into the surrounding soil (from playground equipment, for instance), a risk is also presented by the burning of older CCA timber. The direct or indirect ingestion of wood ash from burnt CCA lumber has caused fatalities in animals and serious poisonings in humans; the lethal human dose is approximately 20 grams of ash. Scrap CCA lumber from construction and demolition sites may be inadvertently used in commercial and domestic fires. Protocols for safe disposal of CCA lumber are not consistent throughout the world. Widespread landfill disposal of such timber raises some concern, but other studies have shown no arsenic contamination in the groundwater.

Mapping of industrial releases in the US
One tool that maps the location (and other information) of arsenic releases in the United States is TOXMAP. TOXMAP is a Geographic Information System (GIS) from the Division of Specialized Information Services of the United States National Library of Medicine (NLM) funded by the US Federal Government. With marked-up maps of the United States, TOXMAP enables users to visually explore data from the United States Environmental Protection Agency's (EPA) Toxics Release Inventory and Superfund Basic Research Programs. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network (TOXNET), PubMed, and from other authoritative sources.

Bioremediation
Physical, chemical, and biological methods have been used to remediate arsenic contaminated water. Bioremediation is said to be cost-effective and environmentally friendly. Bioremediation of ground water contaminated with arsenic aims to convert arsenite, the toxic form of arsenic to humans, to arsenate. Arsenate (+5 oxidation state) is the dominant form of arsenic in surface water, while arsenite (+3 oxidation state) is the dominant form in hypoxic to anoxic environments. Arsenite is more soluble and mobile than arsenate. Many species of bacteria can transform arsenite to arsenate in anoxic conditions by using arsenite as an electron donor. This is a useful method in ground water remediation. Another bioremediation strategy is to use plants that accumulate arsenic in their tissues via phytoremediation but the disposal of contaminated plant material needs to be considered.

Bioremediation requires careful evaluation and design in accordance with existing conditions. Some sites may require the addition of an electron acceptor while others require microbe supplementation (bioaugmentation). Regardless of the method used, only constant monitoring can prevent future contamination.

Arsenic removal
Coagulation and flocculation are closely related processes common in arsenate removal from water. Due to the net negative charge carried by arsenate ions, they settle slowly or not at all due to charge repulsion. In coagulation, a positively charged coagulent such as iron and aluminum (commonly used salts: FeCl3, Fe2(SO4)3, Al2(SO4)3 ) neutralize the negatively charged arsenate, enable it to settle. Flocculation follows where a flocculant bridges smaller particles and allows the aggregate to precipitate out from water. However, such methods may not be efficient on arsenite as As(III) exists in uncharged arsenious acid, H3AsO3, at near-neutral pH.

The major drawbacks of coagulation and flocculation are the costly disposal of arsenate-concentrated sludge, and possible secondary contamination of environment. Moreover, coagulents such as iron may produce ion contamination that exceeds safety levels.

Toxicity and precautions
Arsenic and many of its compounds are especially potent poisons. Small amount of arsenic can be detected by pharmacopoial methods which includes reduction of arsenic to arsenious with help of zinc and can be confirmed with mercuric chloride paper.

Classification
Elemental arsenic and arsenic sulfate and trioxide compounds are classified as "toxic" and "dangerous for the environment" in the European Union under directive 67/548/EEC.

The International Agency for Research on Cancer (IARC) recognizes arsenic and inorganic arsenic compounds as group 1 carcinogens, and the EU lists arsenic trioxide, arsenic pentoxide, and arsenate salts as category 1 carcinogens.

Arsenic is known to cause arsenicosis when present in drinking water, "the most common species being arsenate [HAsO4(2-); As(V)] and arsenite [H3AsO3; As(III)]".

Legal limits, food, and drink
In the United States since 2006, the maximum concentration in drinking water allowed by the Environmental Protection Agency (EPA) is 10 ppb and the FDA set the same standard in 2005 for bottled water. The Department of Environmental Protection for New Jersey set a drinking water limit of 5 ppb in 2006. The IDLH (immediately dangerous to life and health) value for arsenic metal and inorganic arsenic compounds is 5 mg/m3 (5 ppb). The Occupational Safety and Health Administration has set the permissible exposure limit (PEL) to a time-weighted average (TWA) of 0.01 mg/m3 (0.01 ppb), and the National Institute for Occupational Safety and Health (NIOSH) has set the recommended exposure limit (REL) to a 15-minute constant exposure of 0.002 mg/m3 (0.002 ppb). The PEL for organic arsenic compounds is a TWA of 0.5 mg/m3. (0.5 ppb).

In 2008, based on its ongoing testing of a wide variety of American foods for toxic chemicals, the U.S. Food and Drug Administration set the "level of concern" for inorganic arsenic in apple and pear juices at 23 ppb, based on non-carcinogenic effects, and began blocking importation of products in excess of this level; it also required recalls for non-conforming domestic products. In 2011, the national Dr. Oz television show broadcast a program highlighting tests performed by an independent lab hired by the producers. Though the methodology was disputed (it did not distinguish between organic and inorganic arsenic) the tests showed levels of arsenic up to 36 ppb. In response, the FDA tested the worst brand from the Dr. Oz show and found much lower levels. Ongoing testing found 95% of the apple juice samples were below the level of concern. Later testing by Consumer Reports showed inorganic arsenic at levels slightly above 10 ppb, and the organization urged parents to reduce consumption. In July 2013, on consideration of consumption by children, chronic exposure, and carcinogenic effect, the FDA established an "action level" of 10 ppb for apple juice, the same as the drinking water standard.

Concern about arsenic in rice in Bangladesh was raised in 2002, but at the time only Australia had a legal limit for food (one milligram per kilogram, or 1000 ppb). Concern was raised about people who were eating U.S. rice exceeding WHO standards for personal arsenic intake in 2005. In 2011, the People's Republic of China set a food standard of 150 ppb for arsenic.

In the United States in 2012, testing by separate groups of researchers at the Children's Environmental Health and Disease Prevention Research Center at Dartmouth College (early in the year, focusing on urinary levels in children) and Consumer Reports (in November) found levels of arsenic in rice that resulted in calls for the FDA to set limits. The FDA released some testing results in September 2012, and as of July 2013, is still collecting data in support of a new potential regulation. It has not recommended any changes in consumer behavior.

Consumer Reports recommended: A 2014 World Health Organization advisory conference was scheduled to consider limits of 200–300 ppb for rice.
 * 1) That the EPA and FDA eliminate arsenic-containing fertilizer, drugs, and pesticides in food production;
 * 2) That the FDA establish a legal limit for food;
 * 3) That industry change production practices to lower arsenic levels, especially in food for children; and
 * 4) That consumers test home water supplies, eat a varied diet, and cook rice with excess water, then draining it off (reducing inorganic arsenic by about one third along with a slight reduction in vitamin content).
 * 5) Evidence-based public health advocates also recommend that, given the lack of regulation or labeling for arsenic in the U.S., children should eat no more than 1.5 servings per week of rice and should not drink rice milk as part of their daily diet before age 5. They also offer recommendations for adults and infants on how to limit arsenic exposure from rice, drinking water, and fruit juice.

Reducing arsenic content in rice
In 2020, scientists assessed multiple preparation procedures of rice for their capacity to reduce arsenic content and preserve nutrients, recommending a procedure involving parboiling and water-absorption.

Ecotoxicity
Arsenic is bioaccumulative in many organisms, marine species in particular, but it does not appear to biomagnify significantly in food webs. In polluted areas, plant growth may be affected by root uptake of arsenate, which is a phosphate analog and therefore readily transported in plant tissues and cells. In polluted areas, uptake of the more toxic arsenite ion (found more particularly in reducing conditions) is likely in poorly-drained soils.

Biological mechanism
Arsenic's toxicity comes from the affinity of arsenic(III) oxides for thiols. Thiols, in the form of cysteine residues and cofactors such as lipoic acid and coenzyme A, are situated at the active sites of many important enzymes.

Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits lipoic acid, which is a cofactor for pyruvate dehydrogenase. By competing with phosphate, arsenate uncouples oxidative phosphorylation, thus inhibiting energy-linked reduction of NAD+, mitochondrial respiration and ATP synthesis. Hydrogen peroxide production is also increased, which, it is speculated, has potential to form reactive oxygen species and oxidative stress. These metabolic interferences lead to death from multi-system organ failure. The organ failure is presumed to be from necrotic cell death, not apoptosis, since energy reserves have been too depleted for apoptosis to occur.

Exposure risks and remediation
Occupational exposure and arsenic poisoning may occur in persons working in industries involving the use of inorganic arsenic and its compounds, such as wood preservation, glass production, nonferrous metal alloys, and electronic semiconductor manufacturing. Inorganic arsenic is also found in coke oven emissions associated with the smelter industry.

The conversion between As(III) and As(V) is a large factor in arsenic environmental contamination. According to Croal, Gralnick, Malasarn and Newman, "[the] understanding [of] what stimulates As(III) oxidation and/or limits As(V) reduction is relevant for bioremediation of contaminated sites (Croal). The study of chemolithoautotrophic As(III) oxidizers and the heterotrophic As(V) reducers can help the understanding of the oxidation and/or reduction of arsenic.

Treatment
Treatment of chronic arsenic poisoning is possible. British anti-lewisite (dimercaprol) is prescribed in doses of 5 mg/kg up to 300 mg every 4 hours for the first day, then every 6 hours for the second day, and finally every 8 hours for 8 additional days. However the USA's Agency for Toxic Substances and Disease Registry (ATSDR) states that the long-term effects of arsenic exposure cannot be predicted. Blood, urine, hair, and nails may be tested for arsenic; however, these tests cannot foresee possible health outcomes from the exposure. Long-term exposure and consequent excretion through urine has been linked to bladder and kidney cancer in addition to cancer of the liver, prostate, skin, lungs, and nasal cavity.

title: Arsenic