Arsenic acid

Last updated
Arsenic acid
Arsenate.svg
Arsenic-acid-3D-balls.png
Names
IUPAC name
Arsoric acid [1]
Other names
  • Desiccant L-10
  • Orthoarsenic acid
  • Trihydrogen arsenate
  • Zotox
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.029.001 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 231-901-9
KEGG
PubChem CID
RTECS number
  • CG0700000
UNII
UN number 1553, 1554
  • InChI=1S/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5) Yes check.svgY
    Key: DJHGAFSJWGLOIV-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5)
    Key: DJHGAFSJWGLOIV-UHFFFAOYAY
  • O[As+](O)([O-])O
Properties
H3AsO4
Molar mass 141.942 g·mol−1
AppearanceWhite translucent or colorless crystals, hygroscopic
Density 2.5 g/cm3
Melting point 35.5 °C (95.9 °F; 308.6 K)
Boiling point 120 °C (248 °F; 393 K) decomposes
16.7 g/(100 mL)
Solubility soluble in ethanol
Vapor pressure 55 hPa (50 °C)
Acidity (pKa)pKa1 = 2.19
pKa2 = 6.94
pKa3 = 11.5
Conjugate base Arsenate
Structure
Tetrahedral at arsenic atom
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
Extremely toxic, carcinogenic, corrosive
GHS labelling:
GHS-pictogram-acid.svg GHS-pictogram-skull.svg GHS-pictogram-exclam.svg GHS-pictogram-silhouette.svg GHS-pictogram-pollu.svg
Danger
H301, H312, H314, H331, H350, H361, H410
P201, P202, P260, P261, P264, P270, P271, P273, P280, P281, P301+P310, P301+P330+P331, P302+P352, P303+P361+P353, P304+P340, P305+P351+P338, P308+P313, P310, P311, P312, P321, P322, P330, P363, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
4
0
0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
48 mg/kg (rat, oral)

6 mg/kg (rabbit, oral)

Related compounds
Other cations
Sodium arsenate
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)
Three bottles of arsenic acid from the Great Exhibition: impure, pure and distilled. Arseniksyrlighet, prov.jpg
Three bottles of arsenic acid from the Great Exhibition: impure, pure and distilled.

Arsenic acid or arsoric acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but is only found in solution, where it is largely ionized. Its hemihydrate form (2H3AsO4·H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C. [2]

Contents

Properties

It is a tetrahedral species of idealized symmetry C3v with As–O bond lengths ranging from 1.66 to 1.71 Å. [3]

Being a triprotic acid, its acidity is described by three equilibria:

H3AsO4 + H2O ⇌ H2AsO−4 + [H3O]+, pKa1 = 2.19
H2AsO−4 + H2O ⇌ HAsO2−4 + [H3O]+, pKa2 = 6.94
HAsO2−4 + H2O ⇌ AsO3−4 + [H3O]+, pKa3 = 11.5

These pKa values are close to those for phosphoric acid. The highly basic arsenate ion (AsO3−
4) is the product of the third ionization. Unlike phosphoric acid, arsenic acid is an oxidizer, as illustrated by its ability to convert iodide to iodine.

Preparation

Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid. Dinitrogen trioxide is produced as a by-product. [4]

As2O3 + 2 HNO3 + 2 H2O → 2 H3AsO4 + N2O3

The resulting solution is cooled to give colourless crystals of the hemihydrate H3AsO4·0.5H2O (or 2H3AsO4·H2O), although the dihydrate H3AsO4·2H2O is produced when crystallisation occurs at lower temperatures. [4]

Other methods

Arsenic acid is slowly formed when arsenic pentoxide is dissolved in water, and when meta- or pyroarsenic acid (H4As2O7) is treated with cold water. Arsenic acid can also be prepared directly from elemental arsenic by moistening it and treating with ozone.

2 As + 3 H2O + 5 O3 → 2 H3AsO4 + 5 O2

Applications

Commercial applications of arsenic acid are limited by its toxicity. It is a precursor to a variety of pesticides. It has found occasional use as a wood preservative, a broad-spectrum biocide, a finishing agent for glass and metal, and a reagent in the synthesis of some dyestuffs and organic arsenic compounds. [5]

Safety

Arsenic acid is extremely toxic and carcinogenic, like all arsenic compounds. It is also corrosive. The LD50 in rabbits is 6 mg/kg (0.006 g/kg). [6]

Related Research Articles

<span class="mw-page-title-main">Arsenic</span> Chemical element, symbol As and atomic number 33

Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but only the grey form, which has a metallic appearance, is important to industry.

<span class="mw-page-title-main">Phosphoric acid</span> Chemical compound (PO(OH)3)

Phosphoric acid is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H3PO4. It is commonly encountered as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. It is a major industrial chemical, being a component of many fertilizers.

<span class="mw-page-title-main">Pyrophosphoric acid</span> Chemical compound

Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, [(HO)2P(O)]2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 and 71.5 °C. The compound is a component of polyphosphoric acid, an important source of phosphoric acid. Anions, salts, and esters of pyrophosphoric acid are called pyrophosphates.

<span class="mw-page-title-main">Chromium trioxide</span> Chemical compound

Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple solid under anhydrous conditions and bright orange when wet. The substance dissolves in water concomitant with hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser, a mutagen, and a carcinogen.

<span class="mw-page-title-main">Phosphorous acid</span> Chemical compound

Phosphorous acid (or phosphonic acid) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.

<span class="mw-page-title-main">Arsenic trioxide</span> Chemical compound (industrial chemical and medication)

Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound with the formula As
2O
3. As an industrial chemical, its major uses include the manufacture of wood preservatives, pesticides, and glass. It is also used as a medication to treat a type of cancer known as acute promyelocytic leukemia. For this use it is given by injection into a vein.

<span class="mw-page-title-main">Antimony trioxide</span> Chemical compound

Antimony(III) oxide is the inorganic compound with the formula Sb2O3. It is the most important commercial compound of antimony. It is found in nature as the minerals valentinite and senarmontite. Like most polymeric oxides, Sb2O3 dissolves in aqueous solutions with hydrolysis. A mixed arsenic-antimony oxide occurs in nature as the very rare mineral stibioclaudetite.

<span class="mw-page-title-main">Lead hydrogen arsenate</span> Chemical compound

Lead hydrogen arsenate, also called lead arsenate, acid lead arsenate or LA, chemical formula PbHAsO4, is an inorganic insecticide used primarily against the potato beetle. Lead arsenate was the most extensively used arsenical insecticide. Two principal formulations of lead arsenate were marketed: basic lead arsenate (Pb5OH(AsO4)3, CASN: 1327-31-7) and acid lead arsenate (PbHAsO4).

The arsenate is an ion with the chemical formula AsO3−4. Bonding in arsenate consists of a central arsenic atom, with oxidation state +5, double bonded to one oxygen atom and single bonded to a further three oxygen atoms. The four oxygen atoms orient around the arsenic atom in a tetrahedral geometry. Resonance disperses the ion's −3 charge across all four oxygen atoms.

<span class="mw-page-title-main">Arsenous acid</span> Chemical compound

Arsenous acid (or arsenious acid) is the inorganic compound with the formula H3AsO3. It is known to occur in aqueous solutions, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.

<span class="mw-page-title-main">Arsenic pentoxide</span> Chemical compound

Arsenic pentoxide is the inorganic compound with the formula As2O5. This glassy, white, deliquescent solid is relatively unstable, consistent with the rarity of the As(V) oxidation state. More common, and far more important commercially, is arsenic(III) oxide (As2O3). All inorganic arsenic compounds are highly toxic and thus find only limited commercial applications.

Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to protect them from attack by microbes and insects. Like other copper-based wood preservatives, it imparts a greenish tint to treated timber.

<span class="mw-page-title-main">Selenium compounds</span> Chemical compounds containing selenium

Selenium compounds commonly exist in the oxidation states −2, +2, +4, and +6.

<span class="mw-page-title-main">Sodium arsenate</span> Chemical compound

Sodium arsenate is the inorganic compound with the formula Na3AsO4. Related salts are also called sodium arsenate, including Na2HAsO4 (disodium hydrogen arsenate) and NaH2AsO4 (sodium dihydrogen arsenate). The trisodium salt is a white or colourless solid that is highly toxic. It is usually handled as the dodecahydrate Na3AsO4.12H2O.

<span class="mw-page-title-main">Ammonium arsenate</span> Chemical compound

Ammonium arsenate is the inorganic compound with the formula (NH4)3AsO4. It is prepared by treating a concentrated solution of arsenic acid with ammonia, resulting in precipitation of colorless crystals of the trihydrate. Upon heating, it releases ammonia.

Arsenic biochemistry refers to biochemical processes that can use arsenic or its compounds, such as arsenate. Arsenic is a moderately abundant element in Earth's crust, and although many arsenic compounds are often considered highly toxic to most life, a wide variety of organoarsenic compounds are produced biologically and various organic and inorganic arsenic compounds are metabolized by numerous organisms. This pattern is general for other related elements, including selenium, which can exhibit both beneficial and deleterious effects. Arsenic biochemistry has become topical since many toxic arsenic compounds are found in some aquifers, potentially affecting many millions of people via biochemical processes.

<span class="mw-page-title-main">Disodium hydrogen arsenate</span> Chemical compound

Disodium hydrogen arsenate is the inorganic compound with the formula Na2HAsO4.7H2O. The compound consists of a salt and seven molecules of water of crystallization although for simplicity the formula usually omits the water component. The other sodium arsenates are NaH2AsO4 and Na3AsO4, the latter being called sodium arsenate. Disodium hydrogen arsenate is highly toxic. The salt is the conjugate base of arsenic acid. It is a white, water-soluble solid.

<span class="mw-page-title-main">Sodium dihydrogen arsenate</span> Chemical compound

Sodium dihydrogen arsenate is the inorganic compound with the formula NaH2AsO4. Related salts are also called sodium arsenate, including Na2HAsO4 (disodium hydrogen arsenate) and NaH2AsO4 (sodium dihydrogen arsenate). Sodium dihydrogen arsenate is a colorless solid that is highly toxic.

<span class="mw-page-title-main">Monopotassium arsenate</span> Chemical compound

Monopotassium arsenate is the inorganic compound with the formula KH2AsO4. A white solid, this salt is used to prepared other arsenic-containing compounds, mainly pesticides. It is prepared by calcining arsenic oxide and potassium nitrate, followed by extraction with water.

<span class="mw-page-title-main">Arsenic compounds</span> Chemical compounds containing arsenic

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 As3−
4 ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal owing to the influence of the lone pair of electrons.

References

  1. ↑ "Arsenic acid".
  2. ↑ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN   0-12-352651-5.
  3. ↑ Lee, C.; Harrison, W. T. A. (2007). "Tetraethylammonium dihydrogenarsenate bis(arsenic acid) and 1,4-diazoniabicyclo[2.2.2]octane bis(dihydrogenarsenate) arsenic acid: hydrogen-bonded networks containing dihydrogenarsenate anions and neutral arsenic acid molecules". Acta Crystallographica C . 63 (Pt 7): m308–m311. doi:10.1107/S0108270107023967. PMID   17609552.
  4. 1 2 G. Brauer, ed. (1963). "Arsenic Acid". Handbook of Preparative Inorganic Chemistry. Vol. 1 (2nd ed.). New York: Academic Press. p. 601.
  5. ↑ Minerals Yearbook, 2008, V. 1, Metals and Minerals. Government Printing Office. 2010. pp. 6–. ISBN   978-1-4113-3015-3.
  6. ↑ Grund, Sabina C.; Hanusch, Kunibert; Wolf, Hans Uwe (2008). "Arsenic and Arsenic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a03_113.pub2. ISBN   978-3527306732.
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