Arsenic acid

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.^[3]

Arsenic acid

Names
IUPAC name Arsoric acid[1]
Other names Desiccant L-10 Orthoarsenic acid Trihydrogen arsenate Zotox
Identifiers
CAS Number	7778-39-4 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:18231 Y
ChEMBL	ChEMBL2374288
ChemSpider	229 Y
ECHA InfoCard	100.029.001
EC Number	231-901-9
KEGG	C01478 Y
PubChem CID	234
RTECS number	CG0700000
UNII	N7CIZ75ZPN Y
UN number	1553, 1554
CompTox Dashboard (EPA)	DTXSID1034341
InChI InChI=1S/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5) Y Key: DJHGAFSJWGLOIV-UHFFFAOYSA-N Y InChI=1/AsH3O4/c2-1(3,4)5/h(H3,2,3,4,5) Key: DJHGAFSJWGLOIV-UHFFFAOYAY
SMILES O[As+](O)([O-])O
Properties
Chemical formula	H3AsO4
Molar mass	141.942 g·mol−1
Appearance	White 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
Solubility in water	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[2]
Conjugate base	Arsenate
Structure
Molecular shape	Tetrahedral at arsenic atom
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Extremely toxic, carcinogenic, corrosive
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H301, H312, H314, H331, H350, H361, H410
Precautionary statements	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)	4 0 0
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	48 mg/kg (rat, oral) 6 mg/kg (rabbit, oral)
Related compounds
Other cations	Sodium arsenate
Related compounds	Arsenous acid Arsenic pentoxide Phosphoric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Three bottles of arsenic acid from the Great Exhibition: impure, pure and distilled.

Properties

It is a tetrahedral species of idealized symmetry C_3v with As–O bond lengths ranging from 1.66 to 1.71 Å.^[4]

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

H₃AsO₄ + H₂O ⇌ H₂AsO−4 + [H₃O]⁺, pK_a1 = 2.19

H₂AsO−4 + H₂O ⇌ HAsO2−4 + [H₃O]⁺, pK_a2 = 6.94

HAsO2−4 + H₂O ⇌ AsO3−4 + [H₃O]⁺, pK_a3 = 11.5

These pK_a 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.^[5]

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.^[5]

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.^[6]

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).^[7]