Arsenous acid (or arsenious acid) is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the formula H₃AsO₃. It is known to occur in

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

s, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)₃.

Properties

As(OH)₃ is a pyramidal molecule consisting of three

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

groups bonded to arsenic. The ¹H

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...

spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related

phosphorous acid Phosphorous acid (or phosphonic acid (singular)) 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 th ...

H₃PO₃ adopts the structure HPO(OH)₂. The structural analogue of arsenous acid (P(OH)₃) is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners. One tautomer of arsenous acid is HAsO(OH)₂, which is called

arsonic acid Arsonic acid is the simplest of the arsonic acids. It is a hypothetical compound, although the tautomeric arsenious acid (As(OH)3) is well established. In contrast to the instability of HAsO(OH)2, the phosphorus compound with analogous stoichi ...

. It has not been isolated or well-characterized. H3PO3new

Synthesis

The preparation of As(OH)₃ involves a slow

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

arsenic trioxide Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, i ...

in water. Addition of base converts arsenous acid to the arsenite ions sO(OH)₂sup>−, sO₂(OH)sup>2−, and sO₃sup>3−.

Reactions

With its first p''K''_a being 9.2, As(OH)₃ is a weak acid. Reactions attributed to aqueous

are due to arsenous acid and its conjugate bases. Like arsenic trioxide, arsenous acid is sometimes

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphiproti ...

. For example, it reacts with hydrochloric, hydrobromic, and hydroiodic acids to produce arsenic trichloride, tribromide, and triiodide. :As(OH)₃ + 3 HCl ⇌ AsCl₃ + 3 H₂O :As(OH)₃ + 3 HBr ⇌ AsBr₃ + 3 H₂O :As(OH)₃ + 3 HI ⇌ AsI₃ + 3 H₂O Reaction of arsenous acid with

methyl iodide Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one h ...

gives

methylarsonic acid Methylarsonic acid is an organoarsenic compound with the formula CH3AsO3H2. It is a colorless, water-soluble solid. Salts of this compound, e.g. disodium methyl arsonate, have been widely used in as herbicides and fungicides in growing cotton an ...

. This historically significant conversion is the Meyer reaction: :As(OH)₃ + CH₃I + NaOH ⇌ CH₃AsO(OH)₂ + NaI + H₂O Alkylation occurs at arsenic, and the

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of arsenic increases from +3 to +5.

Toxicology

Arsenic-containing compounds are highly toxic and

carcinogenic A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...

. The

anhydride An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...

form of arsenous acid,

, is used as a

herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...

pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...

, and

rodenticide Rodenticides are chemicals made and sold for the purpose of killing rodents. While commonly referred to as "rat poison", rodenticides are also used to kill mice, squirrels, woodchucks, chipmunks, porcupines, nutria, beavers, and voles. Despi ...

References

External links

Case Studies in Environmental Medicine - Arsenic Toxicity
{{DEFAULTSORT:Arsenous Acid Oxoacids Arsenites Arsenic(III) compounds