Arsenous acid

Arsenous acid, also known as arsenious acid, is the hydrolyzed form of arsenic trioxide and has the formula As(OH)3. As(OH)3 occurs in aqueous solutions and has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.

Properties
The molecular weight of arsenous acid, is 125.94 g/mol. As(OH)3 is a pyramidal molecule consisting of three hydroxyl groups connected by single bonds to arsenic. The 1H NMR spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related phosphorus species H3PO3 mainly adopts the structure HPO(OH)2; 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.

As(OH)3 is a weak acid with a pKa of 9.2.

Reactions
The preparation of As(OH)3 involves a slow hydrolysis of arsenic trioxide in water. Addition of base converts arsenous acid to the arsenite ions [AsO(OH)2]-, [AsO2(OH)]-2, and [AsO3]-3. Reactions attributed to aqueous arsenic trioxide are due to arsenous acid and its conjugate bases.

Toxicology
Arsenic containing compounds are highly toxic and carcinogenic. The anhydride form of arsenous acid, arsenic trioxide, is used as a herbicide, pesticide, and rodenticide.

Literary References
Arsenious acid is identifed through the microscope as a "crystalline particle" by the fictitious Dr. Thorndyke following isolation from the grainy arrowroot breakfast of a concerned Mr. Barton in the Moabite Cypher case.