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.
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 1
spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related phosphorus
mainly adopts the structure HPO(OH)2
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.
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
, and [AsO3
. Reactions attributed to aqueous arsenic trioxide
are due to arsenous acid and its conjugate bases.
Arsenic containing compounds are highly toxic and carcinogenic
. The anhydride
form of arsenous acid, arsenic trioxide
, is used as a herbicide
, and rodenticide
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.