Arsenous acid
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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 a solution and has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.
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[edit] 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.
[edit] 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.
[edit] 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.
[edit] References
- Greenwood, N.N.; A. Earnshaw (1997). Chemistry of the Elements. Oxford: Butterworth-Heinemann.
- King, R. Bruce "(ed.)" (1994). Encyclopedia of Inorganic Chemistry. Chirchester: John Wiley & Sons.
- Patnaik, Pradyot (2003). Handbook of Inorganic Chemicals. New York: McGraw Hill.
- Arsenic trioxide. Retrieved on January 29, 2006.
