Properties & Reactions of Arsenic

Other Important Arsenic Compounds
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Arsine:

Arsine, also referred to as As(III) hydride, arsenic hydride or arsenic trihydride, has the simple formula, AsH3. It is both volatile and very toxic. It has a pyramidal structure similar to the As(III) species discussed above. This compound exists only under very reduced conditions and can be found in soil. But because of its volatility and high reactivity, its existence in the soil is transient.

The most commonly used method to accurately measure arsenic in water is to convert the inorganic arsenic to an arsine and then measure the gaseous arsine by atomic absorption or atomic fluorescence spectroscopy.

Methyl Arsenic

The major methyl arsenic compounds are summarized in the table below:

 
As(V)
As(III)
As(III) arsine
monomethyl
CH3AsO(OH)2
Methylarsonic acid
CH3As(OH)2
Methyl-
dihydroxyarsine
AsH2CH3
Methyl-
arsine
dimethyl
(CH3)2AsO(OH)
Dimethylarsinic acid
(CH3)2AsOH
Dimethyl-
hydroxyarsine
AsH(CH3)2
Dimethyl-
arsine
trimethyl
(CH3)3AsO
Trimethylarsine oxide
(CH3)3As
Trimethyl-
arsine
(CH3)3As
Trimethyl-
arsine

As with the corresponding inorganic compounds, the methyl As(III) has pyramidal symmetry, and the methyl As(V) has tetrahedral symmetry. The individual methylarsenic compounds can also dissociate (loss an H+) to form the  corresponding anionic species. For example:

CH3As(OH)2°  >>  H+ + CH3AsO(OH)- CH3AsO(OH)2  >>  H+ + CH3AsO22-

As with the inorganic arsenic compounds, these reactions and the charge of the arsenic species are pH dependent. None of the methyl arsenic compounds is volatile except for the trimethylarsine, (CH3)3As. Click here for more information on how the methyl arsenic compounds are formed.

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