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