Characteristic Reactions of Arsenic Ions (As³⁺)
- Page ID
- 97262
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- Characteristics: Arsenic is a gray, very brittle substance; sublimes at 615º. Combines readily with sulfur and oxygen at high temperatures.
- Characteristic reactions of \(\ce{As^{3+}}\):
Hydrogen Sulfide
In slightly acid solution, yellow \(\ce{As2S3}\) forms on addition of \(\ce{H_2S}\):
\[\ce{2As3+(aq) + 3H2S(aq) <=> As2S3(s) + 6H+(aq)} \nonumber \]
Hydrogen sulfide causes no precipitation from neutral or alkaline solutions. The precipitate is soluble in concentrated \(\ce{HNO3}\) or in ammoniacal \(\ce{H2O2}\):
\[\ce{As2S3(s) + 8H^{+}(aq) + 2NO3^{-}(aq) <=> 2As^{3+}(aq) + 3S(s) + 2NO(g) + 4H2O(l)} \nonumber \]
\[\ce{As2S3(s) + 14H2O2(aq) + 12NH3(aq) <=> 2AsO4^{3-}(aq) + 3SO4^{2-}(aq) + 8H2O(l) + 12NH4^{+}(aq)} \nonumber \]
The precipitate is insoluble in dilute, nonoxidizing acids such as HCl.
Silver Ion
Silver ion will precipitate yellow silver arsenite from neutral or slightly basic solution:
\[\ce{3Ag^{+}(aq) + AsO3^{3-}(aq) <=> Ag3AsO3(s)} \nonumber \]
It is insoluble in water, but soluble in aqueous ammonia and in acids.
Oxidizing Agents
Oxidizing agents readily oxidize arsenites (+3) to arsenates (+5) in alkaline or neutral solutions:
\[\ce{2Cu(OH)2(s) + AsO3^{3-}(aq) <=> Cu2O(s) (red) + AsO4^{3-}(aq) + 2H2O(l) } \nonumber \]
No Reaction
\(\ce{Cl^{-}}\), \(\ce{SO4^{2-}}\), \(\ce{NH3(aq)}\), \(\ce{OH^{-}}\)