Characteristic Reactions of Arsenic Ions (As³⁺)

Last updated
Save as PDF

Page ID: 97262

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Most common oxidation states: +3, +5
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

\(\ce{Cl^{-}}\), \(\ce{SO4^{2-}}\), \(\ce{NH3(aq)}\), \(\ce{OH^{-}}\)