Arsenic pentasulfide
# Arsenic pentasulfide

Arsenic pentasulfide is an inorganic compound containing arsenic and sulfur.

## Uses

Solids of the approximate formula $As_2S_5$ have been used as pigments and chemical intermediates but are generally only of interest in academic laboratories.

## Preparation

Arsenic pentasulfide is prepared by precipitation from an acidic solution of soluble As(V) salts by treatment with hydrogen sulfide. It may be also prepared by heating a mixture of arsenic and sulfur, extracting the fused mass with an ammonia solution and reprecipitating arsenic pentasulfide at low temperature by addition of hydrochloric acid.

Phosphorus pentasulfide with the formula $P_4S_{10},$ is a molecular compound featuring tetrahedral phosphorus(V) centres. Trends in arsenic redox potentials suggest that $As_2S_5$ adopts a similar structure, a plausible alternative being an arsenic polysulfide.

## Reactions

Arsenic pentasulfide hydrolyzes in boiling water, giving arsenous acid and sulfur:

    $As_2S_5$ + 6 $H_2O$ → 2 $H_3AsO_3$ + 2 S + 3 $H_2S$

It oxidizes in air at elevated temperatures producing arsenic oxides, the products and yields of which are variable. In alkali metal sulfide solutions arsenic pentasulfide forms a thioarsenate anion, $[AsS_4]^{3−},$ which contain As(V) centres.

