Arsenic pentasulfide

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Arsenic pentasulfide

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UN number	1557
Template:Longitem	Template:Chembox Elements/molecular formula
Molar mass	Template:Chem molar mass
Appearance	Vivid, dark orange, opaque crystals
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Template:Longitem	Arsenic pentoxide Arsenic trisulfide Phosphorus pentasulfide

Arsenic pentasulfide is an inorganic compound containing arsenic and sulfur.

Uses

Solids of the approximate formula As₂S₅ 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 P₄S₁₀, is a molecular compound featuring tetrahedral phosphorus(V) centres. Trends in arsenic redox potentials suggest that As₂S₅ adopts a similar structure, a plausible alternative being an arsenic polysulfide.

Reactions

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

As₂S₅ + 6 H₂O → 2 H₃AsO₃ + 2 S + 3 H₂S

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, [AsS₄]³⁻, which contain As(V) centres.