Dioxydithioarsenic Acid, H₃AsO₂S₂

Dioxydithioarsenic Acid, H₃AsO₂S₂, has been obtained in dilute aqueous solution by the action of a large excess of hydrogen sulphide on aqueous arsenic acid, and by passing hydrogen sulphide into a solution of sodium orthoarsenate acidified with mineral acid. Normal salts only of this acid have been prepared.

Sodium Dioxydithioarsenate, Na₃AsO₂S₂, may be obtained by the action of aqueous sodium sulphide on arsenious oxide, or of the disulphide on sodium metarsenite

Na₂S₂ + NaAsO₂ = Na₃AsO₂S₂

or on sodium trioxythioarsenate. It has also been prepared by adding sodium hydroxide to a hot solution of sodium thioarsenate

Na₃AsS₄ + 4NaOH = Na₃AsO₂S₂ + 2Na₂S + 2H₂O

and by the action of sodium carbonate on barium dioxydithioarsenate. It yields rhombic crystals with axial ratios a:b:c = 0.769:1:0.555. It is soluble in water; hydrochloric acid decomposes it, giving a yellow precipitate and evolution of hydrogen sulphide. Barium chloride gives a white precipitate of barium dioxydithioarsenate, Ba₃(AsO₂S₂)₂.4 or 6H₂O. This salt is also obtained when a solution of sodium thioarsenate is boiled with baryta-water. Silver nitrate gives a black precipitate.

The potassium salt, K₃AsO₂S₂.nH₂O, is unstable. It has been obtained in an impure form by treating the precipitate resulting from the action of hydrogen sulphide on aqueous potassium orthoarsenate with potassium hydroxide. On evaporation of the solution, the salt separates in yellow, hygroscopic crystals which cannot be recrystallised owing to decomposition to orthoarsenate and thioarsenate.