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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

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.

Potassium salt

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.

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