Sodium Thioarsenates

Sodium Orthothioarsenate, Na₃AsS₄, may be prepared by dissolving arsenic pentasulphide or sodium pyrothio-arsenate in a hot aqueous solution of sodium hydrosulphide and evaporating in vacuo; by adding alcohol to an aqueous solution of the pyro-salt; by passing hydrogen sulphide into a solution of sodium orthoarsenate, or into an alkaline solution of sodium monohydrogen orthoarsenate and pouring the clear liquid into alcohol. It yields pale yellow monoclinic crystals with axial ratios a:b:c = 0.6676:1:1.0393, and β = 100°0'. It is stable in air but, on heating, gives off its water of crystallisation, undergoing partial hydrolysis:

Na₃AsS₄ + 4H₂O = Na₃AsO₄ + 4H₂S

At a higher temperature it turns yellow and a little hydrogen sulphide is given off; finally it melts to a red liquid which, on cooling, yields a residue which consists mainly of the anhydrous salt. It is soluble in water and the solution on boiling gives off hydrogen sulphide. When added to solutions of the salts of heavy metals, precipitates of the thio-arsenates of the latter are formed. The passage of carbon dioxide through the solution expels hydrogen sulphide and causes precipitation of sulphur and arsenic pentasulphide.

Sodium Pyrothioarsenate, Na₄As₂S₇, is obtained as a pale yellow residue when an aqueous solution of sodium monohydrogen arsenate is saturated with hydrogen sulphide and allowed to evaporate. On heating it melts and, at a higher temperature, loses sulphur, forming thioarsenite. Sodium metathioarsenate, NaAsS₃, according to Berzelius, is precipitated when alcohol is added to an aqueous solution of the ortho-salt.

The double salt, Na₃(NH₄)₃(AsS₄)₂, is obtained when alcohol is added to a mixture of the two thioarsenates in molecular proportions, or when a solution containing ammonium chloride and sodium orthothioarsenate is allowed to evaporate; it crystallises as pale yellow hexagonal prisms. A corresponding sodium-potassium salt is obtained by evaporation of a solution of the mixed orthothioarsenates.