Trioxythioarsenic Acid, H₃NaO₃S

Trioxythioarsenic Acid, H₃NaO₃S, is formed in solution when hydrogen sulphide is passed into a cold, dilute, acidified solution of potassium orthoarsenate. Excess of the gas should be avoided but, if present, may be removed either by immediate addition of copper sulphate, or by a vigorous stream of air. Any opalescence due to sulphur may be removed by adding asbestos and filtering. The solution obtained gives no immediate precipitate with hydrogen sulphide, but sulphur ultimately appears. When the solution is boiled, pure sulphur is precipitated, but no hydrogen sulphide or sulphur dioxide is evolved; on cooling, passage of hydrogen sulphide gives an immediate precipitate of arsenious sulphide.

Trioxythioarsenates of ammonium, the alkali metals and the alkaline earth metals have been prepared. Those of the heavy metals are unstable and difficult to isolate; the precipitates obtained when the salts of such metals are treated with a solution of an alkali trioxythio-arsenate undergo immediate decomposition, yielding sulphides.

Ammonium Trioxythioarsenate, (NH₄)₃NaO₃S.3H₂O, may be obtained by heating under pressure on a water-bath an aqueous solution of ammonium arsenite containing the calculated amount of fine sulphur; thioarsenate and arsenate are also formed and the thioxy-salt may be separated by fractional crystallisation from aqueous alcohol. It yields small, colourless plates, which are readily soluble in water. The crystals lose ammonia when kept in air, and the solution also loses ammonia on boiling, the salt being decomposed with formation of arsenious oxide and sulphur. A tetrahydrate, (NH₄)₃AsO₃S.4H₂O, has been obtained by melting together equal parts of arsenious oxide and sulphur and digesting the residue with aqueous ammonia; after keeping the liquid for 24 hours, it was filtered and the filtrate treated with alcohol. An oil separated which, when cooled in ice, set to a white crystalline mass, with properties similar to the trihydrate. The mother liquor from which the oil had separated, on addition of more alcohol, yielded a crystalline precipitate of ammonium monohydrogen trioxythioarsenate, (NH₄)₂HNaO₃S. The crystals turned yellow in air with loss of ammonia, but m an atmosphere of ammonia the white colour was restored.The normal sodium and potassium salts, Na₃NaO₃S.12H₂O and K₃NaO₃S.2H₂O, may be prepared by treating a concentrated solution of the alkali arsenite with sodium thiosulphate at the ordinary temperature. Heat is developed and, in the case of the sodium salt, the trioxythioarsenate crystallises out and sodium sulphite remains in the solution:

Na₃NaO₃ + Na₂S₂O₃ = Na₃NaO₃S + Na₂SO₃

A small quantity of arsenic also separates. In the case of the potassium salt, however, the sulphite is first crystallised out and the trioxythioarsenate then obtained by evaporating the mother liquor over sulphuric acid. The alkali monohydrogen arsenites also yield the above salts, in the cold, with sodium thiosulphate.

Sodium Monohydrogen Trioxythioarsenate, Na₂HNaO₃S.8H₂O, has been obtained as colourless triclinic crystals by fractionally crystallising the liquid resulting when arsenious oxide is boiled with aqueous sodium sulphide; the normal trioxythioarsenate, as well as sodium dioxythioarsenate, Na₃AsO₂S₂.10H₂O, and sodium orthoarsenate, were also obtained from the solution. The corresponding potassium salt, K₂HNaO₃S.2.5H₂O, has been prepared by adding a slight excess more than the theoretical quantity) of potassium hydroxide to a solution of the dihydrogen salt and allowing the mixture to crystallise over sulphuric acid and potassium hydroxide. It yields colourless, very hygroscopic prisms.

Sodium Dihydrogen Trioxythioarsenate, NaH₂NaO₃S.H₂O, is formed when the normal salt, Na₃NaO₃S.12H₂O (1 mol.), is triturated, without addition of water, with salicylic acid (2 mols.) and the product treated with alcohol in a closed vessel until the presence of salicylic acid can no longer be detected. The salt is freed from any remaining normal salt by levigation. It forms colourless prismatic crystals which give an acid reaction with moist litmus paper. It is unstable and decomposes with separation of sulphur, both in the dry state and in the presence of water:

NaH₂NaO₃S = NaAsO₂ + H₂O + S

The decomposition is accelerated by heat. The corresponding potassium salt, KH₂NaO₃S.H₂O,hasbeen obtained by the action of hydrogen sulphide on a cold aqueous solution of potassium mono- or di-hydrogen orthoarsenate, and also by digesting arsenious sulphide in a concentrated solution of potassium carbonate, or by the method described above for the sodium salt. The salt is colourless, stable in air, sparingly soluble in cold water but more soluble in hot. It loses water when heated at 170° C. and at a higher temperature melts and then decomposes. The hot solution also undergoes some decomposition.