Arsenic Pentasulphide, As₂S₅

When arsenic is fused with an excess of sulphur the product contains arsenic, sulphur and arsenic pentasulphide; the last-named may be extracted with liquid ammonia or, by careful fractionation, the arsenic and sulphur may be removed, leaving the sulphide. If the elements are fused together in stoichiometric proportions, a greenish-yellow plastic mass is obtained which gradually hardens and becomes lemon-yellow; if this product is powdered and digested with aqueous ammonia, a yellow solution results and insoluble sulphur remains. After filtering, the addition of an acid to the yellow solution precipitates the pentasulphide.

Borodowski failed to obtain evidence of the formation of this sulphide in his study of the freezing point curves because, with mixtures containing 20 to 60 molar per cent, of arsenic, the products are viscous and do not give definite freezing points.

Arsenic pentasulphide may also be prepared from aqueous arsenic acid or a solution of an arsenate by the action of hydrogen sulphide, but the nature of the product depends upon conditions. Berzelius reported the formation of the pentasulphide when the gas acted on a moderately concentrated solution of arsenic acid, but W^ckenroder stated that the arsenic acid was first reduced by hydrogen sulphide to arsenious acid, even in the presence of hydrochloric acid, and that a mixture of arsenic trisulphide and sulphur was then precipitated. Rose, after passing hydrogen sulphide into a solution of arsenic acid, heated the solution and filtered off the precipitate, and then by the addition of silver nitrate showed that both arsenious and arsenic acids were present in the filtrate. It was therefore accepted that reduction takes place and the reaction was represented thus:

2H₃ASO₄ + 5H₂S = As₂S₃ + S₂ + 8H₂O

Bunsen showed that the passage of a rapid stream of hydrogen sulphide through a hot solution of an alkali arsenate acidified with hydrochloric acid produced a precipitate of arsenic pentasulphide, and that this was a satisfactory method of determining arsenic quantitatively. These results were confirmed by McCay, and led to more systematic investigation of the subject, the result of which showed that the conditions favourable for the formation of arsenic pentasulphide, when hydrogen sulphide acts on aqueous arsenic acid or acid solutions of arsenates, are (a) a considerable excess of hydrochloric acid present, (b) a rapid passage of the gas, and (c) a comparatively low temperature - the liquid should be warm, as precipitation is extremely slow in the cold. Under these conditions arsenic pentasulphide alone is formed:

2H₃ASO₄ + 5H₂S = As₂S₅ + 8H₂O

When these conditions are not fulfilled, in addition to the above, a secondary reaction occurs which proceeds in two stages, thus:

H₃AsO₄ + H₂S = H₃NaO₃ + S + H₂O
2H₃NaO₃ + 3H₂S = As₂S₃ + 6H₂O

Thus, in the absence of hydrochloric acid, a solution containing 0.6 per cent, of arsenic pentoxide gave a precipitate which, after removing the free sulphur, contained 85 per cent, of the pentasulphide. In the presence of 8 per cent, of hydrochloric acid the precipitate consisted of the pure pentasulphide. Solutions containing 0.3664 per cent, of As₂O₅ and varying quantities of hydrochloric acid, after treatment with hydrogen sulphide for 12 hours at 15° C., gave precipitates which, after removal of free sulphur, had the following compositions:

HCl, per cent.	1.8	7.9	10.76	14.34	25.1	32.27
As2O5, per cent.	91	100	100	100	58	0
As2O3, per cent.	9	0	0	0	42	100

A boiling dilute aqueous solution of an alkali thioarsenate on addition of hydrochloric acid yields a precipitate which contains arsenic pentasulphide and may contain sulphur. Under ordinary conditions the precipitation is not quantitative and the excess of sulphur is difficult to remove; for complete precipitation the liquid should be kept overnight before filtration.

A convenient method of preparing arsenic pentasulphide is to boil arsenic pentoxide with piperazine, when a solution is obtained which, by the prolonged action of hydrogen sulphide, yields the compound As₂S₅.3C₄H₁₀N₂.3H₂S; if this is treated with cold dilute hydrochloric or acetic acid, crystals of arsenic pentasulphide are formed.

Arsenic pentasulphide is usually obtained as an amorphous lemon-yellow powder. It is stable in air up to a temperature of about 95° C., but above this temperature a surface film of the trisulphide is formed; nevertheless, it maybe dried at about 110° C. without sensible decomposition. At a higher temperature it melts, the melting point being somewhat higher than that of sulphur. The liquid is rather deeper in colour than the solid. On distillation, the vapour contains arsenic trisulphide and sulphur and the residue becomes continually richer in arsenic trisulphide. The pentasulphide is very slightly soluble in pure water, 1 litre of which at 0° C. dissolves 1.36 mg. of As₂S₅. It is much less soluble in the presence of hydrogen sulphide, and 1 litre of water containing 0.002 per cent, of the gas dissolves only 0.27 mg. As₂S₅. It is insoluble in alcohol or carbon disulphide, but is soluble in aqueous ammonia, alkali hydroxides or alkali sulphides, forming thioarsenates and oxythioarsenates. It also dissolves in aqueous solutions of citric acid and alkali citrates. When the pentasulphide is boiled with water, a solution of arsenic trioxide is obtained, with separation of sulphur. The pentasulphide is insoluble in hydrochloric acid and also in dilute nitric acid; it is attacked by more concentrated nitric acid with evolution of nitric oxide. With acid of 20 to 40 per cent, concentration the action is observed only on boiling or vigorous agitation; with 60 per cent, acid the reaction proceeds in the cold. Ammonia is absorbed by the pentasulphide, but is again liberated on exposure to air. Arsenic pentasulphide is readily reduced by heating in hydrogen or with carbon, or when fused with a mixture of alkali cyanide and carbonate.