Hydrogen Polysulphides

In 1777 Scheele observed that by the rapid addition of much acid to a solution of sulphur in an alkali it was possible to produce a yellow, pungent, oily substance. Better conditions for the preparation of this substance were first described by Berzelius in 1825, who stated that a concentrated solution of " liver of sulphur " should be added in small quantities to dilute hydrochloric acid. The oily substance, of specific gravity approx. 1.7, is very unstable and is rapidly decomposed on contact with water, although dilute acids, especially hydrochloric acid, exert a distinct preservative action. For many years the oil was regarded as hydrogen pentasulphide, H₂S₅. When freshly prepared it is completely soluble in cold benzene; it cannot therefore contain any considerable quantity of free sulphur, and as its composition approximates to that of a penta- sulphide, it was natural to come to the conclusion that it might be an impure hydrogen pentasulphide; now, however, the product is not- regarded as a definite compound. It varies in viscosity according to the percentage of sulphur in the original dissolved metallic polysulphide, and widely divergent views have at various times been expressed as to the formula representing its composition.

A discovery that certain alkaloids were capable of producing definite crystalline compounds with hydrogen polysulphide unfortunately failed to elucidate the mystery of the composition of the latter, since the compounds produced did not yield unanimous indications. Thus strychnine yielded a hexasulphide, (C₂₁H₂₂O₂N₂)₂.H₂S₆, whilst brucine gave two hexasulphides, a red one, (C₂₃H₂₆O₄N₂)₃.(H₂S₆)₂, and a yellow one, (C₂₃H₂₆O₄N₂)₃.H₂S₆.6H₂O, and also an octasulphide, C₂₃H₂₆O₄N₂.H₂S₈.2H₂O. Furthermore, apart from the fact that these additive compounds were not of one type and that their composition was at first wrongly interpreted, there was the additional disadvantage that their indications did not accord well with the earlier views concerning the formula of hydrogen polysulphide.

Solubility of Sulphur in Hydrogen Di- and Tri-sulphides.

At the beginning of the twentieth century, therefore, there was no certainty as to the formula of hydrogen polysulphide, and, indeed, any formula between the limits H₂S₂ and H₂S₈ appeared possible, although the representation H₂S₅ appeared to be more in favour than any other. Three well-defined compounds, however, have now been isolated, hydrogen disulphide, H₂S₂, hydrogen trisulphide, H₂S₃, and hydrogen pentasulphide, H₂S₅; these will be described; there is also evidence of the existence of hydrogen hexasulphide, H₂S₆. The alkaloid compounds already mentioned are well-crystallised, stable bodies, and furthermore, the solubility curves for sulphur in the disulphide and trisulphide respectively have been determined between the temperatures -34.72° C. and 55.3° C. and found to be identical; in both cases there is a marked break in the curve at -1.45° C., at which temperature the liquid has a composition very near to that required by the formula H₂S₆ (i.e. 82.47 per cent, of sulphur not evolved as H₂S); (see fig.).

The crude hydrogen polysulphide, or " hydrogen per- sulphide "as it is frequently termed, is best prepared by heating for three hours at 100° C. a mixture of sodium sulphide, Na₂S.9H₂O, with half its weight of sulphur. The crystalline sulphide melts and the sulphur is gradually dissolved to a deep-red solution of which the solute has a composition between Na₂S₄ and Na₂S₅. This solution is mixed with rather less than its own bulk of water and is then introduced as a thin stream into a mixture of dilute hydrochloric acid and ice. The same yellow, oily product is obtained whatever the composition of the dissolved polysulphide, whether Na₂S₂, Na₂S₃, Na₂S₄ or Na₂S₅.

If the oil is distilled under a pressure of 2 mm. in glass apparatus of which the superficial alkali has been removed by treatment with hydrogen chloride, a distillate amounting to approximately one-sixth of the original bulk can be collected in the usual manner and on analysis proves to be hydrogen trisulphide, H₂S₃. At the same time a more volatile liquid, actually hydrogen disulphide, H₂S₂, can be condensed in a second receiver cooled by a mixture of ether and solid carbon dioxide.

The crude persulphide is also formed together with a large proportion of sulphur, when sulphur dioxide either as gas or in solution is reduced by means of hypophosphorous acid.

The pentasulphide, H₂S₅, has been isolated more recently by decomposing pure anhydrous ammonium pentasulphide with anhydrous formic acid.

All the known hydrogen polysulphides, as well as the crude so-called hydrogen persulphide, are yellow liquids at the ordinary temperature. They are very sensitive towards alkalis, and it is therefore necessary to treat glass vessels intended for their storage (which is often possible only for a few hours) with hydrogen chloride gas. If necessary, the compounds can be dried over calcium chloride, but this also should have received previous treatment with hydrogen chloride.

The compounds burn with a blue flame, giving water and sulphur dioxide. Even at the ordinary temperature they readily decompose into hydrogen sulphide and sulphur, the measurement of the amount of hydrogen sulphide formed from a known weight of a hydrogen polysulphide, when warmed in an atmosphere of hydrogen, supplying the most convenient method of analysis. Dilute acids act as preservatives, but even traces of alkali cause rapid and vigorous decomposition. The addition of alcohols, especially amyl alcohol, also induces rapid decomposition.

Sulphur dissolves in the various hydrogen polysulphides as already described; the solutions deposit crystalline sulphur when cooled or on the addition of benzene. As solutions of sulphur in hydrogen disulphide or trisulphide behave in this way, whereas the fresh crude polysulphide does not yield its excess of sulphur on similar treatment, this supplies further evidence of the existence of a higher polysulphide in the crude " persulphide."

The hydrogen polysulphides are miscible with benzene, toluene, chloroform, bromoform, carbon disulphide, ether and heptane, giving relatively stable solutions, and the use of such solutions has been suggested in place of sulphur chloride for the vulcanisation of caoutchouc at the ordinary temperature. The addition of alcohol to the benzene solutions induces rapid decomposition, with formation of nacreous sulphur, which slowly undergoes conversion into ordinary sulphur. Ketones, nitrobenzene, aniline and pyridine also catalyse the decomposition.