Sodium polysulphides

According to Thomas and Rule, the whole series of polysulphides Na₂S_x exists, x being a whole number with the maximum value 5. Their results obtained by the ebullioscopic method in alcoholic solution favour the simple formula Na₄S_x as against Na₄S₃. On the other hand, Friedrich claims to have prepared polysulphides with the formulae indicated, the melting-points being given in brackets: Na₄S₃ (772° C.), Na₂S₂ (445° C.), Na₄S₅ (345° C.), Na₂S₃ (320° C.), Na₄S₇ (295° C.), Na₂S₄ (255° C.), Na₄S₉ (210° C.), and probably Na₂S₅ (185° C.).

Bloch and Hohn have prepared solutions containing the disulphide Na₂S₂, trisulphide Na₂S₃, tetrasulphide Na₂S₄, and pentasulphide Na₂S₅ by heating sodium sulphide to fusion with appropriate proportions of sulphur, and extracting the melt with water.

Bottger concentrated a solution similarly prepared, and isolated the disulphide as pentahydrate in the form of sulphur-yellow crystals. The anhydrous disulphide was prepared by Rule and Thomas by the action of sodium on an alcoholic solution of sodium tetrasulphide, Na₂S₄. It is a pale-yellow, crystalline powder, becoming darker in colour with rise of temperature, and melting to a dark-red liquid. Friedrich found for the melting-point 445° C. For the heat of formation in solution Sabatier gives 104.6 Cal.

The trisulphide in the form of trihydrate was similarly prepared by Bottger as golden-yellow crystals. The anhydrous substance is produced by the interaction of sodium monosulphide and the calculated amount of sulphur in boiling toluene. Its melting-point is given by Friedrich as 320° C. Its heat of formation in solution is 106.4 Cal.

Sabatier prepared the tetrasulphide by heating the monosulphide with excess of sulphur in a current of hydrogen or its sulphide, the product consisting of transparent red crystals. Rule and Thomas found the tetrasulphide to be the only definite compound produced by the action of sulphur on an alcoholic solution of sodium hydrogen sulphide:

2NaSH+3S =Na₂S₄+H₂S.

It forms microscopic cubes of dark-yellow colour with an olive-green tinge, and is very hygroscopic. At 267° C. it melts to a dark-red liquid. Friedrich gives the melting-point as 255° C. On heating, the aqueous solution deposits sulphur. A pale-yellow hexahydrate and an octahydrate have also been described. The heat of formation in solution is 98.4 Cal. It is the most stable of the polysulphides.

The pentasulphide was prepared by Hugot by the action of excess of sulphur on a solution of sodium in liquid ammonia, but it is probable that his product was not free from excess of sulphur. Friedrich gives the melting-point as 185° C. Schone mentions a hexahydrate, and Bottger a dark orange-yellow octahydrate.

Bloxam recommends the action of hydrogen sulphide on solid sodium sulphide as the best mode of preparing the polysulphides.