Rubidium sulphides

The tetrahydrate of the mono sulphide, Rb₂S,4H₂O, is formed by the interaction of equivalent proportions of rubidium hydrogen sulphide and hydroxide, and is precipitated in colourless crystals by addition of alcohol and ether. The anhydrous salt is produced by the action of excess of rubidium on sulphur, the uncombined metal being removed by distillation in vacuum. It forms microscopic, white needles belonging to the cubic system, and isomorphous with those of the corresponding salt of potassium, but not with those of caesium monosulphide. Its density is 2.912, and it melts at the temperature of softening of glass. It dissolves in water with a hissing sound, the heat of solution being 24.6 Cal. The heat of formation of the solid from its elements is 87.1 Cal., and that from rubidium hydroxide and hydrogen sulphide is 8.0 Cal. Rubidium monosulphide is readily oxidized, is combustible, and weathers in air.

The monosulphide is converted by sulphur in an atmosphere of hydrogen into the pentasulphide, Rb₂S₅, deliquescent, red crystals melting at 223° to 224° C., and of density 2.618 at 15° C. When heated in nitrogen it yields the trisulphide, Rb₂S₃, consisting of hygroscopic, dark-yellow crystals melting at 213° C.; but in hydrogen the hygroscopic disulphide, Rb₂S₂, is formed, a substance melting about 420° C. and boiling above 950° C. Both the disulphide and the trisulphide yield a monohydrate. The tetrasulphide, Rb₂S₄, is formed by heating the monosulphide with the calculated amount of sulphur. It yields a yellow, crystalline dihydrate. Rubidium hydrogen sulphide, RbSH, is produced by saturating a solution of rubidium hydroxide with hydrogen sulphide.