Barium Polysulphides

Barium Trisulphide, BaS₃, is obtained by heating barium sulphide with excess of sulphur at a temperature not greater than 360° C. It forms a yellowish-green mass, fusible at 400° C., above which temperature it loses sulphur, but it is only completely transformed into the monosulphide at dull red heat. In warm water it partially dissolves, forming a reddish-yellow solution which has an alkaline reaction and is readily oxidised in air.

By boiling a mixture of barium sulphide and sulphur with water, or by treating sulphur with a saturated solution of barium hydrosulphide, and evaporating, red crystals of composition of Barium Tetrasulphide, BaS₄.H₂O or 2H₂O separate in the form of rhombic prisms of density 3. By boiling a mixture of barium hydroxide, sulphur, and water a deep brown liquid is obtained which becomes orange-red on cooling. By rapid evaporation, red crystals of BaS₄.H₂O, mixed with sulphur and barium thiosulphate, are formed. The mother liquor contains barium and sulphur in the proportion BaS₅, but this compound is probably not obtained in the solid state, the solution depositing the tetrasulphide and free sulphur.

The tetrasulphide is more stable than the trisulphide. When heated, it loses sulphur at about 110° C., and water and sulphuretted hydrogen above 200° C. At 300° C. it is decomposed into the trisulphide and sulphate. It dissolves in water to the extent of one part in 2.53 of water at 15° C., without decomposition, forming a red solution from which the tetrasulphide can be recrystallised. The solution is not decomposed by sulphuretted hydrogen.

By evaporating a solution of the trisulphide, Schone obtained orange- red monoclinic crystals showing dichroism, of composition Ba₄S₇.25H₂O, or 24H₂O, but he expressed doubt as to whether it was a chemical individual or an isomorphous mixture of the mono- and tetra-sulphides.

No higher polysulphide appears to have been obtained.