Thioantimonates

Compounds of antimony pentasulphide are known to occur naturally; they have also been prepared in a variety of ways. They may be regarded as normal thioantimonates, or salts of an unknown acid, thioantimonic acid, H₃SbS₄. Salts of the heavy metals are best prepared from those of the alkali metals by double decomposition. The most important of these salts is sodium thioantimonate, Na₃SbS₄, known also as Schlippe's salt. It is prepared by the gradual addition of a mixture of antimony trisulphide and sulphur to a boiling solution of sodium hydroxide, according to the equation

4Sb₂S₃ + 8S + 18NaOH = 5Na₃SbS₄ + 3NaSbO₃ + 9H₂O

The sodium antimonate formed at the same time is almost completely precipitated. This reaction may be employed for the preparation of other metallic thioantimonates since carbonates or sulphides of the alkali metals, or hydroxides, carbonates or sulphides of the alkaline earth metals may be used in place of sodium hydroxide.

Thioantimonates may also be prepared by the fusion of antimony pentasulphide (or a mixture of antimony trisulphide and sulphur) with the sulphide or carbonate of an alkali metal, or with sodium thiosulphate. They are also obtained by the action of hydrogen sulphide upon solutions of alkali ortho-antimonates.

Thioantimonates of the alkali or alkaline earth metals are either colourless or slightly yellowish; those of the heavier metals are darker in colour. Salts of the alkali and alkaline earth metals can be heated to red heat, in the absence of air, without decomposition; but in air they decompose gradually. They are soluble in water, but insoluble in alcohol; the aqueous solutions decompose on standing, and also on the addition of acids, or of carbon dioxide. Some solid thioantimonates of the heavier metals are decomposed by the action of mineral acids, with formation of antimony pentasulphide.

When a solution of a thioantimonate of an alkali metal is boiled with powdered sulphur, alkali thioantimonite is obtained.

Some metallic sulphides, among them being the sulphides of copper, cadmium, mercury and iron, are slightly soluble in solutions of alkali thioantimonates.

A number of hydrated forms of thioantimonates of alkali metals has been prepared and described.

Freezing Point Curve of the Syste, Sb2O3-Sb2S3

Thermal analysis of the system antimony trioxide-antimony trisulphide (fig.) indicates the presence of an oxysulphide of antimony, Sb₄OS₅, which decomposes at 522° C. This compound has also been obtained by the action of dry hydrogen sulphide upon antimony pentoxide. The mineral kermesite, 2Sb₂S₃.Sb₂O₃ or Sb₆O₃S₆, corresponds in composition to the eutectic of antimony trioxide and the above-mentioned oxysulphide. This substance is also stated to be formed by heating antimony trisulphide to a red heat in an atmosphere of steam, by the action of dry hydrogen sulphide upon antimony trioxide, and by boiling antimony thioiodide, SbSI, with zinc oxide and water. The natural mineral is a cherry-red or bright red, brilliant, crystalline substance belonging to the monoclinic system, having pseudo-rhombic symmetry:

a:b:c = 3.9650:1:0.8535; β = 90°0'

Its hardness on Mohs' scale is 1.0 to 1.5, its density 4.5 to 4.6. It melts easily when heated in the blowpipe flame. It is decomposed when heated in a current of hydrogen; it is soluble in hydrochloric acid, and in a solution of potassium hydroxide, but is insoluble in a dilute solution of tartaric acid.

Antimonyl Thioantimonate, (SbO)₃SbS₄, is obtained by the action of sodium thioantimonate on potassium antimonyl tartrate.

If the fused oxysulphides of antimony are cooled quickly, they are converted into glass-like substances, known sometimes as "antimony glass."

Many of the substances that have been described as oxysulphides of antimony are most probably mixtures. Some of these, known as kermes mineral, were formerly used medicinally. They were very variable in composition. Many methods of preparation were employed. A somewhat similar preparation was given in the British Pharmacopoeia, under the name of Antimonium Sulphuratum, but has been omitted since 1932.

Allied to these oxysulphides is a potassium compound, K₂HSbO₂S₂.2H₂O (the only metallic compound of this nature that has been described), which may be obtained in the form of yellowish, needle-like crystals by the action of a moderately concentrated solution of potassium hydroxide upon antimony pentasulphide, or upon a mixture of antimony trisulphide and sulphur.

The existence of sulphite of antimony is doubtful.