Antimonites

Antimony trioxide is amphoteric, its basic properties being the more highly developed. Most antimony salts derived from this oxide are, however, hydrolysed by cold water, the exceptions being the trifluoride and some salts of organic acids. Its acidic properties are shown by the formation of antimonites by the action of solutions of alkali and possibly of alkaline earth hydroxides. These are mainly meta-antimonites, derived from the hypothetical acid HSbO₂; but other more complex substances have been obtained.

Sodium meta-antimonite, NaSbO₂, may be taken as typical. It is obtained by fusing together antimony trioxide and excess of sodium carbonate; a trihydrate has also been obtained. It is oxidised by fusion with caustic soda in contact with air, by hydrogen peroxide, and by the halogens. By treatment with hydrogen sulphide a thioantimonite is formed; and with sodium thiosulphate a mixture of sodium pyroantimonate and thioantimonate is formed, the thiosulphate being reduced to sulphite. In solution the sodium salt reacts with solutions of many metallic salts. With copper salts it yields a precipitate, probably of copper antimonite, soluble in nitric acid. With an ammoniacal solution of barium chloride, with solutions of mercurous salts, mercuric chloride and lead and ferric salts precipitates are formed in each case, while with ferric salts a reddish solution is ultimately formed. With silver nitrate a complex reaction occurs yielding a black precipitate, which is a mixture of silver oxide, silver and antimony; while with gold chloride a black precipitate is obtained which is possibly gold antimonite.

A solution of sodium antimonite is reduced by stannous chloride, and is oxidised by potassium permanganate, potassium dichromate or potassium ferricyanide.

Other, and in many cases more complex, antimonites that have been formed include the sodium salt NaSb₃O₅, which is obtained by the prolonged boiling of a mixture of excess of antimony trioxide and a solution of sodium hydroxide, and the salt Na₂Sb₄O₇. A potassium salt of indefinite composition has been described by a number of the early observers. The two salts KSb₃O₅ and K₂Sb₁₆O₂₅.7H₂O have been obtained, the former as minute, prismatic crystals. Antimonites of copper have been obtained by a fusion method, while the mineral thrombolite probably also contains a compound of this type. Antimonites of the alkaline earth metals have not been isolated, but indications of their existence have been obtained, while the mineral romeite may contain an antimonite of calcium. Antimonites of zinc, magnesium and lead, and of iron, cobalt and nickel, have been obtained. In addition, complex compounds of antimony trioxide and alkali molybdates and tungstates have been described. Of the tungstates, the following may perhaps be mentioned: 2BaO.Sb₂O₃.11WO₃.18H₂O; 6(NH₄)₂O.Sb₂O₃.18WO₃.38H₂O; 6(CH₆N₃)₂O.Sb₂O₃.18WO₃.31H₂O; 8K₂O.Sb₂O₃.19WO₃.37H₂O; 2½(NH₄)₂O.2Sb₂O₃.10WO₃.12H₂O; 3K₂O.2Sb₂O₃.10WO₃.6H₂O; 2½(CH₆N₃)₂O.2Sb₂O₃.10WO₃.6H₂O; 2½BaO.2Sb₂O₃.10WO₃.18H₂O.

Antimony trioxide is used under various trade names, such as "Luv extra," "Leukonin" and "Timonox," as an opacifier mixed with paints. The commercial product is usually a mixture of oxides. It can be used with all colours and oxides except those containing lead. The durability, hardness and speed of drying of antimony white paints are stated to be less than for zinc white paints. The trioxide is also employed in enamels, but certain countries have introduced legislation curtailing its use on food receptacles on account of its poisonous nature. It has been shown that whilst antimony in the quinquevalent form is comparatively harmless, it is poisonous in the tervalent form since it is then soluble in dilute acids.