Bismuth Triselenide

Bismuth Triselenide is found in the minerals guanajuatite and frenzelite. It may be obtained by melting together the elements, but on account of the ease with which selenium volatilises, to obtain the pure compound it is necessary to add more selenium to the product first formed and remelt in the absence of air. It is formed when hydrogen selenide is passed into a solution of bismuth nitrate from which excess acid has been almost completely removed, or by the addition of a solution of a bismuth salt to a saturated solution of hydrogen selenide. The hydrogen selenide is best prepared by the action of hydrochloric acid on magnesium selenide in the absence of air. It is probable that by the last-mentioned process bismuth selenide is obtained free from deposited metal and from complexes.

In the mineral form bismuth triselenide is isomorphous with the minerals bismuthinite and antimonite. Its hardness on Mohs' scale is 2.5 to 3.5, and its density 6.2 to 6.6; it is commonly associated with sulphur minerals, and indeed sulphur may partially replace selenium. In the prepared form it is a black or grey powder with a density of 6.82. When heated it loses selenium and absorbs oxygen. It is only very slightly attacked by concentrated hydrochloric acid even on boiling; dilute nitric acid has little action, but concentrated nitric acid and aqua regia decompose it completely with partial separation of selenium. It is insoluble in solutions of potassium hydroxide or potassium sulphide. It is oxidised to the trioxide when fused with potassium nitrate, potassium selenate being formed at the same time. It reacts with the double chloride of bismuth and ammonium when melted, with formation of bismuth selenochloride, BiSeCl.

Thermal investigation of the system Bi₂S₃-Ag₂Se has been undertaken, and from the melting point curve it is deduced that a double compound 3Ag₂Se.4Bi₂S₃ is obtained, the melting point being 773° C.

Two selenites of bismuth have been described, the first, Bi₂O₃.4SeO₂, being formed by the addition of selenious acid to bismuth carbonate, and the second, Bi₂O₃.5SeO₂.H₂O, by the action of excess of selenious acid upon bismuth hydroxide.

A compound which may be bismuth selenate is obtained in the form of very small, colourless prisms by boiling bismuth carbonate with excess of selenic acid and removing the excess acid by heating. It is insoluble in water and is not decomposed by boiling water; it dissolves in mineral acids, but is decomposed by alkalies. The existence of selenites and selenates of bismuth has not, however, been confirmed.