Bismuth Trihydride, BiH₃

Bismuth Trihydride, BiH₃, was first obtained in small quantities by the action of dilute hydrochloric acid (0.2N) on an alloy of magnesium and thorium C (a radioactive isotope of bismuth). It has since been obtained from a non-radio alloy of bismuth and magnesium by the action of more concentrated hydrochloric acid (4N). The best results appear to be obtained when the magnesium is not completely alloyed with the bismuth but only superficially coated with that metal. With non-radio material approximately 5×10^-5 of the bismuth used is converted into the hydride; this is about one-twentieth of the yield obtained when radio materials are employed. The formation of the trihydride could not be detected when active hydrogen reacted with powdered bismuth. It is formed by an oscillating discharge between bismuth electrodes in an atmosphere of hydrogen. In the latter method it is essential that the products should be removed and cooled to room temperature as rapidly as possible, and there must be a complete absence of organic substances.

Bismuth trihydride is a gaseous compound which is almost as stable as antimony trihydride at ordinary temperatures but is readily decomposed on heating. At 160° C. the amount of undecomposed hydride is 35 per cent., at 250° C. 9 per cent, and at 350° C. 6 to 7 per cent. At red heat decomposition is complete. In the heated tube employed in the Marsh test, a mirror similar to the antimony mirror is produced: a strong brown mirror is obtained in front of the heated spot and a fainter ring behind it. The hydride is completely decomposed by concentrated sulphuric acid. The best absorption reagent for the gas is 0.4N solution of silver nitrate; it is also partially absorbed by normal potassium hydroxide solution, 0.4N solution of sodium carbonate, 4N solution of sulphuric acid, and water saturated with hydrogen sulphide. Drying agents such as soda-lime and calcium chloride may also be used for its absorption, but water alone does not absorb it well.

Bismuth trihydride can be distinguished from antimony trihydride by a luminescence method. The gas from the Marsh test is ignited, and bismuth is deposited on a small fragment of calcium carbonate held in the flame. If the calcium carbonate, after cooling, is placed on the edge of a hydrogen flame the bismuth on it will impart to the flame a cornflower blue coloration. Antimony in similar circumstances imparts an azure blue coloration