Bismuth Hydroxide

Bismuth Hydroxide or Hydrated Bismuth Trioxide, three hydrated forms of bismuth trioxide have been described. They are the trihydrate, or bismuth hydroxide, Bi₂O₃.3H₂O or Bi(OH)₃, the dihydrate, Bi₂O₃.2H₂O, and the monohydrate, also known as bismuthyl hydroxide, Bi₂O₃.H₂O or BiO(OH). There is no doubt that hydrated forms of the trioxide can be prepared by a variety of methods, but their existence as definite chemical compounds has been criticised. From a study of the dehydration of hydrated bismuth trioxide at a constant pressure of 10 mm. mercury, it was determined that water is removed in three stages. Assuming the original hydrated oxide to be the trihydrate, water is removed from this continuously as the temperature rises until the composition approximates to that of the dihydrate. At about 320° C. water is removed suddenly and the composition approaches that of the monohydrate. The remaining water is again removed continuously, a temperature higher than 420° C. being required for complete removal. An earlier investigation on the dehydration of the hydrated oxide by heating revealed a slight break in the dehydration curve between 340° and 415° C., but no substance corresponding in composition to a definite hydrate was obtained. If the hydrated oxide is dried over sulphuric acid, dehydration proceeds continuously and there is no indication of a definite hydrate. Further, recent attempts to prepare by various methods a substance that could be identified as the definite compound bismuth hydroxide, Bi(OH), have proved unsuccessful. It is probable, therefore, that if definite hydrates of bismuth trioxide do exist, they are extremely unstable. The substance as usually prepared, and technically known as "bismuth hydroxide," is of indefinite composition. Among the methods adopted for its preparation may be mentioned precipitation from solutions of bismuth salts by ammonium hydroxide, followed by washing with a succession of volatile solvents and evaporation, and electrolysis of a dilute solution of sodium chlorate, containing carbon dioxide, using bismuth anodes and cathodes of carbon, zinc, iron or aluminium. A description follows of the three hydroxides that have been reported.

Bismuth Trihydrate, Bi₂O₃.3H₂O or Bi(OH)₃, is most conveniently prepared by adding a solution of a bismuth salt containing glycerol to one of sodium hydroxide, and neutralising the excess of alkali with nitric or acetic acid. The product is frequently contaminated with traces of the bismuth salt employed and with the carbonate (through absorption of carbon dioxide). The hydrate is not precipitated from solutions by alkalis in the presence of tartaric acid or citric acid. It is a white substance, and is converted to yellow bismuth trioxide on boiling with alkalis; at lower temperatures, in contact with alkalis, it is said to be converted to the monohydrate. At 20° C. it is practically insoluble in a normal solution of sodium hydroxide and more dilute solutions; the solubility is slightly higher in 4N NaOH, and increases appreciably with higher concentrations; it is greater at 100° C., but still low in dilute solutions. Thus bismuth hydroxide resembles the trioxide in possessing only very slight acidic properties. It is soluble in sodium hydroxide in glycerol, and by heating this solution, and those in tartaric or citric acid, with grape sugar, the hydroxide is reduced to metal.

The so-called dihydrate, Bi₂O₃.2H₂O, is prepared as a pale yellowish- white, flocculent body by precipitation from a solution of so-called bismuthic acid in hydrochloric acid by potassium hydroxide after passing a current of sulphur dioxide through the liquid. When, however, sulphuric acid is used in place of hydrochloric acid, the precipitate is more probably the monohydrate. Two dimorphous forms of the dihydrate have been described.

The monohydrate, or bismuthyl hydroxide, or "bismuthi hydroxidum," Bi₂O₃.H₂O or BiO(OH), is prepared by pouring a solution of bismuth nitrate in dilute nitric acid rapidly into ammonium hydroxide and drying the precipitate at a temperature not exceeding 70° C. This method has, however, been criticised on the grounds that the hydroxide always contains either an oxy-salt or the trioxide; it is also stated that the only true hydroxide is the trihydrate, Bi₂O₃.3H₂O. On heating to 110° C. the trihydrate loses water, the residue containing 94 per cent, of the trioxide. The monohydrate is soluble in ordinary distilled water to the extent of 1.44 milligrams per litre at 20° C. It will precipitate the hydroxides of aluminium, chromium and ferric iron from neutral solutions of their salts, but is apparently without effect upon solutions of copper, zinc, ferrous iron, nickel, cobalt, manganese and lead; but according to other investigators the monohydrate precipitates most metals as oxides or basic salts. It can be oxidised in alkaline solution by many oxidising agents and is reduced by stannous solution to bismuth monoxide.

Colloidal hydrated bismuth oxides, known as "bismon," have been obtained, while hydrosols have also been prepared.