Bismuth Oxychloride, BiOCl

Bismuthyl Chloride, or Bismuth Oxychloride, BiOCl - An oxychloride, which is probably bismuthyl chloride, occurs in the mineral daubreit, which is found in Bolivia.

Bismuthyl chloride is the product obtained by the reaction of bismuth trichloride with water, the decomposition being effected by the action of either hot or cold water on the solid, or by the addition of water to a moderately concentrated solution in hydrochloric acid. It is also obtained by the addition of a solution of an alkali chloride to a solution of bismuth nitrate. Many other methods may also be adopted for the preparation of this substance. (1) When bismuth trichloride is heated in steam, bismuthyl chloride is left as a residue. (2) It is formed in small quantities when bismuth trichloride is sublimed in air. (3) It is the stable compound always produced by the action of water, sulphur dioxide, chromyl chloride, nitrogen peroxide and other oxidising agents upon bismuth trichloride. (4) It is formed by the action of dilute hydrochloric acid on bismuth trioxide; thus

Bi₂O₃ + 2HCl = 2BiOCl + H₂O

Colourless tetragonal crystals may be obtained by a suitable modification of this method. (5) When bismuth sulphate is heated with sodium chloride and the mixture treated with water, bismuthyl chloride remains undissolved.

Bismuthyl chloride is a white, crystalline powder. On heating, the colour changes to yellow, and in parts brown, this disappearing only partially on cooling; the colour change thus appears to be partly physical and partly chemical. The compound also darkens on exposure to light. The crystals belong to the tetragonal system, have a density of 7.717 at 15° C. and a molecular volume of 33.7; the density of the precipitated form is 7.2 at 20° C. Melting occurs at red heat without decomposition, and on solidifying a pale yellow, crystalline mass is formed. Prolonged heating at a high temperature causes slight volatilisation, possibly of bismuth trichloride, although it is also suggested that true sublimation occurs to a slight degree. The oxychloride is almost insoluble in water and in liquid ammonia. It dissolves readily in both hydrochloric and sulphuric acids, with the formation of the trichloride and sulphate respectively; it also dissolves in nitric acid on heating, and from this solution it is re-deposited by evaporation. It reacts with alkalis, especially when the solutions are concentrated. It is probable that with a dilute solution of potassium hydroxide a reversible reaction takes place according to the equation

BiOCl + KOH ⇔ BiO.OH + KCl

With concentrated solutions the action is much more complex. When heated with mercuric oxide it is partially converted to bismuth trioxide. It is reduced to metal by fusion with potassium cyanide, and also by heating in a current of hydrogen; in the latter case some trichloride volatilises.

Two hydrates have been reported: the monohydrate, BiOCl.H₂O, and the trihydrate, BiOCl.3H₂O, both of which have been stated to become anhydrous at 100° C.; but from a study of the equilibrium of the system Bi₂O₃-HCl-H₂O it would appear that no hydrate of bismuthyl chloride can exist at 25° C.; the stable phase which separates out from solutions containing 25 to 33.67 per cent. HCl (up to the point 33.7 per cent. HCl, 58.7 per cent. Bi₂O₃, 7.6 per cent. H₂O) being anhydrous bismuthyl chloride. The solubility product of the monohydrate is, however, stated to be 1.58×10^-31.

The existence of other oxychlorides to which the formulae Bi₂O₃Cl₂, Bi₃O₂Cl₃ and Bi₄O₃Cl₄ have been ascribed has been suggested from time to time, but it has not been definitely established that these are chemical entities. A study of the hydrolytic dissociation of bismuth trichloride indicates that only one oxychloride exists.

From the experimental result shown by the equation

BiCl₃ + Aq. → BiOCl + 7830 gram-calories

the heat of formation of bismuthyl chloride is calculated to be 88,180 gram-calories.