Thallic chloride, TlCl₃

Thallic chloride, TlCl₃, cannot be obtained in the pure state by the chlorination of thallous chloride except when the reaction is carried out in a sealed tube and the pressure of the chlorine is several atmospheres. Thallic chloride thus prepared is described by Thomas as consisting of white crystals, which melt at 60° to 70° (in a sealed tube).

Anhydrous thallic chloride may be obtained by dehydrating its tetra- hydrate at the ordinary temperature over sulphuric acid, potash, or phosphoric anhydride, or, according to Meyer, by the decomposition in vacuo of the compound TlCl₃.(C₂H₅)₂O. The chloride thus obtained forms small, six-sided plates, and melts at 25°. It commences to lose chlorine at 40°, and the decomposition is rapid at 100°. It is extremely hygroscopic, rapidly absorbing moisture with the formation of an aqueous solution of thallic chloride; and it is soluble in alcohol, ether, and other organic solvents. With a small quantity of water; thallic chloride forms a clear solution, which is considerably hydrolysed by dilution.

When thallous chloride is suspended in water and chlorine is passed through the liquid for a sufficient length of time, a clear solution of thallic chloride is obtained. If the solution be evaporated below 60° to a syrup, and then cooled in a freezing mixture, colourless, orthorhombic (1) crystals of the tetrahydrate, TlCl₃.4H₂O, separate. This hydrate melts at 43° to 45°, and in dry air slowly effloresces (more rapidly at 55°) to the monohydrate, TlCl₃.H₂O, while according to M'Clenahan, a dihydrate, TlCl₃.2H₂O also exists. The monohydrate crystallises from a saturated aqueous solution of the trichloride at ordinary temperatures.

From a solution of thallic chloride acidified with nitric acid, silver nitrate quantitatively precipitates the chlorine as silver chloride; but from a neutral solution, in addition to the precipitate of silver chloride, a brown precipitate of thallic hydroxide is obtained.

Thallic chloride combines with ammonia, producing a white, crystalline compound, TlCl₃.3NH₃, decomposed by water. This compound was prepared by Willm in a number of ways. According to Thomas, a compound, TlCl₃.5NH₃, is produced by the action of an excess of ammonia on anhydrous thallic chloride. No compound richer in ammonia is known, and the compound TlCl₃.5NH₃ is insoluble in liquid ammonia. Thallic chloride combines with ether, pyridine, aniline, etc.

Thallic chloride tetrahydrate readily absorbs one molecular proportion of hydrogen chloride, a liquid being produced from which, on standing over phosphoric anhydride, hydrogen thallic chloride, TlCl₃.HCl.3H₂O or HTlCl₄.3H₂O, separates in deliquescent crystals.