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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.

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