Thallous chloride, TlCl

Thallous chloride, TlCl, may be obtained by the action of chlorine on thallium. It is prepared by adding hydrochloric acid or a solution of a soluble chloride to a solution of a thallous salt, and forms a white, crystalline precipitate.

Thallous chloride is a white, crystalline solid of density 7.02. It melts to a yellow liquid at 426° C., and boils at 708° to 731° C. Its compressibility coefficient is 4.8×10^-4 per atmosphere. The chloride crystallises in isometric cubes and is isomorphous with potassium chloride. From 830° to 1030° its vapour density is in agreement with the molecular formula TlCl; and the molecular formula is also TlCl in solution in fused mercuric chloride. Thallous chloride is slightly soluble in cold, and more readily soluble in hot water. The solubility, in grams per litre, is as follows: -

Temperature, °C	0°	10°	20°	25°	30°	40°	50°	60°	80°	100°
Grams of TlCl	1.7	2.4	3.4	4.0	4.6	6.0	8.0	10.2	16.0	24.1

The chloride is less soluble in dilute solutions of hydrochloric acid and alkali chlorides than in pure water; it is insoluble in alcohol. The influence of various salts on the solubility of thallous chloride in water at 25° is shown by the following table of results, due to Bray and Winninghoff, solubilities being expressed in milli-equivalents per litre: -

Conc. Of added salt in milli-eqs. Per litre.	Added Salt
	HCl	KCl	BaCl2	TlNO3	Tl2SO4	KNO3	K2SO4
0	16.07	16.07	16.07	16.07	16.07	16.07	16.07
20	...	...	...	...	10.34	17.16	17.79
25	8.66	8.69	8.98	8.80	...	...	...
50	5.83	5.90	6.18	6.24	6.77	18.26	19.42
100	3.83	3.96	4.16	4.22	4.68	19.61	21.37
200	2.53	2.68	2.82	...	...	...	...
300	...	...	...	...	...	23.13	26.00

The specific conductivity of the saturated aqueous solution is 1514×10^-6 reciprocal ohms per cm. cube at 18°, and 2176×10^-6 at 25° C.

Fused thallous chloride is miscible with many other molten chlorides, and from a study of the freezing-point curves it has been shown that various double chlorides exist, a number of which are stable at the melting-point. The compounds at present known containing thallium and the metals of Groups I. and II. are as follows: 2AgCl.3TlCl; TlCl.2ZnCl₂ (m.p. 226°); TlCl.CaCl₂ (m.p. 683°); TlCl.CdCl₂ (m.p. 436°); TlCl.SrCl₂; TlCl.HgCl₂ (m.p. 224°); TlCl.2MgCl₂; 4TlCl.HgCl₂; 2TlCl.ZnCl₂ (m.p. 352°).

Thallous chloride forms mixed crystals with the chlorides of potassium, rubidium, zinc, and mercury. The double salt TlCl.HgCl₂ crystallises in needles, and was originally prepared by Carstanjen, who also prepared thallous aurichloride, TlAuCl₄.