Thallous nitrate, TlNO₃

Thallium dissolves readily in nitric acid; and by dissolving the metal, thallous oxide, hydroxide, or carbonate in nitric acid, a solution of thallous nitrate is obtained from which the salt crystallises in the anhydrous state in orthorhombic prisms (a:b:c = 0.5109:1:0.6507) of density 5.55. At 72.8° the rhombic substance changes into a rhombohedral form, which at 142.5° changes to a cubic modification; and this, at 205°, melts. On cooling, the reverse changes take place. The transition at 72.8° is a very slow process. At temperatures above 300° decomposition of the nitrate takes place; it is rapid at 450°, crystalline thallic oxide being left, and oxygen, nitrogen, and oxides of nitrogen being evolved. A little of the nitrate volatilises unchanged.

Thallous nitrate is insoluble in alcohol. Its solubility in water is given in the following table: -

t °C.	0°	10°	20°	30°	40°	50°	60°	70°	80°	90°	100°	105°
a	3.76	5.86	8.72	12.51	17.33	23.33	31.55	41.01	52.60	66.66	80.54	85.59
b	3.91	6.22	9.55	14.3	20.9	30.4	46.2	69.5	111.0	200.0	414.0	594.0

A saturated solution in contact with excess of the salt boils at 105°.

Mixtures of molten thallous nitrate and silver, mercurous, or mercuric nitrate are useful liquids in which to effect the separation of minerals by density differences. The following melting-points and densities are due to Retgers: -

Composition of salt.	TlNO3	TlAg(NO3)2	TlHg(NO3)2	Tl2Hg(NO2)4
Density of liquid.	5.3	4.8	5.3	5.0
Melting-point °C	205°	70°	76°	110°

When thallous nitrate is dissolved in cold nitric acid (density, 1.5) and the solution is cooled below 0° C. acid thallous nitrate, TlNO₃.2HNO₃, crystallises out. The compound melts below the ordinary temperature.

Equilibrium diagram for the system thallous nitrate-potassium nitrate.

With sodium nitrate, thallous nitrate forms neither a compound nor mixed crystals.

From a study of the crystals deposited from a solution containing both potassium nitrate and thallous nitrate, it has been concluded that these two salts are isodimorphous. The fusion curve indicates that two series of solid solutions separate out. The series containing excess of thallous salt is cubic, the other series being rhombohedral. These mixed crystals are transformed at lower temperatures into other crystalline modifications, and the various regions of stability are indicated in the diagram (fig.).

Rubidium nitrate, and also caesium nitrate, forms a continuous series of cubic mixed crystals with thallous nitrate. The cubic crystals become rhombohedral at lower temperatures, and those with a very high thallium content become orthorhombic at still lower temperatures. Cubic crystals always separate from the fused mixtures, except those very rich in rubidium, which crystallise first in rhombohedral crystals different from those already mentioned.

Ammonium nitrate forms a continuous (?) series of cubic mixed crystals with thallous nitrate. These crystals separate from fused mixtures of the nitrate on cooling. Two other series of mixed crystals are stable at lower temperatures.

Silver nitrate forms no mixed crystals with thallous nitrate, but forms a compound, TlNO₃.AgNO₃, which melts at 82.8°, and only separates as the initial solid phase from fused mixtures containing 48 to 52 molecules per cent, of thallous nitrate. One or two other double nitrates are known.