Uranyl Nitrate, UO₂(NO₃)₂

Uranyl Nitrate, UO₂(NO₃)₂.6H₂O, is the most important compound of uranium ; it has received much attention from investigators and is the most widely used uranyl salt of commerce. It is obtained directly from uraninite, and the commercial preparation, which generally contains a small amount of impurity, especially alkali, may be purified by recrystallising several times from water.

It may be prepared by dissolving any of the oxides or hydroxides of uranium in nitric acid and concentrating the solution ; or by double decomposition of uranyl chloride with silver nitrate. It crystallises in lemon-yellow prisms, the form of which varies according to the nature of the solvent. The crystals exhibit a yellowish-green fluorescence which does not increase, as is usually the case with phosphorescent substances, even at such a low temperature as -190° C. They also possess the remarkable properties of triboluminescence, emitting flashes of light when mechanically shaken or crushed, of detonation, or even in some cases of violent explosibility. The phenomena were first described by Ivanov, but subsequent investigators observed that the crystals were only explosive when free nitric acid was present and when they had been obtained by crystallisation from ether. Explosions never occurred with preparations recrystallised from water, although the crystals generally showed triboluminescence. This property appears to diminish with time, but it has been observed in a specimen of the salt which had been kept in a stoppered bottle for forty-one years. Moreover, it does not appear to be connected with the radioactive properties of the salt. If crystallisation is allowed to proceed under the influence of Rontgen rays no effect on the properties of the crystals is observed. A specimen of sodium diuranate, with no triboluminescence, may be converted into a strongly triboluminescent uranyl nitrate. Siemssen suggests that the property is due to electrical tension in the crystals. The less frequently observed explosibility appears to be due to the formation of an unstable compound of the uranyl salt with a lower oxide of nitrogen and ether. According to Andrews some of the water of crystallisation of the hexahydrate is replaced by ether. Eichhorn observed that uranyl nitrate extracted from aqueous solution by means of ether contained less water of crystallisation than the ordinary salt, and explosive specimens of the salt are never obtained when prepared in absence of nitric acid and ether. Two actual compounds of uranyl nitrate with ether have been obtained by drying the ethereal solution over calcium nitrate and cooling to about -10° and -70° C. respectively ; when the ether was removed from these compounds by means of a current of dry air, the dihydrate remained. Analogous compounds, in which the water molecules are replaced by molecules of ammonia, have also been obtained.

The crystals of uranyl nitrate hexahydrate, which have density 2.807 and are diamagnetic, deliquesce in moist air. At 15° C. the vapour pressure is practically negligible. The melting-point is 60.2° C., and the liquid boils at 118° C.

Uranyl nitrate is readily soluble in water, ether, ethyl and methyl alcohols, and other organic solvents. Water at 18° C. dissolves twice its weight of the salt.1 The density of the saturated solution at 17° C. is 1.7536, and its refractive index, n_D¹⁷ =1.41155.

The equivalent conductivities of solutions containing ½UO₂(NO₃)₂ in v litres at 25° C. are as follows:

v =	32	64	128	256	512	1024	2048	4096	Λ1024 - Λ32
Λ (i) =	100.5	107.4	111.7	122.6	130.8	140.2	150.0	161.8	39.7
Λ (ii) =	103.9	-	117.8	-	131.3	139.5	-	-	35.6
Λ (iii) =	-	110.8	-	120.0	128.6	136.1	146.3	-	-

As in the case of the sulphate, the difference Λ₁₀₂₄ - Λ₃₂ is abnormally high for a normal salt of a divalent anion, apparently owing to hydrolysis; the solution has an acid reaction. Dittrich has shown the presence of hydrogen ions in solution by the inversion of sugar, and by cryoscopic measurements has determined the degree of dissociation a and the molecular weight of the solute at varying dilutions; thus for solutions containing the molecular weight (UO₂(NO₃)₂ = 394.2) in v litres:

v =	1	2	4	8	16
α =	0.78	0.80	0.82	0.88	0.94
Mol. wt. =	153.4	151.6	148.9	142.5	137.8

It may be shown by extrapolation that the apparent molecular weight at infinite dilution is almost exactly one-third the real molecular weight, so that ionisation appears to proceed according to the scheme:

UO₂(NO₃)₂ ⇔ UO₂^•• + 2NO₃'.

Up to an equivalent dilution of 4096 litres, the values of A do not converge to a limit. In concentrated solution the salt is ionised to a considerable extent, and it is doubtful whether it may be considered as a simple ternary electrolyte. The addition of ammonia to the solution precipitates ammonium diuranate, thus indicating the probable presence of the anion U₂O₇''.

The following conductivity measurements have been obtained for solutions containing ½UO₂(NO₃)₂ in v litres at 50° C.:

v =	4	16	64	256	512	1024	2048
Λ (1) =	134.3	159.3	189.5	-		246.3	-
Λ (2) =	121.1	149.3	174.8	201.5	216.4	226.1	250.0

By the dehydration of the hexahydrate, tri-, di-, and mono-hydrates have been obtained. The anhydrous salt has also been described.

When uranyl nitrate hexahydrate is placed in an enclosed space with sulphuric acid at ordinary temperature and pressure it undergoes dehydration in two stages, yielding after a few days the trihydrate, UO₂(NO₃)₂.3H₂O, and after about a month the dihydrate, UO₂(NO₃)₂.2H₂O. Reduction of pressure or increase of temperature only accelerates the change. The trihydrate may also be prepared by drying the hexahydrate over lime in a vacuum; by evaporation of a neutral solution of the nitrate at 65° C.; or by evaporation of an acid solution in vacuo. It yields monoclinic crystals which melt at 121.5° C.

The dihydrate is formed by evaporation at ordinary temperature of an ethereal solution of the hexahydrate which has been dried with calcium nitrate; or by crystallisation of the hexahydrate from concentrated nitric acid solution.5 It yields small lustrous plates, thick and square, probably rhombic, and possessing a green fluorescence. It melts at 179.3° C. It is much more stable than the trihydrate, and can be kept in a vacuum desiccator with caustic alkali or phosphorus pentoxide without any loss of water. It dissolves readily in ether. If the dihydrate is heated in a current of carbon dioxide at 98° C. a product corresponding very nearly in composition to the monohydrate, UO₂(NO₃)₂.H₂O, is obtained; at 160° C. under the same conditions the anhydrous salt, UO₂(NO₃)₂, is obtained. The latter may also be obtained by passing a current of dry nitric anhydride over the trihydrate carefully heated at 170° to 180° C. It is a yellow amorphous powder, readily soluble in water with evolution of heat. It reacts violently with ether. When heated to 200° C. it decomposes and leaves a mixture of uranic acid, UO₃.H₂O, and uranic anhydride.

By an investigation of the cooling and heating curves for solutions of uranyl nitrate, indications have been obtained of the existence of an icositetra-hydrate, UO₂(NO₃)₂.24H₂O.

The following values for the heats of solution of anhydrous uranyl nitrate and its hydrates have been obtained:

	de Forcrand	Marketos
UO2(NO3)2.	+ 19,000 calories	+ 16,000 calories
UO2(NO3)2.H2O.	+ 11,870 calories
UO2(NO3)2.2H2O.	+ 5,050 calories	+ 5,420 calories
UO2(NO3)2.3H2O.	+ 1,350 calories	+ 2,000 calories
UO2(NO3)2.6H2O.	- 5,450 calories	- 4,760 calories

The following values for the heat of formation of uranyl nitrate are due to de Forcrand:

UO₂ + 3O₂ + N₂ = UO₂(NO₃)₂ (solid) + 67,250 calories,
or = UO₂(NO₃)₂aq. + 86,250 calories.

The heats of solution in dilute nitric acid of

UO₃ = 19,803 calories.
UO₃.H₂O = 14,846 calories
UO₃.2H₂O = 12,375 calories

By dissolving the dihydrate of uranyl nitrate in fuming nitric acid and treating the cooled solution with a mixture of nitric anhydride and excess of nitrogen peroxide, a light yellow precipitate, of composition UO₂(NO₃)₂.2NO₂, is obtained. It is decomposed by water with liberation of nitrogen peroxide. At 163° to 165° C. it yields the anhydrous uranyl nitrate.

Uranyl nitrate forms double salts of the type R'UO₂(NO₃)₃, where R' = K, NH₄, Rb, Cs, or Tl. They may be prepared by crystallising a solution of the mixed nitrates in concentrated nitric acid; or by crystallisation from a solution of the alkali uranate in excess of nitric acid. The crystals are all anhydrous, and in the case of the potassium salt are orthorhombic, whilst the rubidium and caesium salts are rhombohedral and isomorphous with each other. The ammonium salt yields both orthorhombic and rhombohedral crystals. All exhibit a yellowish-green fluorescence; they are hygroscopic and readily decomposed by water into their components. The thallium salt is particularly unstable, being decomposed in moist air. On the other hand, the rubidium salt dissolves in water at 80° C. without decomposition. The corresponding salts of sodium, lithium, or of divalent metals have not been prepared.

The following unstable complex nitrates have been obtained:

Cadmium uranyl nitrate, CdUO₂(NO₃)₄.30H₂O, yellow needles, which lose 10H₂O when dried in vacuo over sulphuric acid;

Nickel uranyl nitrate, 3UO₂(NO₃)₂.10Ni(NO₃)₂, greenish-yellow needles; and

Rhodium uranyl nitrate, RhUO₂(NO₃)₅.10H₂O, orange leaflets, which lose 5H₂O in vacuo.

Ethylenediammonium uranyl nitrate, C₂H₄(NH₂)₂.2HNO₃.UO₂(NO₃)₂.2H₂O, has been prepared.