Calcium Nitrite, Ca(NO₂)₂

Calcium Nitrite, Ca(NO₂)₂, may be obtained by careful calcination of the nitrate, with or without a reducing agent, but this is not satisfactory owing to the fact that the temperature at which decomposition of the nitrate commences is 500° C., whereas for the nitrite it is 220° C. It may also be prepared in solution by the action of silver nitrite on calcium chloride. From this it crystallises in very faintly yellow hexagonal needles which contain 1 molecule of water of crystallisation and are isomorphous with barium and strontium nitrites.

Dehydration over sulphuric acid appears to result in the formation of a half-hydrate, Ca(NO₂)₂.½H₂O, and Oswald obtained a tetrahydrate Ca(NO₂)₂.4H₂O, of density 1.674 at 0° C. The hydrated nitrite can be dehydrated in vacuo over phosphorus pentoxide without decomposition. The concentrated aqueous solution undergoes hydrolysis at 100° C., but neither the solid nitrite nor the saturated solution is oxidised by oxygen at atmospheric pressure. Oxidation only occurs in the presence of acids, and is then due to the oxidation of the decomposition products of nitrous acid.

Calcium nitrite is very soluble in water, but not in alcohol. A solution saturated at 16° C. contains 42.3 per cent, of nitrite, and has a density of 1.4205.

Oswald studied the system, calcium nitrite: water, from the point of view of the phase rule and obtained the following values for the concentration of the saturated solution of calcium nitrite at different temperatures: -

Temperature, ° C.	Percentage Concentration of Solution.	Solid Phase.
-4	16.7	Ice.
-9.3	25.5
-11.3	28.4
-12.5	29.5
-14.5	32.0
-9.5	36.2	Ca(NO2)2.4H2O
0	38.3
18.5	43.0
42	51.8
54	55.2	Ca(NO2)2.H2O
64	58.4
70	60.3
73	61.5
91	71.2

The eutectic point is at -17.5° C. at a concentration of 35 per cent, of nitrite. The transition from the tetra- to the mono-hydrate takes place at a temperature of 44°±2° C., and a concentration of 53.5±1 per cent.

The monohydrate is unchanged in air at ordinary temperatures, and the tetrahydrate effloresces, or is unchanged, according to the pressure of water vapour in the air. Over phosphorus pentoxide, under a pressure of a few centimetres of mercury, the tetra- changes to the mono-hydrate.

A double nitrite of calcium and caesium is known, Cs₂Ca(NO₂)₄.H₂O, and also one of calcium and mercury, Hg(NO₂)₂.Ca(NO₂)₂.5H₂O, which Ray regarded as containing the complex divalent anion Hg(NO₂)₄^2-.

Triple nitrites with potassium and copper have been described.