Ferrous nitrate, Fe(NO₃)₂

The anhydrous salt has not been prepared. The hexahydrate of Ferrous nitrate, Fe(NO₃)₂.6H₂O, is very unstable, and is, in consequence, somewhat difficult to isolate. It may be obtained in solution by dissolving either ferrous sulphide or metallic iron in dilute nitric acid, of density ranging from 1.034 to 1.115. If metallic iron and acid of lower density be employed, the iron dissolves without any evolution of gas, and with the formation of ferrous nitrate and ammonium nitrate. By using iron and acid within the above-mentioned range of densities, ferrous nitrate is obtained, together with some ferric nitrate, the percentage of the latter increasing with the density of the acid. If the acid density is greater than 1.115, ferric nitrate is the main product.

Ferrous nitrate may also be obtained in solution by grinding lead nitrate with an equivalent quantity of ferrous sulphate in the presence of dilute alcohol; double decomposition takes place, the ferrous nitrate passing into solution. Upon evaporation at room temperature the salt crystallises out as bright green rhombic prisms, which are faintly stable at 0° C. They melt at 60.5° C., but rapidly decompose when the temperature is raised by even a very small amount above this point. At higher temperatures a residue of ferric oxide is obtained.

The solubility of ferrous nitrate in water at various temperatures is given as follows: -

Temperature, °C	-9	0	18	24	25	50	60.5
Grams Fe(NO3)2 per 100 grams solution.	39.68	41.53	45.14	46.51	44.44	46.94	62.50
					1.48	1.50

Between -9° and 24° C. the composition of the saturated solution is given by the expression: -

Fe(NO₃)₂ + (14.198-0.11242t)H₂O.

A break in the solubility curve at -12° C. indicates the formation of another hydrate, namely, the nonahydrate, Fe(NO₃)₂.9H₂O, which, however, has not been isolated. Its solubility in water is as follows: -

Temperature, °C.	-27	-21.5	-19	-15.5
Grams Fe(NO3)2 per 100 grams solution	35.66	36.10	36.56	37.17

The heat of formation of the dissolved salt is: -

[Fe] + (N₂) + 3(O₂) + Aq. = Fe(NO₃)₂.Aq. + 119,000 calories.
2HNO₂. Aq. + [Fe(OH)₂] = Fe(NO₃)₂. Aq. + 21,500 calories.