Ferric nitrate, Fe(NO₃)₃

Ferric nitrate, Fe(NO₃)₃, is conveniently obtained by dissolving iron in nitric acid of density 1.3 until the colour of the solution changes through green to dark red. An equal bulk of nitric acid of density 1.43 is now added, and the liquid allowed to cool.

The salt separates out in hydrated crystals, which are usually pale violet in colour, the extent of hydration varying according to circumstances. Two definite hydrates are known: namely, the nonahydrate, Fe(NO₃)₃.9H₂O, which yields pale red deliquescent monoclinic crystals, of the following crystallographic elements: -

a: b: c = 1.1296: 1: 1.9180, β = 131° 34',

and the hexahydrate, Fe(NO₃)₃.6H₂O, which crystallises in cubes, melting at 60.5° C.

The crystals readily dissolve in water to a yellowish brown solution. In nitric acid they are much less soluble, and admit of ready recrystallisation from this acid. The crystals of nonahydrate melt at 40° C., yielding a dark red liquid (Ordway).

Although ferric nitrate crystals are usually pale violet in colour, when perfectly pure the salt is colourless. Ordway mentions having obtained colourless crystals, and this is confirmed by the more recent work of Lambert and Thomson, who took elaborate precautions to prepare a very pure salt. A pure specimen of ferric chloride, free from arsenic, sulphate, alkali, or alkaline earth metals, was dissolved in conductivity water and electrolysed between iridium electrodes. The deposit of iron on the cathode was dissolved in pure diluted nitric acid, and the solution concentrated on the water-bath. The salt was crystallised from solution in concentrated nitric acid, separated from the mother liquor, and recrystallised several times from the concentrated acid. The crystals so obtained were colourless, or white when seen in bulk.

Ferric nitrate in aqueous solution is reduced by metallic silver to the ferrous condition, equilibrium being ultimately attained according to the equation

Ag + Fe(NO₃)₃ ⇔ AgNO₃ + Fe(NO₃)₂.

In dilute solution ferric nitrate is hydrolysed, yielding colloidal ferric hydroxide and free nitric acid. Such solution gives no coloration with potassium ferrocyanide. In less dilute solutions, to which potassium ferrocyanide has already been added, the blue colour gradually intensifies owing to the continued re-formation of ferric nitrate, as the equilibrium represented by the equation

Fe(HO)₃ + 3HNO₃ ⇔ Fe(NO₃)3 + 3H₂O

is disturbed, owing to removal of ferric nitrate as insoluble Prussian blue.

Aqueous solutions of ferric nitrate containing less than 0.1 per cent, are quite colourless.

The heat of formation of ferric nitrate is given by Berthelot as follows: -

2[Fe] + 3(N₂) + 9(O₂) + Aq. = 2Fe(NO₃)₃.Aq. + 314,300 calories.

Numerous basic ferric nitrates have been described. They are uncrystallisable substances, and contain varying amounts of water. Hausmann, by boiling solutions of ferric nitrate, obtained the following supposed compounds: - 8Fe₂O₃.2N₂O₅.3H₂O, 36Fe₂O₃.N₂O₅.48H₂O, and 8Fe₂O₃.N₂O₅.12H₂O.

Scheurer-Kestner identified 2Fe₂O₃.N₂O₅.H₂O, 2Fe₂O₃.N₂O₅.2H₂O, and 4Fe₂O₃.N₂O₅.3H₂O. Cameron and Robinson, however, have proved that no definite basic nitrates of iron are formed from solution at 25° C., the solid phase under these conditions consisting of a solid solution of ferric oxide, nitric acid, and water. It seems unlikely, therefore, that the substances described by Hausmann and Scheurer- Kestner are definite compounds.

An acid nitrate, Fe₂O₃.4N₂O₅.18H₂O or 2Fe(NO₃)₃.2HNO₃.17H₂O, appears also to have been prepared, although later attempts to obtain the salt proved abortive.

With caesium nitrate the double salt, CsNO₃.Fe(NO₃)₃.7H₂O, is formed as pale yellow deliquescent crystals, melting at 33° to 36° C.