Aluminium nitrate, Al(NO3)3.9H2O, is prepared by dissolving aluminium in nitrie acid of density 1.42 and crystallising the solution. It is dimorphous, crystallising in flat, orthorhombic crystals (a:b:c = 0.8925:1:1.0202) or in monoclinic prisms (a:b:c = 1.1340:1:1.9191, β = 131°36') isomorphous with the corresponding ferric salt. It melts at C. 70°, and at 140° is completely decomposed, amorphous aluminium hydroxide being left. The aqueous solution when heated deposits basic salts.
At the ordinary temperature nitric acid of density 1.5 transforms the ennea-hydrate into the hexahydrate, Al(NO3)3.6H2O, which is then the stable phase. The hexahydrate often separates from its solution in nitric acid of density 1.42, but it is then metastable with respect to the ennea-hydrate. The transformation from hexa- to ennea-hydrate can be made to proceed in two stages, an intermediate hydrate, 2Al(NO3)3.15H2O (or 16H2O?), being formed. Ditte has described a dihydrate, Al(NO3)3.2H2O. |