Aluminium nitrate, Al(NO₃)₃

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.