Aluminium selenate is an inorganic compound of aluminium and the selenate ion, with the chemical formula Al2(SeO4)3. Hydrated and basic aluminium selenates are also known.
Preparation Aluminium selenate can be prepared by oxidation of aluminium selenite with hydrogen peroxide. Refluxing aluminium selenite with approximately 30% hydrogen peroxide converts Se(IV) to Se(VI) and gives aluminium selenate.
It can also be prepared by reacting aluminium hydroxide with a slight excess of selenic acid.
Properties Several hydrated forms are known. A hexadecahydrate, Al2(SeO4)3·16H2O, has been described, together with the basic selenate AlOH(H2O)5SeO4.
Aluminium selenate also forms double salts with alkali-metal selenates. For example, potassium aluminium selenate can be crystallized from mixed solutions of aluminium selenate and potassium selenate.
Structure Anhydrous Al2(SeO4)3 is isostructural with anhydrous iron(III) sulfate, Fe2(SO4)3.Perret, René; Rosso, Bernard. "Crystalline parameters of anhydrous trivalent sulfates and selenates. II. Aluminum and gallium selenates." Bulletin de la Société Chimique de France. 1968. 7: 2700–2701.
The structure consists of AlO6 octahedra and SeO4 tetrahedra linked through shared oxygen atoms, as is typical of trivalent-metal selenates.
Thermal decomposition Aluminium selenate is thermally less stable than the corresponding sulfate. On heating, it decomposes rather than melting cleanly.Selivanova, N. M.; Samplavskaya, K. K.; Strel'tsov, I. S.; Mazepova, V. I. "Thermal decomposition of aluminum selenate". Zhurnal Neorganicheskoi Khimii. 1963. 8 (7): 1645–1653.
Thermal decomposition proceeds through loss of selenium oxides and formation of aluminium oxide at higher temperature.
References