Aluminium sulfate and alums

   The most important soluble oxosalts of Al are undoubtedly Al₂(SO₄)₃.16H₂O and the double sulfates Mal(SO₄)₂. 12H₂O (alums). In alums, M‏ is usually K‏, Rb‏, Cs‏ or [NH₄]⁺, but Li‏, Na‏ and Tl‏ compounds also exist; Al3‏ may be replaced by anotherM3‏ ion, but its size must be comparable and possible metals are Ga, In (but not Tl), Ti, V, Cr, Mn, Fe and Co. The sulfate ion in an alum can be replaced by [SeO₄]^2-.     Alums occur naturally in alum shales, but are well known in crystal growth experiments; beautiful octahedral crystals are characteristic, e.g. in colourless KAl[SO₄]₂.12H₂O or purple KFe)SO₄(₂.12H₂O. The purple colour of the latter arises from the presence of the [Fe(H₂O)₆]^3+‏ ion and, in all alums, the M³⁺‏ ion is octahedrally coordinated by six water ligands. The remaining water molecules are held in the crystal lattice by hydrogen bonds and connect the hydrated cations to the anions. Aluminium sulfate is used in water purification for the removal of phosphate and of colloidal matter, the coagulation of which is facilitated by the high charge on the Al³⁺‏ cation. Intake of Al salts by humans, however, is suspected of causing Alzheimer’s disease.

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