The hydrolysis of aluminium salt solutions has been studied using 27AI and 'H n.m.r. and this has led to reliable values for the concentrations of the species of AI(H,O)z+, A12(OH)2(H,0)~84+, and A11304(OH)24(H20),,7+ at different degrees of hydrolysis. These ions do not account for all the aluminium in solution and another species, probably A18(OH),,(H,0),4+ is thought to be present. It is shown that previous controversy over the constitution of these solutions arose because of the different methods used for preparing the solutions : only the first two species exist when hydrated aluminium(lll) chloride is dissolved in water whereas the highly polymeric species appear only after considerable hydrolysis has occurred at higher pH. The cations, particularly the dimer, may also be more hydrolysed than the above formulae indicate and this has probably confused the previous interpretation of the potentiornetric results.

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