27Al nuclear magnetic resonance studies of the hydrolysis and polymerisation of the hexa-aquo-aluminium(III) cation

Abstract

The hydrolysis of aluminium salt solutions has been studied using ²⁷AI and ¹H n.m.r. and this has led to reliable values for the concentrations of the species of AI(H₂O)₆ ³⁺, AI₂(OH)₂(H₂O)₈ ⁴⁺, and AI₁₃O₄(OH)₂₄(H₂O)₁₂ ⁷⁺ at different degrees of hydrolysis. These ions do not account for all the aluminium in solution and another species, probably Al₈(OH)₂₀(H₂O)_x 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(III) 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 potentiometric results.