Nuclear magnetic resonance and molar-volume studies of the complex formed between aluminium(III) and the sulphate anion

Abstract

Aluminium-27 n.m.r. spectroscopy monitors the concentration of a first-sphere sulphate complex which is formed in aqueous aluminium sulphate solutions. The proportion of complexed aluminium increases markedly with temperature and a heat of reaction for complexation of 33.1 kJ mol^–1 is obtained. Line broadening occurs as the temperature is increased and this gives a value of 49.8 kJ mol^–1 for the activation energy of exchange of the sulphate complex. The lifetime of the aqua cation is 0.47 s at 298 K, ca.⅓ that in AlCl₃ solution. At constant temperature the proportion of first-sphere complex formed is insensitive to concentration > 0.01 mol dm^–3 and its formation constant is much greater than is calculated from the n.m.r. integrals of the two resonances observed. This paradox can be resolved if it is assumed that the first-sphere complex signal simply monitors the amount of aluminium with sulphate in the second sphere, it being known that the second-sphere complex is very strongly formed. Molar-volume measurements confirm that the more concentrated solutions are highly associated. Spectral changes obtained on varying the proportion of sulphate to aluminium indicate that up to two sulphate anions may reside in the second sphere. Line broadening of the resonances which is observed at the highest dilutions is associated with auto-hydrolysis.