Aluminium-27 nuclear magnetic resonance studies of the hydrolysis of aluminium(III). Part 7.—Spectroscopic evidence for the cation [AlOH]2+ from line-broadening studies at high dilution

Abstract

The ²⁷Al nuclear magnetic resonance spectrum of aluminium salt solutions broadens markedly as these are diluted below 0.05 mol dm^–3 owing to the formation of increasing amounts of [AlOH]²⁺. This latter cation has a broadened line and fast exchange between this and the aquocation causes the observed changes in linewidth. By assuming that [AlOH]²⁺ has the same line-width as the isoelectronic [AlF]²⁺ it is possible to obtain the proportion of [AlOH]²⁺ formed and so calculate its formation constant. Suitable treatment of the data, including results from partially acidified solutions, shows that [AlOH]²⁺ is the only fast-exchanging hydrolysed species formed and gives a pK value of 4.93, in close agreement with earlier potentiometric studies. The present method, however, has the advantage that it avoids some of the sources of error in the earlier work. The way the formation constant varies with temperature is also briefly discussed.