Proton chemical shifts of aqueous aluminium salt solutions

Abstract

Proton chemical shifts have been measured in 1M-aqueous solutions of aluminium nitrate, perchlorate, and sulphate over a range of temperature. The effects of hydrolysis and of acid addition have been studied in detail and have a marked influence on the shifts which arise as the exchange average between bulk water and three ionic environments: the cationic hydration complex Al(H₂O)₆³⁺, which has a temperature-independent low-field shift, its second hydration sphere which has a small electric-field-induced shift, and broken water structure around the anion which thus introduces an upfield temperature-independent shift. Exchange averaging is not complete in the region in which measurements were made and this has introduced errors into previous determinations of hydration numbers. The measurements are shown to be consistent with a primary cationic hydration number of six.