An improved procedure for the complexometric titration of aluminium

Abstract

Aluminium ions in solution show a marked tendency to hydrolyse with the formation of soluble polynuclear hydroxo complexes and precipitates of the trihydroxide, both of which are substantially unreactive. As the stability constant of the aluminium-EDTA complex is rather low, relatively high pH values are necessary to ensure complete formation of the complex; these pH values approach the critical range for hydroxide precipitation. Under these circumstances direct macro-scale titrations are of uncertain accuracy and direct micro-scale titrations become practically impossible to carry out. These problems can be overcome by the use of back-titration procedures coupled with carefully controlled pH adjustment in order to avoid locally exceeding the critical value for hydroxide precipitation. With macro-scale titrations the pH can be adjusted satisfactorily in a stepwise manner, but for micro-scale titrations the pH must be increased homogeneously. This type of increase can be achieved by using hexamine, which slowly releases ammonia in boiling aqueous solutions. Based on these observations, a procedure is presented that provides for the complexometric determination of aluminium at concentrations down to 0·5 p.p.m., with an accuracy of 1 per cent. or better.