Inorganic complexes of aluminium in aqueous solution

Abstract

The ability of aluminium to form complex ions in aqueous acidic solutions is well known, the study of hydrolysed species of aluminium, for example, being of considerable current interest. Investigations of the hydrolytic equilibria have been made using numerous techniques including potentiometric and conductivity measurements and such methods have also shown the presence of soluble complexes of aluminium fluoride, phosphate I and fluorophosphate In suitable binary and ternary aqueous systems. The role of such complexes and their equilibria is of importance in many fields. The study of aluminium hydrolysis is of interest to the water chemist for the clarification of industrial and potable water. The agricultural chemist requires knowledge of these species for investigations of plant nutrition and predogenesis. Recently it has been suggested that complexes of aluminium containing phosphate and/or fluoride may be important contaminants in by-product gypsum produced during the manufacture of phosphoric acid from phosphate rock. In order to assess the chemistry of aluminium in such Situations it is necessary to first obtain a complete understanding of the nature of the complexes and their equilibria in pure binary and tertiary aqueous systems. Such knowledge is not yet complete and considerable speculation exists as to the equlibria and species formed. Two spectroscopic techniques are particularly suitable for the study of complexes in aqueous media namely nuclear magnetic resonance (n.m.r.) and Raman spectroscopy. The former can supply considerable information when more than one magnetic nucleus is available for study in the same system. Despite the suitability of the above aluminium complexes for such studies, investigators have limited their attention to the aquated aluminium ion A1(H2O)n 3+ (1H and 17O) and to the alumino fluorides (19F and 27AI). This present investigation uses n.m.r. spectroscopy to examine the aqueous aluminium ion, AI(H20)n3+ and the soluble complexes formed from its hydrolysis I the study then being extended to other complexes including those in aqueous binary and ternary systems containing fluoride and phosphate ions. The complexes are detected by their 1H 19F 27Al or 31P resonances where applicable. A limited Raman investigation is also included and in both studies work is confined to soluble complexes in acidic solution. Before presenting the results of these investigations it 1s first convenient to review the literature available on these systems. This is followed by a short discussion on those properties of the nucleus which are important in the study of inorganic complexes, chapter 2, and the experimental techniques employed, chapter 3. Following the results of the n.m.r. studies, chapters 4 to 8, a separate chapter is presented on the results of Raman investigations.