Abstract
A review of previous work on the phosphates of aluminium and indium has shown that on several points the conclusions reached by the various workers are rather conflicting. The present work was undertaken in the hope that application of pH-titration and ion-exchange methods would enable more precise conclusions to be drawn. Similar studies were made of the indium phosphates so that a comparison could be made between the results for the two systems. To make the comparison more complete, the system indium oxide - phosphoric oxide - water was also studied, both at 25 and at 70 C. , and as a result several stable solid phases have been identified. Studies using different metal forms of a cation-exchanger enabled the existence of phosphato-, sulphato-, and chloro- complexes of aluminium to be demonstrated. Cation-exchange studies with hydrogen form resin and solutions containing metal, chloride, and phosphate ions indicated the presence of polynuclear complex ions such as [M3 (PO4)2(OH)]2+ in these solutions. In the case of aluminium, such complexes appear to be less stable than the corresponding indium complexes. Anion-exchange studies of metal phosphate solutions in phosphoric acid confirmed the existence of anionic phosphate complexes of both aluminium and indium. For indium, the sorption of a complex ion [InH2(PO4)3]4- is indicated. The results of pH-titration studies supported these conclusions and further indicated that hydroxy-complexes were present (at least initially) in the metal chloride solutions. A comparative discussion is given of the results of the studies of aluminium and indium phosphates. Attempts are made to account for the weaker sorption of aluminium phosphate complexes by ion-exchangers, taking into account the smaller ionic radius of aluminium as compared with the ionic radii of indium and iron(III). Methods are developed for the interpretation of the quantitative results of the ion-exchange studies.