Abstract
In the past, very little work has been reported on the phosphates of the tervalent metals, and the present work was undertaken to improve the position regarding these. The phosphates of iron, chromium, and aluminium have been studied from two points of view; firstly to elucidate the type and number of compounds formed in the systems metal oxide, phosphoric oxide, and water by use of the tertiary phase-diagram, and secondly, to study the type of ionic complex present by use of ion-exchange resins. The ferric phosphate system was studied at 25°C. and a complete phase-diagram given for a range of solutions containing up to 66% of phosphoric oxide. A route by which the stable tertiary phosphate is obtained via a metastable primary salt and a compound characterised by a series of parallel tie-lines has been established end, in the more concentrated regions, the existence of a tetraphosphate of iron, Fe2P4O13,has been confirmed. On the basis of ion-exchange studies with acidic and basic exchangers, the main complex species in solution has been shown to be [Fe(HPO4)3]3-. At 0°C., chromic phosphate has been shown to give a range of purple solutions and solids in which the chromium is in the form of the hexaquochromic ion ([Cr(H2O)6]3+); a complete phase-diagram has been given for the range of solutions containing up to 52% of phosphoric oxide. The corresponding system at 40°C. has been studied and shown to be complex-containing. All solutions and solids are green and the existence of the ions [Cr(PO4)2(H2O)2]3- and [Cr(HPO4)(H2O)4]+ has been postulated on the evidence obtained from ion-exchange experiment carried out on stable solutions containing up to 25% of phosphoric oxide. The aluminum phosphate system has been studied at 25°C., both on the basis of the ternary phase-diagram and ion-exchange experiments carried out on stable solutions containing up to 20% of phosphoric oxide. On the basis of the results thus obtained, the existence of the ion [AL(HPO4)3]3- has been postulated, as has also that of a cationic complex of indeterminate composition. All three systems have been shown to exhibit a negative solubility coefficient in the more dilute regions of the system, namely that corresponding to the region of stability of the tertiary phosphate. A discussion has been given of the behavior of phosphate-groups as ligands and also of the possible formation of salts of polyphosphoric acids in the more concentrated solution.