Abstract
The aim of the work described in this thesis has been to further the knowledge of the solid phase, as well as the complex species in solution of the system chromium (III)-arsenic acid. A gray-violet modification of chromium (III) arsenate believed to be the hexaquo compound, was prepared by the double decomposition between hexaquochromium (III) nitrate and tertiary ammonium arsenate in aqueous alcoholic solution below 10°C. This compound was found to be unstable and reverted to an amorphous green modification polymeric in nature. This process of decomposition is discussed and an explanation has been put forward. A phase diagram study of the system at 25°C, showed a marked similarity to that for the Cr[2]O[3] - P[2]O[5] - H[2]O system at 70°C in that it yielded parallel tie lines. The solid phase was found to exhibit cation-exchange properties. The solids in the system were studied by means of thermogravimetric, magnetic, infra-red and X-Ray methods. The thermal studies were found to indicate that on dehydration, a process of olation condensation was taking place in the solid phase. The complex species in solution seemed to be exclusively cationic and aggregation of simple species was found to occur at high concentrations especially when these solutions were left in contact with the solid green modifications. The species in fresh and aged solutions were investigated by means of pH and spectrophotometric measurements, as well as ion-exchange studies involving batch techniques and chromatography, The presence of polynuclear species in the aged solutions was indicated, but characterisation was made difficult due to rapid acid attack and hydrolysis. Attempts to overcome this were unsuccessful. Structures for the solids and the species in solution have been put forward and the whole system has been discussed in terms of an extended concept of olation involving oxy-anion groups.