Studies of Complex Formation in the System Chromium(III) - Phosphoric Acid.

The aim of the work described in this thesis has been to further the existing knowledge of the chromium (lll) - phosphoric acid system in aqueous media. Because of the length of time required for the system to attain equilibrium at room temperature, the majority of the solutions investigated were aged at 70°C. Methods for deciding whether equilibrium had in fact been reached were determined. At 70°C, a phase diagram study of the system Cr[2]O[3]-P[2]O[5]-H[2]O gave a series of parallel tie-lines and no compound could be isolated as such, A preliminary investigation into the nature of the solid material indicated that it behaved as an ion exchanger. Possible structures have been put forward. The results of ion-exchange studies carried out on both saturated and unsaturated solutions of the system indicated the existence of polynuclear species. The batch studies using cation exchange resins of varying degrees of cross-linking show that there is a marked 'sieving out' effect where the cross-linking is heavy. From the chromatographic separations employing the technique of gradient elution the PO[4] : Cr ratio of a number of these complexes have been evaluated. Probable structures have been allocated to these 7 complexes. Modes of formation have also been discussed. The structure of the cationic complexes existing in solution and the nature of the solid phase in contact with a saturated solution are discussed in terms of a concept of "-olation" which is held to be capable of extension.