Crystalline insoluble acid salts of tetravalent metals—XXXIII: Investigations on the unusual initial shape of the titration curves of layered exchangers of the α-zirconium phosphate type

Abstract

Several titration curves of crystalline acid salts of tetravalent metals show an evident decrease in the pH of the supernatant solution with an increasing addition of metal hydroxide. This phenomenon, very unusual for common organic ion-exchangers, seems to be quite general for inorganic ion-exchangers with layered structure of α-type. In order to throw light on this phenomenon, a detailed investigation was carried out on the titration curves of α- Zr(HPO 4) 2·H 2O with various metal hydroxides, particularly KOH. To obtain the presence of a distinct minimum in the titration curve, three conditions seem to be necessary: (1) high activation energy for H + M z+ exchange (which, in turn, depends on the relative size of M Z+ and size of the windows connecting the cavities (2) formation of solid solution having high M-content and (3) formation of a phase, with a large inter-layer distance, in the external parts of the crystals. This last point is particularly important since the enlargement of the external part of the crystals lowers the activation energy for the exchange of large cations. Thus, once started, the exchange can take place at lower pH′ values.