The Synthesis and Ion-exchange Properties of TiH2(PO4)2·1/2H2O

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Abstract
In order to develop an ion-exchanger for industrial use, a new compound, titanium dihydrogen bis (phosphate) hemihydrate, was prepared by means of a reflux between titanium phosphate and concentrated phosphoric acid; then the conditions of synthesis and the ion-exchange properties of the products were investigated. The amorphous titanium phosphate used as the starting material was formed by the reaction of titanium tetrachloride with phosphoric acid in diluted hydrochloric acid. The phase equilibrium of the TiO2–P2O5–H2O system has been studied at the refluxing temperatures of the mother liquors. The solid phase in equilibrium with the mother liquor containing 13–64% P2O5 had the components of TiO2 (29–32%), P2O5 (53–54%), and H2O (15–16%); its composition was TiH2(PO4)2·H2O, which had already been known, but the solid phase in equilibrium with the mother liquor containing more than 64% P2O5 had as components TiO2 (31–32%), P2O5 (57–58%), and H2O (10–11%); therefore, its composition corresponded to TiH2(PO4)2·1/2H2O. The X-ray diffraction pattern of this compound was entirely different from that of monohydrate, and some large d-values were recognized. From the DTA and TGA, the processes of the dehydration of monohydrate and hemihydrate were also different from each other; however, both compounds were converted to titanium pyrophosphate (TiP2O7) at 800 °C. TiH2(PO4)2·H2O can adsorb sodium ions, but the ions of a large radius, such as potassium and cecium, are not adsorbed. In contrast to this, TiH2(PO4)2·1/2H2O had an excellent ion-exchange property; its ion-exchange capacities were 4.0 meq/g to K+ and 2.0 meq/g to Cs+ at pH 4. These capacities were about eight times those of monohydrate. The solubilities of the phosphate used as an ion-exchanger were remarkably low. Therefore, the titanium dihydrogen bis (phosphate) hemihydrate thus obtained may be utilized as a useful inorganic ionexchanger for potassium and cesium ions.