ON MECHANISM OF ION EXCHANGE IN CRYSTALLINE ZIRCONIUM PHOSPHATE .2. LITHIUM ION EXCHANGE OF ALPHA-ZIRCONIUM PHOSPHATE
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The ion-exchange behavior of -zirconium phosphate crystals titrated with lithium ion and the behavior of the lithium-exchanged crystals back-titrated with HCl have been determined. The titration curves exhibit a hysteresis loop which arises from the presence of different phases during the forward and back titrations. When lithium ion replaces protons, two phases, Zr(LiPO4)1.33(HPO4)4H2O and the unexchanged crystals, are present up to 66% of exchange. At higher levels of exchange only the latter phase is present, the excess lithium ion being held in solid solution. However, at about 80% of exchange the fully exchanged phase, Zr(LiPO4)24H2O, forms. These two phases persist, in concentrations dependent on the total lithium content of the solid, until 100% replacement of the protons, whereupon only the latter phase is obtained. When HCl is added to Zr(LiPO4)24H2O, about 5% of the lithium ions are replaced without phase change. Thereafter, a two-phase region is obtained. The two phases are the slightly protonated fully exchanged phase and Zr(LiPO4)1.33 (HPO4)0.674H2O. When greater than 33% of the Li+ has been replaced by protons, gelatinous zirconium phosphate begins to form. At the completion of the titration the solid phase is completely converted into gel. These results are incompatible with the idea proposed earlier that -zirconium phosphate contains two types of protons of differing acidity.