MECHANISM OF ION-EXCHANGE IN ZIRCONIUM-PHOSPHATES .18. EFFECT OF CRYSTALLINITY UPON K+-H+ EXCHANGE OF ALPHA-ZIRCONIUM PHOSPHATE
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abstract
K+H+ ion exchange isotherms for -zirconium phosphates of widely varying crystallinity were determined. The least crystalline exchanger formed a single solid solution requiring a steady increase in solution pH with increased K+ uptake. However, with exchangers of intermediate crystallinity replacement of hydrogen ion by potassium ion occurs in two stages. Initially, some K+ goes into solid solution but when the solubility limit is reached a half exchanged phase of approximate composition ZrKH(PO4)2H2O forms. Actually this latter phase can exist over a range of potassium ion content ( 3 - 4 mequiv/g). The foregoing exchange process occurs at pH 3 and covers the first 50% of exchange. At pH 7 the remaining hydrogen is exchanged with formation of a phase of ideal composition Zr(KPO4)23H2O. With highly crystalline -zirconium phosphate the phases formed as a result of exchange were quite similar to those described for exchangers of intermediate crystallinities but equilibrium is approached very slowly. Rate measurements indicated particle diffusion controlled reactions with interdiffusion coefficients of 7.6 10-13 and 4.9 10-13 cm2sec-1 for the two exchange processes. These rates must be followed by an additional much slower reaction, such as a recrystallization process, to account for the observed time required to achieve equilibrium. 1978.
