The sodium ion-hydrogen ion, hydrogen ion-sodium ion exchange titration curves of «-zirconium phosphate crystals, ( 04)2• 20, exhibit a hysteresis loop. This results from the presence of different phases in the forward and backward titrations. When the crystals are titrated with sodium hydroxide two solid phases are present up to 50% of exchange. They are the unexchanged «-zirconium phosphate and a half-exchanged phase whose formula is ZrCNaPChXHPCh) -5H20. This latter phase is the only one present at exactly 50% of exchange but at higher levels of exchange another two-phase region is obtained. The two phases are the half-exchanged crystals and a fully exchanged phase of composition Zr(NaP04)2.3H20. The interconversion of the half and fully exchanged phases is reversible. However, when the half-exchanged phase takes up protons it forms an unexchanged phase which is more highly hydrated than the original «-zirconium phosphate. Dehydration of the exchanged phases leads to the formation of several new phases. The phase changes during exchange and dehydration are explained on the basis of the swelling and contraction of the layers in the «-zirconium phosphate crystals.

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