Crystalline α-titanium(IV) hydrogenphosphate and its sodium- and n-butylammonium-intercalated forms: ion exchange and thermochemistry

Abstract

Crystalline α-titanium(IV) hydrogenphosphate and its sodium- and n-butylammonium-intercalated forms were used as exchangers with MX ₂ (M = Zn or Cu; X = Cl, NO ₃ or MeCO ₂ ) in aqueous medium at 298.2 ± 0.2 K. An enhancement of the exchange capacity was observed for α-Ti(HPO ₄ ) ₂ and its sodium form when acetate was used as the counter ion for both zinc and copper cation solutions. Thermodynamic determinations for all ion-exchange processes were performed by titrating calorimetrically a suspension of a given exchanger in water (2.0 cm ³ ) with an aqueous cation solution in a heat-flow microcalorimeter at 298.15 ± 0.01 K. In all cases the net thermal effects were calculated after subtracting the corresponding dilution thermal effects. With the exception of exothermic ion-exchange processes involving CuCl ₂ with the α-Ti(HPO ₄ ) ₂ and sodium forms, and Zn(NO ₃ ) ₂ with α-Ti(HPO ₄ ) ₂ , which gave -2.40 ± 0.12, -1.01 ± 0.01 and -12.26 ± 0.15 kJ mol ^-1 , respectively, all other exchange processes were endothermic in nature with low enthalpy values, varying from 0.010 ± 0.001 to 8.59 ± 0.14 kJ mol ^-1 for the Zn(NO ₃ ) ₂ and Zn(O ₂ CMe) ₂ systems, with the sodium form. The number of moles exchanged did not correlate with the corresponding enthalpies. However, the thermodynamic data are in agreement with the occurrence of a spontaneous and entropically favourable ion-exchange process for all systems considered.