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

Crystalline alpha-titanium(IV) hydrogenphosphate and its sodium- and n-butylammonium-intercalated forms were used as exchangers with MX2 (M = Zn or Cu; X = Cl, NO, or MeCO2) in aqueous medium at 298.2 +/- 0.2 K. An enhancement of the exchange capacity was observed for alpha-Ti(HPO4)(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 CuCl2 with the alpha-Ti(HPO4)(2) and sodium forms, and Zn(NO3)(3) with alpha-Ti(HPO4)(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(NO3)(2) and Zn(O2CMe)(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.