Some features of crystalline α-titanium hydrogenphosphate, modified sodium and n-butylammonium forms and thermodynamics of ionic exchange with K+ and Ca2+

Crystalline alpha-titanium phosphate (TPH) was modified to sodium (TPNa) and n-butylammonium-intercalated (TPBA) forms. These materials were characterized and then used as exchangers with potassium and calcium in aqueous medium by the batch method at 298.2 +/- 0.2 K. Thermodynamic determinations for all ion-exchange processes were performed by calorimetrically titrating suspensions of each exchanger in water (2.0 cm(3)) with an aqueous cation solution in a heat-flow microcalorimeter at 298.15 +/- 0.02 K. The enthalpy of the ion exchange with the general exchanger TPY can be represented as: nTPY(aq) + M-aq(n+) = (TP)(n)M-aq + nY(aq)(+) Delta(r)h, where Y = H, Na, BA and M = K+, Ca2+. In all cases, the net thermal effects were calculated after subtracting the respective thermal effect of dilution. The enthalpy of the exchange process for potassium gave the following exothermic values for all matrices: -0.54 +/- 0.01, -2.67 +/- 0.05 and -3.97 +/- 0.04 kJ mol(-1) for TPH, TPNa, TPBA, respectively. These values contrast with those obtained for calcium, which are endothermic for the same sequence of ionic exchangers: 5.82 +/- 0.24, 3.74 +/- 0.05 and 0.74 +/- 0.02 kJ mol(-1), respectively. The Gibbs free energy values indicated that the processes are spontaneous. During the ionic exchange process, the water hydration sphere of the cation promoted deorganization by increasing the molecules of solvent in the media, entropically favoring the system. (C) 1999 Elsevier Science B.V. All rights reserved.