Thermodynamic data of ion exchange on amorphous titanium(IV) phosphate

The proton of amorphous titanium(IV) phosphate was exchanged with divalent cations (Zn, Ni, Co, Cu, Cd, Ph, and Hg) in aqueous solution at 298.15 +/- 0.02 K. For these cations, the values for the number of moles exchanged were in the sequence: Hg > Pb > Cu > Co > Ni > Cd > Zn, which is also that of the hydration volume of the cation. A suspension of the exchanger was calorimetrically titrated with a cation solution in a heat-flow microcalorimeter. After subtracting the thermal effect of dilution, the thermodynamic data for this process were calculated. All enthalpies obtained are endothermic in nature; the values were: (Zn) 13.20+/-0.35; (Hg) 6.93+/-0.21; (Cd) 5.36+/-0.21; (Ni) 3.85+/-0.09; (Cu) 3.47+/-0.10; (Cd) 3.41+/-0.07 and (Ph) 1.87+/-0.06 kJ mol(-1), which can be ordered as Zn > Hg > Cd > Ni > Cu > Co > Pb. With the exception of mercury, this sequence of enthalpy is the reverse of that obtained for the number of moles exchanged.