Raman- and infrared spectroscopic investigation of aqueous ZnSO4 solutions from 8°C to 165°C:: Inner- and outer-sphere complexes

Raman and infrared spectra for aqueous ZnSO4 and (NH4)(2)SO4 solutions have been recorded over broad concentration and temperature ranges. Whereas the v(1)(a(1))-SO42- band profile is symmetrical in (NH4)(2)SO4 solutions, in ZnSO4 solutions a shoulder appears on the high frequency side which increases in intensity with increasing concentration and temperature. The molar scattering coefficient of the v(1)(a(1))-SO42- band is the same for all forms of sulfate in (NH4)(2)SO4 and ZnSO4 solutions and is independent of temperature up to 165 degrees C. The high frequency shoulder is attributed to the formation of a 1.1 contact ion pair [Zn2+OSO32-]. Also the v(3)(f(2))-SO42- antisymmetric stretching mode shows a splitting in the ZnSO4 solution. Additionally, the bending modes, v(2)(e)-SO42- and v(4)(f(2))-SO42- normally forbidden in the isotropic spectrum show with increasing ion pair formation a gain in intensity. A polarized band at 275 cm(-1) has been assigned to the Zn2+-OSO32- Ligand vibration. Further spectroscopic evidence for contact ion pair formation at 25 degrees C is provided by i.r. spectroscopy. No higher associates or anionic complexes are required to interpret the spectroscopic data. In concentrated ZnSO4 the majority of the sulfate is occurring in the outer-sphere. With temperature increase, the amount of inner-sphere complex is increasing (entropically driven reaction). The degree of association has been measured as a function of concentration and temperature. At concentrations above 0.8 mol kg(-1) the concentration quotient, Q(A), has a relatively high and constant value of 0.06+/-0.02. The thermodynamic association constant and the enthalpy of formation for the contact ion pair are estimated to be K-A = 30+/-8, Delta H-O = (12+/-2) kJ . mol(-1) and Delta S-O = (70+/-10) J . K-1 . mol(-1).