Hydration and water–ligand replacement in aqueous cadmium(II) sulfate solution
A Raman and infrared study
Abstract
Raman and infrared spectra of aqueous CdSO4 and (NH4)2SO4 solutions have been recorded over a broad concentration and temperature range. Whereas the ν1(SO4)2− band profile is symmetrical in (NH4)2SO4 solutions, in CdSO4 solutions a shoulder appears on the high-frequency side at 989 cm−1 , which increases in intensity with increasing concentration and temperature. The relative molar scattering coefficient of the ν1(SO42−) band is the same for all forms of sulfate in (NH4)2SO4 and CdSO4 solutions and is independent of temperature up to 109 °C. The high-frequency shoulder is attributed to the formation of a 1:1 inner-sphere complex [Cd2+(OH2)5OSO32−]. The ν3(SO42−) antisymmetric stretching mode, normally forbidden in isotropic scattering, is present in spectra in concentrated CdSO4 solutions. The bending modes, ν2(SO42−) and ν4(SO42−), forbidden in isotropic scattering, are also present. A polarized band at 240 cm−1 has been assigned to the Cd2+-OSO32− ligand vibration. Further spectroscopic evidence for contact ion pair formation at 25 °C is provided by IR spectroscopy. Higher associates or anionic complexes are not required to interpret the spectroscopic data, but at room temperature the favourable complex is an outer-sphere complex. The reaction outer-sphere/inner-sphere complex is entropically driven and above 109 °C the inner-sphere complex becomes the favourable one. The Raman spectroscopic results confirm the stepwise reaction mechanism of sulfato-complex formation in aqueous CdSO4 solution.
The degree of association has been measured as a function of concentration and temperature. At concentrations >1.0 mol kg−1 the concentration quotient, QA equal to QII, has a relatively high and constant value of 0.14 ± 0.02. The thermodynamic association constant, enthalpy and entropy for the outer-sphere/inner-sphere complex formation, have been estimated to be: KII = 3.3 ± 0.3, ΔH° = 6.2 ± 0.2 kJ mol−1 and ΔS° = 33 ± 1 JK−1 mol−1.