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, Q_A equal to Q_II, 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: K_II = 3.3 ± 0.3, ΔH° = 6.2 ± 0.2 kJ mol⁻¹ and ΔS° = 33 ± 1 JK⁻¹ mol⁻¹.