Thermodynamics of Mn(II) and La(III) sulphate complexes in aqueous solutions

Abstract

Data of ultrasound velocity dispersion are interpreted as being due to the process of association between the cation Mn²⁺ or La³⁺ and anion SO₄^2-. The thermodynamic parameters for the formation of inner sphere complexes have been calculated on the basis of the numerical values of relaxation frequencies and of the ultrasound velocity dispersion at different temperatures. The activation energy Ea of the backward reaction of complexation, e.g. the release of the ligand from the first coordination sphere of the cation in MnSO₄ solutions is larger than Ea in La₂(SO₄)₃ solutions. It may be explained by the difference in the coordinated numbers of the cation. The positive value in the internal energy ?H₀ in MnSO₄ solution is interpreted in terms of the hydrational changes during the association process and negative value ?H₀ for the [LaSO₄]_aq⁺¹ complexes is proposed that the lanthanum-ligand interaction is dominant factor. The amount of inner sphere complexes in MnSO₄ solution is very insignificant occurring only at high concentrations. In the rate determining reaction for complex formation different mechanisms for individual cations are proposed, e.g. the associative interchange Ia for Mn(II) and the dissociative interchange I_d for La(III) sulphates.