Thorium(IV) complex formation with benzoic, phenylacetic and 3-phenylpropionic acids in aqueous solution.

The stability and the heat of formation of the 1:1 complexes of thorium(IV) with benzoate, phenylacetate and 3-phenylpropionate have been determined in 1.0 mol dm(-3) aqueous solution of Na(ClO4) at 25 degrees C by means of potentiometric and calorimetric measurements. The stability of the 1:1 complexes formed by phenylacetate and 3-phenylpropionate with thorium(IV) is in agreement with the general linear correlation between the logarithm of the formation constant and the pK(a) of the ligand acid, while the stability of the benzoate complex is higher than that expected on the basis of such a relationship. The results are discussed in terms of the different solvation of the species in solution