Uranyl(VI) complexes of 3-aminopropanoic acid and 4-aminobutanoic acid in aqueous solution: a potentiometric and calorimetric study

Abstract

Using potentiometric and calorimetric measurements, the stability constants and the enthalpy changes of the uranyl(VI) complexes with 3-aminopropanoic and 4-aminobutanoic acids in aqueous 1 mol dm^–3 sodium perchlorate at 25°C have been determined. The uranyl(VI) ion has been found to form three successive entropy-stabilized mononuclear complexes with both the amino acids. The values of the stability constants indicate that the interaction of the amino acids proceeds without any involvement of the amino group. In addition, due to the ionic character of these interactions, a simple equation, based on electrostatic interaction between cation and anion, is shown to reproduce well the free energy changes relative to the formation not only of the first, but also of the second and third uranyl(VI)–amino acid complexes. A comparison between the entropy and enthalpy changes for the complexation of uranyl(VI) by amino acids and some monocarboxylate anions indicates that the amino acid complexes are generally more solvated.