Thermodynamic considerations in co-ordination. Part IX. Heat capacity investigations into complex formation between some lanthanide(III) ions and histidine

Abstract

The changes in free energy, enthalpy, and entropy for the formation of tripositive lanthanum, praseodymium, neodymium, samarium, gadolinium, dysprosium, erbium, and ytterbium complexes with histidine (histH) have been determined at 37·0 °C and I= 3·00 mol l^–1(with sodium perchlorate). The formation constants, and thus the free-energy changes, were measured potentiometrically, and the enthalpy changes were measured by calorimetry. The entropy changes were calculated from the relationship ΔG_n°=ΔH_n°–TΔS_n°. The experimental results for the lanthanides studied are interpreted in terms of the formation of three complexes: [Ln(hist)]²⁺, [Ln(hist)₂]⁺, and [Ln(histH)]³⁺. By combining data at 25 °C and at 37 °C, heat capacity changes (ΔC_p) for the complexing reactions have been calculated at the mean temperature of 31 °C. The trends reported in ΔC_p data as one crosses the lanthanide series could be explained by changes in the hydration numbers of the lanthanide ions.