Thermodynamic considerations in co-ordination. Part VIII. A calorimetric and potentiometric study of complex formation between some lanthanide(III) ions and histidine
The changes in free energy, enthalpy and entropy for the formation of tripositive lanthanum, praeseodymium, neodymium, samarium, gadolinium, dysprosium, erbium, and ytterbium complexes with histidine (histH) have been determined at 25·0° and l= 300 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 ΔGn°=ΔHn°–TΔSn°. The calorimeter described for the measurement of enthalpies has a built-in electrode pair so that potentiometry and heat measurements may be made simultaneously. The experimental results for all the lanthanide complexes are interpreted in terms of the three complexes: [Ln(hist)]2+, [Ln(hist)2]+, and [Ln(histH)]3+.