Stability of Alkaline‐Earth Citrate Complexes

Abstract

The stability constants of the Ca, Sr and Ba complexes with citrate ion have been determined as a function of the temperature, in the presence of 0.2 M NH,CI. There is also evidence for 1 : 2 complexes, the constants of which are reported. The stability constants for the Sr and Ba monocomplexes show a minimum at 30'C. Above this temperature, there exists a linear relationship between the enthalpy and the entropy of complex formation, the isokinetic temperature being 104 ' C . EXPERIMENTALThe radiochemical method of Schubert [ I ] was employed because for these complexes the method gives optimal accuracy and reliability, not attainable by other known methods.Reagent grade chemicals, not otherwise purified, were used throughout the work. The tracers were checked for radiochemical purity by gamma ray spectroscopy, except for which is a pure 1 3 emitter. They were furnished by the Belgian Nuclear Centre, Mol.The cation exchanger, Dowex 50, X12, 200-400 mesh, was converted t o the ammonium form, washed and dried in air. Assay solutions were prepared by equilibrating 0.2000+0.0005 g resin with 50.00 ml aqueous solution, containing tracer and ligand, during about five hours in a constant temperature bath. Longer periods did