Electrostatic and non-electrostatic thermodynamic quantities of reaction of bivalent metal ions with ethylenediaminetetraacetate

Abstract

The thermodynamic functions for the formation of bivalent metal complexes with ethylenediaminetetra-acetate in aqueous solution have been analyzed in terms of electrostatic (el) and nonelectrostatic (non) parts. The rather smaller exothermic reactions than expected from the high stability of the complexes have been found to arise because the exothermic ΔH non value is compensated by the endothermic ΔH el value. The ΔH non increases linearly with a measure of softness of metal ion introduced by Glopman. The correlation has been pointed out between the ΔH el and the heat of hydration of the corresponding metal ion. The average interionic distance of metal and charged donors in the complex molecule calculated from the ΔG el value was compared with the effective ionic radius of metal ion on hydration and with the crystal ionic radius. This comparison suggested similarity in the complex structures throughout the metal series studied.