Voltammetric and potentiometric investigations on the complexation of Cd(II) by glycine in seawater

Abstract

Complexation of Cd(II) by glycine has been studied at the ionic strength of seawater by two independent methods, potentiometry and differential pulse polarography. Moreover, stability constants of Cd-glycine complexes have been determined in artificial seawater and in seawater from the open Pacific at the low level of 7×10 −7 M by differential pulse polarography. The influence of alkaline earth metals present in seawater and competing for the ligand has been elucidated. Fairly good agreement of the values of the stability constants obtained by the two electrometric methods ensures the accuracy of the results. The stability constants are practically the same in artificial seawater and in real seawater, indicating that all relevant complexation equilibria have been taken into account. From the measured stability constants and other complex equilibria relevant for Cd-speciation in the sea, the distribution of Cd among the various species in seawater is evaluated. It has been found that in sea water the Cd-glycine complexes will be only detectable, i.e. present at 2%, at a total glycine concentration of 4×10 −3 M. As the stability constants of the complexes of Cd with other amino acids do not differ substantially from those of glycine, the distribution presented will generally be valid for the Cd-speciation in seawater containing amino acids as the only natural organic, ligand. Thus, amino acids such as glycine cannot contribute significantly to the speciation of heavy metals in the open sea. However, for other types of natural waters containing smaller amounts of competitive ions, especially chloride ions, the limiting glycine concentrations for a detectable complexation of Cd lies much lower, at 2×10 −5 M.