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

Abstract

Stability constants of Zn-glycine complexes have been determined by two independent methods, i.e. potentiometry and differential pulse polarography at the ionic strength of seawater in 0.7 M NaClO 4. Moreover, these stability constants have been determined in artificial seawater and in seawater from the open Pacific at the low level of 10 −6 M Zn by differential pulse polarography. Also, the stability constants of complexes of Ca and Mg needed in the calculations have been determined potentiometrically at the ionic strength of 0.7 M. The specific influences of alkaline earth metals and of traces of other heavy metals present in seawater and competing for the ligand have been elucidated. The potentialities of both methods are compared with respect to sensitivity, accuracy and possible interferences. 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 natural seawater, indicating that all relevant complexation equilibria existing in seawater have been taken into account. From the measured stability constants of zinc-glycine complexes and stability constants of other complex equilibria relevant for Zn speciation in the sea, the distribution of Zn among the various species in seawater was evaluated. It has been concluded that in seawater the Zn-glycine complexes will only be detectable, i.e. present at 2%, at total glycine concentrations above 10 −4 M. As the stability constants of the complexes of Zn with other amino acids do not differ substantially from those of glycine, the distribution presented will be generally applicable for the Zn speciation in seawater containing amino acids as the only natural organic ligand. Bearing in mind that the concentration of total DOM in seawater is usually lower than 1 mg kg −1, amino acids like glycine cannot contribute significantly to the speciation of heavy metals in most parts of the open sea. However, for other types of natural waters containing smaller concentrations of competitive ions, especially chloride ions, the limiting glycine concentration for a detectable complexation of Zn lies much lower at 10 −6 M.