Conditional stability constants of metal complexes of organic ligands in sea water: past and present, and a simple coordination chemistry model

Abstract

The conditional stability constants of complexes of organic ligands with trace metals (Cu and Zn) that have been determined by solution chemistry techniques since 1980 are summarized. The present common understanding on the chemical speciation of trace metals is that > 99% of copper dissolved in sea water is complexed with natural organic ligands; more than strongly binding organic ligand exists in sea water. A simple coordination model, including two non-specific discrete strong organic ligands such as EDTA, is introduced to understand the complex features of recent metal speciation. The model, consistent with the result of the ligand titration, permits the wide range of conditional stability constants determined by metal titrations and the present concentration levels of several trace metals in sea water to be explained. These findings suggest that organic metal complexes play a significant role in the geochemical cycle of reactive trace metals.