Copper complexing ability of strong ligands isolated from seawater is investigated by means of thermodynamic calculations using the experimental results of two different measurements. Model parameters are derived from published data for copper complexation of natural ligands in seawater. Analyses by the model in the present study indicated that the strong ligand has been maintained quantitatively in the form of the complex with copper during the course of both experiments and that its copper complex is remarkably stable. The tentative calculation based on the results of both experiments suggests that the values for conditional stability constants are in the ranges of 1014.9-1016.4 at an ionic strength lower than 10-5M, pH5.71, and a temperature of 4°C.
The kinetics of competitive effects of side reactions for the complexation reaction between copper and the strong organic ligand isolated from seawater is examined with regard to other metals that were possibly present in the equilibrium systems and also a chelating agent, EDTA (ethylenediaminetetraacetic acid) that was added exogenously in the procedure developed by Midorikawa and Tanoue (1994). The kinetic problems in the ligand-exchange reaction of the strong ligand with EDTA is negligible because of quantitative removal of the alkaline earth metal ions in the pretreatment of seawater samples. It is found that the assumption for the concentration of copper-EDTA complex in the sample, introduced by Midorikawa and Tanoue (1994) in their equilibrium model, may lead to a conservative estimate of the conditional stability constant of the strong ligand, taking into account the kinetic reactivity of the strong ligand for copper complexation. When trace metals other than copper are present, the model calculation demonstrates that these metal-competition effects involving the side reactions of EDTA finally lead to an overestimation of the conditional stability constant for the strong ligand. The magnitude of the overestimation is as small as the errors in its estimation based on the uncertainties of measurements. Possible metal competitions for the strong ligand may result in the conservative estimate of the total concentration of the strong ligand and would have no serious influence on the estimation of its conditional stability constant.
The systems of natural waters are too complicated to investigate low concentrations of strong ligands in their media. It is important to establish experimental conditions where un-biased measurements of intrinsic complexing properties of strong ligands can be performed, in due consideration of the aspects of both thermodynamics and kinetics on the nature peculiar to strong ligands. The accumulation of information on properties and characteristics of strong ligands in the simpler systems should be very helpful in developing the model and estimating metal speciation in more complex systems of natural waters. Our methodology involves the processes of concentration and desalting of natural ligands in sample waters, and of the removal of metals bound to natural ligands. These processes are especially effective in detecting the low levels of strong ligands in sample waters which contain the high levels of trace metals. The application of this approach to the actual natural waters allows us to compare the complexing properties of natural ligands between samples from various sources with each other and well-known synthetic complexing reagents on the same basis without serious influence of the enormous complexity of the equilibrium systems in natural waters. The results obtained by our method from different marine environments indicated that three classes of natural ligands were commonly detected and the sources of these ligands would be of marine origin. The horizontal and vertical characteristics of these ligands suggested their refractory nature in the marine environments.