The Effect of Metal Concentration on the Parameters Derived from Complexometric Titrations of Trace Elements in Seawater—A Model Study

In this study we examine the impact of dissolved metal concentrations on the parametersthat are commonly determined from complexometric titrations in seawater. We use thenon-ideal competitive adsorption (NICA) model within the framework of the chemicalspeciation program visual MINTEQ with iron as a model metal. We demonstrate thatdissolved iron concentrations effect the determined parameters for a heterogeneousbinding site distribution with a fixed concentration of dissolved organic carbon. Thecommonly used terms “ligand concentration” and “binding constant” are thereforedependent on metal concentration, so we adopt the terminology suggested by Town andFilella (2000) and use the terms ligand quotient and stability quotient here. The systematicincrease in the ligand quotient with dissolved iron concentration likely contributes towardthe trend of increasing ligand quotient with dissolved iron concentration observed infield studies, and makes it hard to assign an objective meaning to the parameter. Wesuggest that calculation of the side reaction coefficient, a parameter that describesthe probability that any added metal will be complexed, could be less prone to biasand misinterpretation than calculation of conditional stability and ligand quotients. Weexplore the impact of experimental design on side reaction coefficients by applyingdifferent detection windows, and multiwindow and reverse titration approaches. Weidentify the method that results in the best estimates of side reaction coefficients over arange of iron concentrations between 0.1 and 1.5 nmol L−1. We find that single windowtitrations can only reliably estimate side reaction coefficients over a limited range of ironconcentrations. Multiwindow titrations provided estimates of side reaction coefficientswithin the 99% confidence interval of the values calculated directly from the NICAmodel at all iron concentrations examined here. We recommend that future reportsof speciation measurements consider the potential influence of metal concentrationson the determined parameters and future studies focus on developing and applyingexperimental designs that improve the robustness and rigor of chemical speciationanalysis in the marine environment