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

Abstract

In this study we examine the impact of dissolved metal concentrations on the parameters that are commonly determined from complexometric titrations in seawater. We use the non-ideal competitive adsorption (NICA) model within the framework of the chemical speciation program visual MINTEQ with iron as a model metal. We demonstrate that dissolved iron concentrations effect the determined parameters for a heterogeneous binding site distribution with a fixed concentration of dissolved organic carbon. The commonly used terms "ligand concentration" and "binding constant" are therefore dependent on metal concentration, so we adopt the terminology suggested by Town and Filella (2000) and use the terms ligand quotient and stability quotient here. The systematic increase in the ligand quotient with dissolved iron concentration likely contributes toward the trend of increasing ligand quotient with dissolved iron concentration observed in field studies, and makes it hard to assign an objective meaning to the parameter. We suggest that calculation of the side reaction coefficient, a parameter that describes the probability that any added metal will be complexed, could be less prone to bias and misinterpretation than calculation of conditional stability and ligand quotients. We explore the impact of experimental design on side reaction coefficients by applying different detection windows, and multiwindow and reverse titration approaches. We identify the method that results in the best estimates of side reaction coefficients over a range of iron concentrations between 0.1 and 1.5 nmol L −1 . We find that single window titrations can only reliably estimate side reaction coefficients over a limited range of iron concentrations. Multiwindow titrations provided estimates of side reaction coefficients within the 99% confidence interval of the values calculated directly from the NICA model at all iron concentrations examined here. We recommend that future reports of speciation measurements consider the potential influence of metal concentrations on the determined parameters and future studies focus on developing and applying experimental designs that improve the robustness and rigor of chemical speciation analysis in the marine environment.

Fe‐Binding Organic Ligands in the Humic‐Rich TransPolar Drift in the Surface Arctic Ocean Using Multiple Voltammetric Methods

“…In the Barents Sea, logK′ Fe′L using TAC is similar to the other subsets at 12.1 ± 0.2, whereas here the SA method results in a lower logK′ Fe′L of 11.2 ± 0.2. Figure shows the relation between the TAC and SA methods in terms of log α ′ Fe′L , and excess L. Log α ′ Fe′L is less prone to bias and therefore a good parameter for comparison (Gledhill & Gerringa, ). For the SA method log α ′ Fe′L is invariably near 3; for the TAC method log α ′ Fe′L is near 3 outside the TPD; inside the TPD values decrease to below 1 (Table and Figure a).…”

The Size Fractionation and Speciation of Iron in the Longqi Hydrothermal Plumes on the Southwest Indian Ridge

“…In addition, besides the employed detection windows, the dissolved Fe concentrations present in the water samples can influence the logK′ FeL and [L] (Gledhill & Gerringa, ; Town & Filella, ). The increasing [DFe] results in increased [L] and decreased logK′ FeL , which is associated with the fact that the stronger ligands are complexed first at low concentrations of DFe, and the comparatively weaker ligands will be available for complexation at high [DFe] (Gledhill & Gerringa, ; Town & Filella, ). This might be another interpretation for the high [L] and comparatively lower K′FeL in this study and others (Hawkes, Connelly, et al, ; Kleint et al, ).…”

Perspective on identifying and characterizing the processes controlling iron speciation and residence time at the atmosphere-ocean interface

“…These ligands could contribute to the stabilization of aerosol DFe in seawater, though strong ligands in seawater are usually assumed to have an oceanic origin (Hunter and Boyd, 2007). Electrochemical titration studies, used for the operational definition of the ligand concentrations, show that strong ligand concentrations are typically low in the open ocean (~0.44 nM or lower; concentrations of true siderophores as determined by LC-MS can even be at the 1 pM level, Mawji et al, 2008), though weaker ligand concentrations are slightly higher (~1.5 nM) (Gledhill and van den Berg, 1994;Rue and Bruland, 1995;Vraspir and Butler, 2009;Gledhill and Gerringa, 2017). Thus, atmospherically delivered DFe deposition events at the surface of the ocean can quickly deplete the number of available "excess" L 1 and L 2 ligands.…”