Binding Curve from Normalized Limiting Currents of Labile Heterogeneous Metal‐Macromolecular Systems. The Case of Cd/Humic Acid

Abstract

A set of titrations of different amounts of humic acid with cadmium are performed at pH 6.0 and fixed ionic strength (0.1 M KNO 3 ). Both reverse pulse polarograms, which minimize the electrodic adsorption effects on the limiting current, and potentiometric measurements with an ion selective electrode are recorded along the titrations. The free metal concentration is determined from the polarographic measurement by means of an iterative numerical method recently developed. The resulting binding curve is found to agree very well with the one obtained through the potentiometric data obtained along the same titration. After application of the Donnan model to correct for polyelectrolytic effects, different complexation isotherms have been used to fit the specific binding. Complexation parameters for the Cd/HA system obtained with a Langmuir-Freundlich and Gaussian isotherms are briefly discussed.

Gibbs–Donnan and specific-ion interaction theory descriptions of the effect of ionic strength on proton dissociation of alginic acid

“…(12), has been considered. This assumption is in contrast, for example, with the typical expression used in the NICA-Donnan model for humic substances [40,41]:…”

AGNES: a new electroanalytical technique for measuring free metal ion concentration

“…For its well known reversibility, Cd has been chosen. The selected ligand, nitrilotriacetic acid (NTA), is commonly used in speciation studies with Cd [14,[24][25][26][27][28]. Obtaining a very low limit of detection with AGNES is left for further work (probably with another kind of electrode), while here we describe a safe combination of the parameters so that AGNES requirements clearly apply.…”

“…Among the typically claimed advantages, a low level of detection is particularly attractive and is usually achieved by stripping techniques [11][12][13] relying on some pre-concentration step. However, a major problem in voltammetric techniques (such as anodic stripping voltammetry, stripping chronopotentiometry or pulse polarography) is the complex interpretation of the response functions in order to determine the free metal concentration [14]. So, we aim here at the design of a technique addressed to the sensing of the free ion concentration of amalgamating metals with a less complex interpretative framework.…”

Voltammetry of heterogeneous labile metal–macromolecular systems for any ligand to metal ratio: part IV. Binding curve from the polarographic waves

“…By repeating this process until consistency of the recovered binding curve, the heterogeneous effects of the ligand are fully included in the interpretative framework and the binding curve is recovered with accuracy depending only on the accuracy of the experimental measurements. Experimental application of this procedure to the Cd/humic acid system has been reported [4].…”

“…In our previous papers [1][2][3][4], the influence of the heterogeneity on the limiting currents has been analysed. From this analysis, the inverse problem, i.e., the determination of the binding curve from a titration experiment when the complex behaves as fully labile has been considered.…”