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

Abstract

AGNES (absence of gradients and Nernstian equilibrium stripping) is a stripping technique consisting of two conceptual steps: (i) application of a potential program (e.g. a step at a fixed potential) generating a known concentration gain between the outer and inner concentrations of the metal at the mercury electrode surface together with null gradients of the concentration profiles (inside and outside the mercury electrode) and (ii) determination of the concentration of reduced metal inside the amalgam in a stripping step. In the present implementation, the stripping step under diffusion limited conditions leads to a measured current just proportional to the free metal ion concentration. In this paper we present the basic principles of the technique, analytical expressions for a simplified model of its voltammetric implementation and a numerical study for a more refined model together with preliminary experimental results in the Cd(II)/nitrilotriacetic acid system showing how this technique can be used as an alternative to other techniques (such as ion selective electrodes) in order to determine free metal activities or concentrations in the presence of complex mixtures avoiding complications such as electrodic adsorption or complexation kinetics.

Stability of core/shell quantum dots—role of pH and small organic ligands

“…A differential pulse polarography experiment with the largest mercury drop (radius 2.03×10 −4 m, so that the planar electrode expression (Galceran et al 2004) applies), and with a modulation amplitude of 0.04995 V, was performed along with the calibration in order to obtain the E 1 value (corresponding to a desired Y) for the AGNES experiment. The potential program for Cd consists in applying two potential steps in the first stage: (1) E 1,a under reduction diffusion-limited conditions, corresponding to Y 1,a =1×10 8 for t 1,a =100-600 s (with stirring); (2) E 1,b corresponding to Y=100 for t 1,b =300-1,800 s. The waiting time (without stirring) was t w =50 s; (3) E 2 corresponding to Y 1,a =1× 10 −8 under re-oxidation diffusion-limited conditions for 50 s, with the response current being read at t 2 =0.20 s. The program Visual MINTEQ (version 2.5.3) (Allison et al 1999) was used to determine the free metal concentration along the calibration in order to calculate the h-value (a proportionality factor).…”

The use of microelectrodes with AGNES

“…Absence of gradients and nernstian equilibrium stripping (AGNES) is a recently electroanalytical technique [1] specifically designed for the determination of free metal ion concentration of amalgamating elements. One essential feature is the need to reach a special situation (called ''target'' for convenience) by the end of the deposition (or first stage), so that key parameters of this technique, such as deposition time and deposition potential have to be selected judiciously, especially if one desires to achieve the target within the minimum deposition time (or to reach the lowest limit of detection for a given deposition time).…”

Experimental verification of the metal flux enhancement in a mixture of two metal complexes: the Cd/NTA/glycine and Cd/NTA/citric acid systems

“…20 The potential program for the AGNES experiment consisted in applying three potential steps: 23 (i) E 1,a under reduction diffusion limited conditions, corresponding to Y 1,a = 1 Â 10 8 for a time t 1,a (with stirring). The suitable t 1,a depends on the desired gain Y (or Y 1,b ) applied: from previous experiments, it is known that t 1,a = 35 s for Y 1,b = 50;…”

“…19 An additional and independent measurement of the free metal concentrations can be obtained from the voltammetric stripping technique AGNES. 20 It is the aim of this paper to provide experimental evidence of this enhancement by measuring the metal flux in different systems with a mercury electrode by using SSCP. Two systems have been analyzed: Cd/NTA/glycine and Cd/NTA/citric acid, as two examples of a fixed inert complex (CdNTA) with two different labile complexes.…”