Determination of Zn2+ concentration with AGNES using different strategies to reduce the deposition time

Abstract

Absence of gradient and Nernstian equilibrium stripping (AGNES) senses the free ion concentration of Zn(II) in solutions containing different ligands, being unaffected by the lack of reversibility of the Zn 2+ /Zn 0 couple under the conditions assayed. In the presence of oxalate, the determination of [Zn 2+ ] agrees with the stability and solubility constants of this sparingly soluble salt, once the precipitation kinetics are taken into account. Different strategies have been analysed and implemented in order to reduce the preconcentration time with the standard electrode of the polarographic stand (smallest drop radius around 0.141 mm): (i) using a lower preconcentration factor when there is no need of enhanced limit of detection; (ii) splitting the deposition stage into two, with a first potential step under diffusion limited conditions; (iii) the analysis of the chronoamperometric response in the deposition stage allows its duration to be adjusted, especially if non-inert complexes contribute to the arriving flux of metal to the mercury electrode. The two-potential-steps strategy is assessed as the most suitable in a general case.

The use of microelectrodes with AGNES

“…6, we can see that -for a fixed Y-the current initially increases with increasing deposition time up to a practically constant I-value, which indicates us the minimum time needed for accepting that AGNES conditions have been achieved. For example, using Y = 5000, we achieve AGNES conditions with the microelectrode in around 400 s. This deposition time is clearly less than the one required by the HMDE (r 0 = 1.41 · 10 À6 m) to reach the same Y, which can be estimated, from extrapolation of values reported in experiments previously published [2]: Y = 50 usually requires t 1 -t w = 350 s, so Y = 5000, given the proportionality between Y and t 1 -t w seen in Eq. (4), would require t 1 -t w = 35,000 s to which we have to add the standard resting time t w = 50 s, to finally reach 35,050 s. This reduction (from 35,050 s to 400 s) is consistent with the theoretical reduction factor of around 95 (see Section 2).…”

“…2 shows the potential program of the simplest AGNES experiment (E, referred to the right axis, versus t) depicted as a thick solid line in the plot, together with the measured current (I, referred to the left axis versus t). A strategy (called ''2 pulses'' or ''2P'') developed to reduce the time of the experiment [2] (and applied here with HMDE) consists in splitting the first deposition stage into two substages, with diffusion limited conditions for the deposition along the first substage. The subtraction of the blank current is required to obtain the faradaic current.…”

“…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). In previous works [2,3], we have developed different strategies to reduce the deposition time such as the use of a lower gain factor Y (controlled via the deposition potential) or the use of two potential steps along the deposition stage.…”

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

“…The SSCP (Pinheiro and van Leeuwen 2004;Domingos et al 2007Domingos et al , 2008 and AGNES techniques (Companys et al 2005;Domingos et al 2008) are described in the Supporting information. SSCP and AGNES experiments were performed using an Eco Chemie μAutolab III potentiostat in conjunction with a Metrohm 663VA stand and a personal computer using the GPES 4.9 software (Eco Chemie).…”

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;…”