AGNES in irreversible systems: The indium case

Josep Galceran,Jose Paulo Pinheiro,Encarna Companys,Jaume Puy,Elise Rotureau

doi:10.1016/j.jelechem.2021.115750

Abstract

• In 3+ /In 0 irreversibility is by-passed due to the lability of In hydroxy species. • AGNES conditions are met in the Nernstian region of the foot of an SSCP wave. • They can be recognized as a linear segment when plotting ln (τ/τ*) vs E d. Free indium concentrations can be determined using AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) with no relevant hindrance from the irreversibility of the In 3+ /In° redox couple at a mercury electrode. The electroactivity, high lability and mobility of the In hydroxy species help in reaching AGNES conditions for a relatively moderate ratio (deposition time):(gain), which decreases with increasing pH due to the growing contribution of the hydroxy species. In the related technique SSCP (Scanned Stripping ChronoPotentiometry), points corresponding to more positive deposition potentials are said to be “at the foot of the wave”. Some of these points can reach equilibrium along the fixed SSCP deposition time, so that these points are effectively AGNES experiments, and free metal ion concentrations (such as those of free indium) can also be computed from them provided sufficiently accurate data are available. A new representation of the SSCP wave allows for the diagnosis of the potential region where AGNES conditions are fulfilled.

Highlights

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is an electroanalytical technique that typically quantifies the concentration of free metal ions in solution [1,2]
When –for instance- the stripped charge Q does not increase despite increasing the deposition time, it means that the deposition process has ceased due to the fulfillment of equilibrium, including the electrodic process
Free indium concentrations can be obtained from AGNES measurements, as experimentally found in systems containing NTA, oxalate, precipitated indium hydroxide, etc

Summary

Introduction

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is an electroanalytical technique that typically quantifies the concentration of free metal ions in solution [1,2]. The fundamentals of AGNES are quite similar to those of an ion selective electrode (ISE), since the main requirement is the attainment of Nernstian equilibrium at the interphase solution/electrode. In AGNES, the accumulated reduced element in the electrode has to reach Nernstian equilibrium with ions in solution by the end of the deposition stage (or first stage), which typically is of the order of hundreds of seconds, in a sufficient amount to be comfortably quantified in the stripping stage (or second stage). AGNES has been successfully implemented for divalent cations such as Zn2+, Cd2+ and Pb2+ [3,4,5,6], whose reduced forms amalgamate in a mercury electrode. AGNES has been extended beyond divalent analytes, such as antimony hydroxide [8] and the free (i.e. hexaaquo) ionic form of Indium (III) [9,10,11]

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