Pseudo-indicator behaviour of platinum electrode explored for the potentiometric estimation of non-redox systems

A pseudo-indicator electrode based potentiometric method for estimation of non-redox metal ions is presented. In the proposed method, nature and concentration specific impact of analyte over the redox potential of ideally polarisable Pt/pregenerated-redox-couple interface forms the basis of quantification. Utility of the method in estimation of six non-redox metal ions viz. Zn2+, Cu2+, Ni2+, Cd2+, Pb2+, Al3+ in the concentration range of 10−1–10−3moldm−3, individually and as binary mixtures is also presented. Three types of potentiometric behaviours, which we ascribe to the nature specific thermodynamic and kinetic aspects of metal-EDTA binding, were observed. While Cu2+, Ni2+, Pb2+ and Al3+ were found to bind EDTA efficiently, without exchanging Fe3+; Zn2+ and Cd2+ were observed to replace Fe3+ from EDTA. In contrast, Ca2+ and Mg2+ were found to show no binding affinity to EDTA in the pH range employed in the present work. The proposed method was also used to explore the reversibility and the Nernestian behaviour of ferricyanide/ferrocyanide redox couple through spectroelectrochemical titration of Zn2+ with ferrocyanide. The presented method is presaged to be a reliable and low cost future replacement for costly and delicate ion selective electrodes (ISE) in the estimation of non-redox species like Zn2+, Cu2+, etc. ► We propose pseudo-indicator model for estimation of non-redox systems. ► The shape of potential curves is a function of working pH. ► Potentiometric response is dictated by metal-EDTA stability constant data. ► Non-redox metal ions are classified on the basis of potentiometric response.