Novel electroanalytical methods

Abstract

This work involves the characterisation of novel electroanalytical techniques, and the combined use of these and more standard methods in studying different electrochemical systems. Ferrocene carboxylic acid (FCA) was studied extensively using cyclic voltammetry, rotating disk electrochemistry, differential pulse voltammetry, a microelectrode, UV-Vis and I.R. spectroelectrochemistry, and convolution. In chapter 2, various qualitative and quantitative studies were performed on aqueous solutions of FCA using the above techniques in an effort to explain the occurrence of an oxidation post-peak in the cyclic voltammogram. It was found that the conditions necessary for the post-peak to occur were; high pH (>9), and the presence o f oxygen. Although attempts to gain spectral characterisation of the species responsible were unsuccessful, it is proposed that the post-peak is a result of the oxidation of oxygen to ozone - a reaction which is mediated by the ferrocene carboxylate. In chapter 3, a three-electrode electrochemical cell with a platinum microelectrode of radius 5 x l0 “6m as the working electrode was used for the study o f ferrocyanide. This system was examined using both direct and differential pulse voltammetry experiments. Theory was postulated for applying a differential pulse waveform to a micro electrode and simulations were carried out and compared with the experimental results. While Cottrell behaviour was assumed for the first set of simulations, it later became necessary to modify the theory to allow for a steady state current occurring at the microelectrode after the pulse. This modified theory concurred well with the experimental results, and the sensitivity of the experiment was found to be 8.1 x 1CT7 AM' 1, while that predicted from the simulations was 8.4 x 10~7 AM '1. Concentrations as low as 5 x 10”5M [Fe(CN) 6]4 were clearly observed. In chapter 4, a ferrocyanide system was examined using both direct and differential pulse voltammetry experiments at a rotating disk electrode. Two theoretical models were postulated for applying a differential pulse waveform to a rotating disk electrode, and simulations were carried out and compared with the experimental results. It was found that Model 1 applies when pulse widths are short and/or when rotation rates are low, and conversely, Model 2 applies when pulse widths are long and/or when rotation rates are high. In this case the experimental results seemed to be governed by an overlap of both models. In chapter 5, the semidifferentiation of cyclic voltammograms for the further elucidation of electrochemical processes was explored, with ferrocyanide and FCA as the analytes. Many simulations were carried out and compared to experimental data. The simulation of a bulk species with a subsequent catalytic current and an irreversible adsorbed species seemed to fit the experimental cyclic voltammogram and semiderivative of FCA in 0.2M Na2HP0 4 /H20 best. Also in chapter 5, HPLC experiments using an electrochemical detector for the detection o f ascorbic acid were carried out. Both direct and differential currents were applied, with the direct current experiments being used as a standard technique for the characterisation of the differential current experiments. Although both techniques did yield peaks for ascorbic acid, a much larger signal was observed for the direct current experiments