Dual electrode micro-channel flow cell for redox titrations: Kinetics and analysis of homogeneous ascorbic acid oxidation

Abstract

A channel flow cell, fabricated using microsterolithography (MSL), has been employed to determine the rate constant for the homogenous oxidation of ascorbic acid (AA) by an electrogenerated oxidant in aqueous solution under physiological pH conditions using a dual gold microband electrode format. Specifically, the oxidation of ferrocenylmethyl trimethyl-ammonium (FcTMA+) at an upstream electrode produce FcTMA2+, which oxidises AA in solution, yielding FcTMA+ which can undergo further heterogeneous oxidation at the upstream electrode. In addition, by setting the downstream electrode potential to a value where FcTMA2+ is detected at a transport-limited rate, collection efficiency measurements are also possible in a redox titration format. The benefits of studying homogeneous kinetics, and particularly this catalytic EC′ process, in a flow through system using a dual electrode format are highlighted. Collection efficiency measurements are shown to be particularly attractive for the detection of low levels of AA and are much more sensitive than measurements at a single electrode, particularly as a range of device parameters, such as flow rate, electrode size and electrode separation can be tuned to optimise detection. Various electrode sizes operating in the generation/collection (GC) mode have been assessed, with the optimum geometry of those studied (for the highest collection efficiency in the absence of homogenous reactions) being a small upstream generator electrode (25μM long), a large downstream collector electrode (400μM long), separated by a few tens of microns, in a channel of 200μm height and 4mm width. A full model for the cell hydrodynamics and mass transport has been developed using finite element modelling and extensive experiments are reported assessing the collection efficiency for different electrode geometries and AA concentrations. The homogeneous rate constant for the reaction between FcTMA2+ and AA is 5.5 (±1.6)×105M−1s−1.