Theory and practice of electrochemical titrations with dual microband electrodes

Dual parallel microband electrodes, operated as a generator–collector pair and made by a simple, inexpensive mass-production method, have been used to implement a ‘titration’ method. The solution contains the electro-inactive analyte and a reagent from which the titrant can be generated electrochemically. The galvanostatically generated titrant is detected at the collector amperometrically. The collection efficiency is affected by the reaction between the titrant and the analyte. Determination of ascorbic acid by titration against ferricyanide is given as an example. The measurement is performed as follows: After application of the collector potential, the boundary conditions between measurements are renewed by a quick pulsed motion of the electrode assembly. Then the generator current is applied. Following an initial delay, the collector current increases as t1/2, with slope and collection efficiency dependent on analyte concentration. This results in a fast and effective method for implementing some standard titration methods without the need for accurate volume measurement and reagent preparation. The accuracy is determined by the reproducibility of electrodes. The present work shows concentration measurements in the mM scale to ≈±10%, in a time of a few seconds. The highly stable extrapolation method used for numerical simulation of the generator–collector experiment, which takes into account the non-uniform current distribution over a microband electrode with a galvanostatic boundary condition, is developed using a conformal map. A good agreement between simulations and experimental results was obtained in voltammetric, potential step and generator–collector measurements. It is shown that a useful approximate calculation can be made rather easily by representing the problem in terms of a reaction layer in the conformal space.