Titrations with electrogenerated halogens in the diffusion layer of an interdigitated microelectrode array

Abstract

A technique based on diffusion layer titration was developed for the iodimetric determination of low concentrations of thiosulfate and the bromimetric determination of allyl alcohol. The diffusion layer titrations utilize chemical reactions proceeding quantitatively only in the close vicinity of the electrode. One set of segments of an interdigitated array (IDA) microelectrode serves for galvanostatic anodic generation of titrant (iodine or bromine) and the second set, immersed in the diffusion layer of the generator, detects its unreacted flux. The detector (collector) is potentiostated to the potential of the limiting diffusion current of iodine/bromine cathodic reduction. The diffusion layer titration curves (collector current versus generator current plots) measured ‘point by point’ or by slowly scanning the generator current, show very good reproducibility. Since no bulk phase chemical reaction actually proceeds, the experiment can be repeated extensively in the same solution. The sensitivity of this method is 1424 µA l mol^–1 and the determination limit is 6 × 10^–7 mol l^–1 for thiosulfate determination. The substantially lower sensitivity compared with the rotating ring-disc diffusion layer titration is compensated for by the possibility of a many-fold decrease in the sample volume for IDA microelectrode diffusion layer titration. The technique was applied to the trace determination of thiosulfate in analytical-reagent grade potassium iodide. The thiosulfate content found was slightly lower than that specified by the manufacturer. A sensitivity of 486 µA l mol^–1 and a determination limit of 2 × 10^–5 mol l^–1 were found for the bromimetric determination of allyl alcohol. Analytically favourable titration curves with negligible curvature around the end-point were obtained in IDA diffusion layer titrations, in contrast to the rotating ring-disc electrode measurement where considerable curvature was encountered owing to the lower rate of the bromination of allyl alcohol. The absence of almost any curvature in IDA experiments is explained by a significantly slower allyl alcohol flux compared with convective diffusion at a rotating electrode. The slower flux results in more time for the titration reaction to proceed close to equilibrium.