TITRATIONS WITH ELECTROGENERATED HALOGENS IN THE DIFFUSION LAYER OF AN INTERDIGITATED MICROELECTRODE ARRAY

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 titrat ions utilize chemical reactions proceeding quantitatively only in the close vicinity of the electrode. One set of segments of an interdigita ted array (IDA) microelectrode serves for galvanostatic anodic generat ion of titrant (iodine or bromine) and the second set, immersed in the diffusion layer of the generator, detects its unreacted flux, The det ector (collector) is potentiostated to the potential of the Limiting d iffusion current of iodine/bromine cathodic reduction, The diffusion l ayer titration curves (collector current versus generator current plot s) measured (point by point' or by slowly scanning the generator curre nt, show very good reproducibility. Since no bulk phase chemical react ion actually proceeds, the experiment can be repeated extensively in t he same solution, The sensitivity of this method is 1424 mu A l mol(-1 ) and the determination limit is 6 x 10(-7) mol l(-1) for thiosulfate determination, The substantially lower sensitivity compared with the r otating ring-disc diffusion layer titration is compensated for by the possibility of a many-fold decrease in the sample volume for IDA micro electrode diffusion layer titration, The technique was applied to the trace determination of thiosulfate in analytical-reagent grade potassi um iodide, The thiosulfate content found was slightly lower than that specified by the manufacturer; A sensitivity of 486 mu A l mol(-1) and a determination limit of 2 x 10(-5) mol l(-1) were found for the brom imetric determination of allyl alcohol, Analytically favourable titrat ion curves with negligible curvature around the end-point were obtaine d in IDA diffusion layer titrations, in contrast to the rotating ring- disc electrode measurement where considerable curvature was encountere d owing to the lower rate of the bromination of allyl alcohol. The abs ence of almost any curvature in IDA experiments is explained by a sign ificantly 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.