VISIBLE TIME-RESOLVED SPECTROELECTROCHEMISTRY - APPLICATION TO STUDY OF THE REDUCTION OF SULFUR (S-8) IN DIMETHYLFORMAMIDE

We have studied the reduction of sulfur in dimethylformamide (DMF) by time-resolved spectroelectrochemistry coupling electrochemical techniques (cyclic voltammetry and chronoamperometry) with intensified multichannel detection spectrophotometry. The measurement system allows up to 2000 spectra to be recorded in real time during the electrochemical perturbation and the derivative of absorbance to be calculated at all wavelengths of the visible range in order to compare electrochemical and spectroscopic measurements. The comparison between experimental data obtained under semi-infinite linear diffusion conditions and under thin layer conditions, between -40 and +60 degrees C, shows that S-8 is reduced to S-8(2-) and that the disproportionation of S-8(2-) is weak but leads rapidly to the formation of a low concentration of S-3(-). We show that the reduction prewave, observed at a potential just lower than that of the reduction of S-8 and assigned by various authors to acidic impurities, is well resolved under thin layer conditions and gives a high concentration of S-3(-) near the working electrode. This reduction prewave is interpreted as the reduction of S-8(2-) and/or of its monomer S-4(-).