A POLAROGRAPHIC, VOLTAMMETRIC AND SPECTROSCOPIC STUDY OF 2-MERCAPTONICOTINIC ACID AND ITS CHROMIUM(III) COMPLEX

The redox behaviour of the ligand 2-mercaptonicotinic acid and its chromium(III) complex has been studied in dimethyl sulphoxide. Oxidation of this compound to the corresponding disulphide at +0.90 V vs. SCE is displaced at +0.20 V vs. SCE and -0.05 V vs. SCE upon addition of one and two equivalents of base respectively. The addition of chromium(III) to the ligand, both in its neutral and mono-anionic form, does not produce any change in the spectrum or in the cyclic voltammograms, which indicates that there is very weak or no interaction from chromium. If the proton mercapto group is neutralized, a 1:3 complex is formed and the dimerization does not occur. This complex has been characterized by polarography, cyclic voltammetry, controlled-potential electrolysis, UV-visible spectrometry and magnetic susceptibility measurements. The conditional formation constant was determined by polarographic and spectroscopic methods, giving the values of 2.5 x 10(14) and 1.5 x 10(14) (25 degrees C), respectively.