Abstract
The oxidation of Cu(II) complexes with tetra, penta and hexaglycine in borate buffer aqueous solution, by dissolved oxygen is strongly accelerated by sulfite. The formation of Cu(III) complexes with maximum absorbances at 250 nm (e = 9000 mol-1 L cm-1) and 365 nm (e = 7120 mol-1 L cm-1) was also characterized by using rotating ring-disk voltammetry, whose anodic and cathodic components were observed in voltammograms recorded in solutions containing Cu(II). Voltammograms, obtained at various rotation speeds, showed that the Cu(III) species electrochemically generated is not stable over the entire time window of the experiment and in solutions containing tetraglycine the overall limiting current is controlled by the kinetics of an equilibrium involving Cu(II) species.The calculated first order rate constant of the decomposition was 4.37x10-3 s-1. Electrochemical experiments carried out in Cu(II) solutions after the addition of relatively small amounts of sulfite demonstrated that the Cu(III) species formed in the chemical reaction is the same as the one collected at the ring electrode when Cu(II) is oxidized at the disk electrode in ring-disk voltammetry. The concentration of Cu(III) complexes is proportional to the amount of added sulfite and the results indicated that indirect analytical methods for sulfite may be developed by means of spectrophotometric or amperometric detection of the chemically generated product.
Highlights
The present article is a comparative study of the sulfite induced oxidation of Cu(II) complexes with tetra, penta and hexaglycine by dissolved oxygen
During rotating ring-disk electrode (RRDE) experiments, the disk electrode potential was scanned between the limits 0 and 0.6 V, the ring being maintained at 0.1 V to collect the material generated at the disk. 0.1 mol L-1 KNO3 in borate buffer was used as supporting electrolyte
Rotating ring-disk electrochemical experiments were carried out using an analytical rotator (AFMSRX) connected to an AFCBP1 bipotentiostat (Pine Instrument Company), recording current potential curves typically at a 50 mV s-1 potential scan rate with a data acquisition software made available by the manufacturer (ASWCV2, PineChem)
Summary
The present article is a comparative study of the sulfite induced oxidation of Cu(II) complexes with tetra, penta and hexaglycine by dissolved oxygen. The representations CuIIG and CuIIIG n n refer to all complexes species present in solution This reaction is catalyzed by initial traces of [CuIII(H-3G4)]- (10-7 mol L-1), with an induction period which becomes smaller by addition of strong oxidants as [CuIII(H-3G4)]- (electrochemically generated). Cu(III) formation was followed when sulfite was added to air saturated solutions of CuIIG4, G5 and G6 complexes in borate buffer in aqueous medium. These new comparative studies, using spectrophotometric and electrochemical measurements bring a new contribution to better understanding of the mechanism and development of a new alternative analytical method. The final concentration in air saturated solutions after mixing the reactants, with or without sulfite, are indicated in all figures
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