Studies on Redox Interaction among the Dihydroxybenzene Isomers

Abstract

Redox interaction of catechol, hydroquinone and resorcinol has been investigated in different pH (3-9) media using acetate buffer, phosphate buffer and in only supporting electrolyte containing KCl media at platinum (Pt), glassy carbon (GC) and gold (Au) electrode. The study has been carried out using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques at various scan rates and various concentrations. The results suggest that the all three electrodes exhibited an excellent electrocatalytic effect on the redox behaviors of the dihydroxybenzene isomers. Catechol and hydroquinone show one pair of redox peaks in buffer solution of pH 7 at GC and Pt electrode. But catechol and hydroquinone show two pairs of redox peaks at Au electrode at different pH. In KCl media, catechol and hydroquinone also shows two pairs of peaks at GC and Pt electrode. Resorcinol shows irreversible anodic peak at GC electrode but it shows quasi-reversible voltammogram in Pt and Au electrodes in all electrolytic media. The electrochemical process in all the isomers was controlled by diffusion process. The electrochemical behavior of catechol, resorcinol and hydroquinone in presence of sulfanilic acid has been studied by cyclic voltammetry and differential pulse voltammetry using glassy carbon (GC), gold (Au) and platinum (Pt) electrodes. Voltammetric appearance reflected that sulfanilic acid has been formed adduct with catechol and hydroquinone. But no reaction has been occurred between sulfanilic acid and resorcinol. The peak position of the redox couple is also found to be dependent upon pH. Electron transfer is most favorable in neutral media. The slopes of the peak potential, Ep vs pH plot was determined graphically as the anodic peak of dihydroxybenzene isomers or adducts at 0.1V/s at different electrode. The value of slope is close to the theoretical value (60mV/pH) for one-electron, one-proton transfer process. This indicates that the oxidation reaction of dihydroxybenzene isomers proceeded via the 1e−/1H+ process. This also suggests that during the reaction not only electron but also proton are released from the oxidation of dihydroxybenzene isomers. Among the dihydroxybenzene isomers, the electron transfer of hydroquinone is easier than the catechol and catechol is easier than the resorcinol. As -OH sites in resorcinol are connected via meta linkage, so the electron transfer at meta position is unfavorable (electron deficiency) to that at a para (hydroquinone) or ortho (catechol) –position. The interaction energy of catechol and hydroquinone in KCl media are 37.63J and 38.60J respectively. So the extent of delocalization of charge through phenyl ring of hydroquinone is higher than the catechol. Resorcinol shows only one anodic peak, -OH sites in resorcinol are connected via meta linkage, so the electron transfer at meta position is unfavorable due to the deficiency of electron. The redox interactions of the dihydroxybenzene isomers are media dependent.