Spectroelectrochemical Studies of the Oxidation of Dimethyl‐p‐phenylenediamine by Reflection‐Spectroscopy at SnO2‐Electrodes

The two step oxidation of dimethyl‐p‐phenylenediamine was studied by attenuated total reflection spectroscopy (ATR) at transparent SnO2‐electrodes. The equilibrium constant for the formation of the semiquinone diimine (Wurster's Red) could be determined from the potential dependence of the ATR‐absorption as well as from differential absorption measurements (K = 13 at 3.5 < pH < 6 and K = 18 at pH < 2.5). The formation of the semioxidized species from the diamine and the diimine, which was formed by short oxidation pulses, was observed. From the time dependence of the ATR‐absorption (after 0.2 ms pulse) we obtained a rate constant of 5.2 . 105 IM−1 cm−1 at pH = 3.