Determination of the second-order rate constant for the reduction of dopamine quinone with ascorbic acid

The voltammetric determination of dopamine in the presence of ascorbic acid is strongly dependent on the local transport conditions because of the reduction of the product quinone with the ascorbate. Rotating ring-disk electrodes (RRDE) may be applied both to the kinetics of this system and to analytical advantage in detecting trace amounts of ascorbate. In the present work, RRDE techniques were used to determine the rate constant by which oxidized dopamine, generated at the disk, was reduced by ascorbate present in solution. This was accomplished by comparing the current generated at the ring by reduction of unreacted dopamine quinone to the amount predicted by theory for an infinitely fast reaction rate. Rate constants were obtained for ascorbate concentrations from 100 to 400 mu M and dopamine concentrations from 2 to 10 mM using three different electrodes. A second-order rate constant of 1.5 x 10(6) M-1 s(-1) was found.