Anodic voltammetry of dopamine, noradrenaline and related compounds at rotating disc electrodes of platinum and gold

Abstract

The anti-Parkinson agent dopamine, its dosage forms levodopa, methyldopa and carbidopa, and its hydroxy derivatives adrenaline and noradrenaline have been investigated by anodic rotating disc electrode voltammetry in 0.1 mol l^–1 sulphuric acid; levodopa has also been examined in perchloric acid, neutral phosphate buffer and sodium carbonate and adrenaline has been examined in buffer media at unit pH intervals in the pH range 0–11.5. All of the compounds proved amenable to rapid voltammetric determination with a mean relative standard deviation of 1.66%. All displayed a single four-electron wave at platinum electrodes, but there was evidence of two two-electron waves at gold electrodes. Apart from some non-zero intercepts in the limiting current versus square root of frequency plots, the Levich dependence was good to excellent. Electrode kinetic parameters, half-wave potentials, mass-and charge-transfer rate constants and charge-transfer coefficients have been determined. The reaction mechanism has been examined, and consists of three steps: Faradaic oxidation to the quinone, ring closure and regeneration of the catechol and Faradaic re-oxidation to the quinone, an ECE mechanism.