The effect of the buffering capacity of the supporting electrolyte on the electrochemical oxidation of dopamine and 4-methylcatechol in aqueous and nonaqueous solvents

Dopamine was electrochemically oxidized in aqueous solutions and in the organic solvents N,N-dimethyl-formamide and dimethylsulfoxide containing varying amounts of supporting electrolyte and water, to form dopamine ortho-quinone. It was found that the electrochemical oxidation mechanism in water and in organic solvents was strongly influenced by the buffering properties of the supporting electrolyte. In aqueous solutions close to pH 7, where buffers were not used, the protons released during the oxidation process were able to sufficiently change the localized pH at the electrode surface to reduce the deprotonation rate of dopamine ortho-quinone, thereby slowing the conversion into leucoaminochrome. In N,N-dimethylformamide and dimethylsulfoxide solutions, in the absence of buffers, dopamine was oxidized to dopamine ortho-quinone that survived without further reaction for several minutes at 25 °C. The voltammetric data obtained in the organic solvents were made more complicated by the presence of HCl in commercial sources of dopamine, which also underwent an oxidation process.