Formation of a stable adduct between 1,2-benzoquinone and dimethylamine and its application to the spectrophotometric determination of 1,2-benzoquinone

A stable pink adduct (Absmax = 520 nm, molar absorption coefficient = 5000) is produced when dimethylamine is added to the buffer solution containing enzymatically produced 1,2-benzoquinone from catechol, and spectrophotometric analysis of the unstable 1,2-benzoquinone based on the measurement of the absorbance of this adduct has been proposed. The reaction time required for complete change of the color is about 5 min. This adduct can be reduced by reducing agents such as l-ascorbic acid or dithiothreithol and gives quasi-reversible redox waves at a carbon felt electrode. The oxidation and reduction peak potentials (vs. SCE) at pH 6.5 are −0.05 V and −0.20 V, respectively. The reduction potential of the adduct is more negative than that of 1,2-benzoquinone (+0.30 V vs. SCE) at the same pH indicating that the stability of this adduct against reduction is much stronger than that of 1,2-benzoquinone. Different from 1,2-benzoquinone, 1,4-benzoquinone does not produce a stable adduct at low concentrations of dimethylamine (< 2 mM). Selective spectrophotometric and electrochemical determinations of 1,2-benzoquinone can be easily performed by the present method. The optimum pH of the formation of the adduct is pH 6.5–7.0 and the detection limit of 1,2-benzoquinone was found to be 2 × 10−6 M.