Determination of biologically active acids based on the electrochemical reduction of quinone in acetonitrile plus water mixed solvent

The reduction peak of quinone in aprotic media appears at a positively shif ted potential in the presence of acids, which play a role as proton sources . Since the height of the newly developed peak shows a good correlation wit h the concentration of the acid, it can be utilized to determine the concen tration of weak acids. Although the magnitude of the peak potential shift ( DeltaE(p)) decreases as the water content in the solution increases, the pr esence of a Bronsted acid still leads to a significantly large DeltaE(p) in an acetonitrile + water mixture (9/1, v/v) when a proper quinone derivativ e is chosen. DeltaE(p) depends on not only the acidity of proton sources bu t also the basicity in quinone derivatives. Among the various quinones exam ined in the present study, tetramethyl-1,4-benzoquinone (duroquinone) exhib its the largest DeltaE(p) and is found to be a suitable redox-active additi ve for the selective analysis of a variety of organic acids. Based on these results, conventional voltammetric techniques, e.g, differential pulse vol tammetry, square-wave voltammetry and linear sweep voltammetry, can be empl oyed to determine various biologically important acids with protonated amin e or carboxylate groups. The quantitative analyses of histamine, maleic aci d and pyruvic acid are successfully demonstrated in the 10(-5) M range with a high sensitivity of 17 similar to 19 muA mM(-1) by linear sweep voltamme try. (C) 2001 Elsevier Science B.V. All rights reserved.