Development of a regression equation to estimate a quantifiable basic compound by the reduction prepeak measurement...

We have previously reported the electrochemical analysis that is coupled with the concept of acid-base back titration and the electrochemical sensing for acids based on the reduction of 3,5-di-tert-butyl-1,2-benzoquinone (DBBQ) for determining inorganic basic compounds and amino acids. The purpose of this study is to develop a regression equation to estimate quantifiable basic compounds by this electrochemical analysis. Through the analysis of 23 compounds with various basic and acidic groups, we found that there are some quantifiable and non-quantifiable basic compounds. For example, 4-aminophenol is quantifiable because the reduction prepeak current height of DBBQ derived from surplus HCl after neutralization with 4-aminophenol was linearly decreased with increasing 4-aminophenol concentration (r² = 0.988). In contrast, 4-aminobenzoic acid is non-quantifiable because the above voltammetric behaviors were not observed. From the relationships between the predicted pK_b and pK_a values of the quantifiable and non-quantifiable compounds, we found that a quantifiable basic compound without an acidic group satisfies pK_b < 12, and a quantifiable basic compound with an acidic group satisfies the following equation: pK_b < -0.0399(pK_a)² + 1.25pK_a + 2.12. These findings are useful to predict interfering substances in analyses of real samples with complicated matrixes and to estimate quantifiable basic compounds by this electrochemical analysis.