Spectrophotometric kinetic and determination of quinones and barbiturates

The kinetics of 1,3-dimethylbarbituric acid with some quinones, namely 1,4-benzoquinone, 1,4-naphthoquinone and p-chloranil in 50% methyl alcohol-water mixture have been investigated spectrophotometrically at 30-50 degrees C. The reaction follows overall second-order kinetics, first order each in reactant. From the dependence of the rate constants on temperature, activation parameters have been calculated. A plot of deltaH# versus deltaS# for the reaction gave a good straight line with an isokinetic temperature of 387.66 K. The rate of reaction increases with increasing dielectric constant of the medium. Based on this reaction, a spectrophotometric determination method of quinones is described. Beer's law was obeyed within the concentration range 2.7-61.5 microg ml(-1) quinone. The method was applied for determination of barbituric, thiobarbituric and 1,3-dimethylbarbituric acids with 1,4-naphthoquinone within a concentration range of 3.2-39.5 microg ml(-1) barbiturate. The reaction mechanism and reactivity have been discussed.