The kinetics of 1,3-dimethylbarbituric acid with some quinones, namely 1,4-
benzoquinone, 1,4-naphthoquinone and p-chloranil in 50% methyl alcohol-wate
r mixture have been investigated spectrophotometrically at 30-50 degreesC.
The reaction follows overall second-order kinetics, first order each in rea
ctant. 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 38
7.66 K. The rate of reaction increases with increasing dielectric constant
of the medium. Based on this reaction, a spectrophotometric determination m
ethod of quinones is described. Beer's law was obeyed within the concentrat
ion range 2.7-61.5 mug ml(-1) quinone. The method was applied for determina
tion of barbituric, thiobarbituric and 1,3-dimethylbarbituric acids with 1,
4-naphthoquinone within a concentration range of 3.2-39.5 mug ml(-1) barbit
urate. The reaction mechanism and reactivity have been discussed. (C) 2001
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