KINETIC DETERMINATION OF METHOTRIMEPRAZINE, THIORIDAZINE AND THEIR MIXTURE BY USE OF A MODULAR STOPPED-FLOW DIODE-ARRAY DETECTION SYSTEM

The oxidation reactions of methotrimeprazine and thioridazine by iron( III) were used to develop kinetic methods based on measurements of the formation rate of the corresponding coloured phenothiazinyl free radi cals. The calibration graphs were linear over the range 0.8-60 mu g/ml for methotrimeprazine and 1.1-60 mu g/ml for thioridazine. The relati ve standard deviation was 1.1-3.4%. The method was used to determine t he analytes in commercially available pharmaceutical preparations. The different absorption maxima of the oxidation products formed and also the different, high initial rates of the reactions for both analytes allow their simultaneous determination by using a stopped-flow module coupled to a diode-array spectrophotometric detector. Reaction rate an d absorbance increment measurements of the kinetic curves obtained wer e used to resolve mixtures of both phenothiazines by using the proport ional-equation method and establishing three different equation system s that allowed the results obtained in each case to be compared. Mixtu res of methotrimeprazine and thioridazine in ratios between 8:1 and 1: 8 were satisfactorily resolved.