Kinetics and Mechanisms of Catalysed and Uncatalysed Oxidation of Organic Dyes by Inorganic Species and Indicator Reaction for Determination of Cations
Ngugi, Sironik Stephen
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The rate of oxidation of both the uncatalysed and Vanadium (V) catalysed reaction between indigo carmine (IC) and Chromium (VI) as well as the uncatalysed and Ruthenium (III) catalysed reaction between methylene blue (MB) and cerium (IV) have been studied. Both reactions were performed in sulphuric acid media by monitoring the absorbances of IC and MB at 610 nm and 665 nm respectively at 25°C. The kinetics and the probable mechanisms of these reactions have been elucidated. For the former reaction, the order with respect to IC, H+ and Cr(VI) were 1, 2 and 1.5 respectively and 1 for V(V) in the case of catalyzed reaction while for the latter reaction the partial orders with respect to MB, Ce(IV) and H+ were 2, 1 and 0 respectively and 1 for Ru(III) in the case for the catalysed. The mechanism for the ICCr( VI) reaction involves an initial two electron process which is confirmed by the limiting induction factor of 0.5 from the iodide ion induction reaction. Further the presence of V(V) ion in the reaction mix!ure caused an increase in the rate of the oxidation of IC that was affected by traces of Ru(III) and Mo(VI). The interfering effects of the Ru(III) and Mo(VI) were effectively masked by traces of thiourea and thiocyanate respectively without any appreciable effect on the rate of the uncatalysed reaction. The uncatalyzed and RU(III)-catalyzed reaction between MB and cerium (IV) sulphate in acidic media occurred by means of a chain reaction. The presence of free radicals in the reaction media were established by means of introducing acrylamide which formed a co-polymer with MB whose rate of formation was drastically reduced using 1,4-benzoquinone. In addition a high stoichiometry of MB:Ce(IV) 1:40 was observed for both the catalyzed and uncatalyzed reactions. The presence of Mo(VI), W(VI) and Co(III) ions was found to interfere with the rate of the indicator reaction. The interfering effect of W(VI) and Co(III) were masked by ammonium hydrogen phosphate and sodium fluoride respectively while that of Mo(VI) could be effected by extraction using 2-Methylpropanol followed by ethylacetate prior to oxidation. Further, a fixed time method is described for the determination of V(V) and Ru(III), based on their ability to catalyze the oxidation of IC by Cr(VI) and MB by ceric sulphate respectively and the interference of other cations on their determination studied using synthetic mixtures.