Kinetic-fluorimetric study of the catalytic effect of manganese(II) on the air oxidation of morin

Abstract

A kinetic study of the air oxidation of morin catalysed by trace amounts of manganese(II) has been made. The effect of ammonia, ethanol, morin and manganese(II) concentrations was examined, and the partial orders with respect to these reagents and the kinetic constants were obtained. The catalysed reaction is of zero order with respect to morin (for the range 1.48 × 10^–5–2.07 × 10^–5M), 0.75 order with respect to ammonia (between 0.40 and 6.68 M) and first order with respect to manganese(II). The effect of temperature was also studied. The activation energy was calculated by applying the Arrhenius equation (linear between 20 and 40 °C). The reaction was followed by measuring the decrease in fluorescence at 548 nm, with an excitation wavelength of 415 nm. A kinetic-fluorimetric method for the determination of trace amounts of manganese(II) is proposed. Calibration graphs were obtained by applying the fixed time, fixed concentration, initial rate and rate constant methods. The method is linear up to 70 ng ml^–1 of manganese(II); the detection and quantitation limits were found to be 0.54 and 1.81 ng ml^–1, respectively. The precision was calculated for the different methods applied, and varies between 1.9%(initial rate) and 4.5%(fixed time at 5 min). Be^II, Cu^II and Zr^IV increase the catalysed reaction rate, and Mg^II, Cr^III and Fe^III decrease the reaction rate.