Kinetics of degradation of allura red, ponceau 4R and carmosine dyes with potassium ferrioxalate complex in the presence of H2O2

The degradation of title dyes with ferrioxalate and H2O2 couple has been investigated spectrophotometrically. In the absence of either one of the oxidizing agents no degradation occurred. The reaction rate was proportional to moderate concentrations of the dye and H2O2. At high concentration of the dye and H2O2 the reaction rate decreased. With regard to the concentration of the ferrioxalate complex the rate of reaction increased even over a wide range of complex concentration. Degradation of dyes does not occur in acidic medium. It is slow in neutral but thoroughly fast in alkaline medium. The reaction rate reaches a maximum value at pH 11.5. This behavior was again observed if HCl or NaOH were added. With HCl the reaction rate decreases with increasing acid concentration but greatly increases with NaOH concentration. Isopropanol showed inhibiting effect due to scavenging the in situ generated hydroxyl radical ((OH)-O-center dot). Oxalate ion enhanced the rate, confirming an outer sphere mechanism. The activation parameters of the reaction are estimated and a possible mechanism is proposed. The mechanism is well confirmed with data simulation procedure. (C) 2009 Elsevier Ltd. All rights reserved.