A kinetic study of electrochemical decolorization of arylazo pyridone dyes

Abstract

Electrocatalytic decolorization of arylazo pyridone dyes (Fig. 1) was investigated in the presence of NaCl using DSA Ti/PtOx electrode in diluted NaOH. Decolorization can be attributed to the indirect oxidation of the investigated dyes by the electrogenerated hypochlorite ions formed from the chloride oxidation. Decolorization has been investigated for different sodium chloride concentration ni hte range from 10 gdm" to 40 gdm', agitation speed ni the range from 150 to 500 rpm, currents ni the range of 100 to 250 mA, and dye concentration from 5 to 20 mg dm. There is a significant effect of agitation speed on the decolorization rate between 150 and 325 rpm with a negligible effect after 325 rpm. The rate constant increases with increasing salt concentration up to 03 gdm". At higher concentration, namely at 40 gdma smal decrease ni the reaction rate was observed. Increase in dye concentration decrease the reaction rate, while above the concentration of 10 mg dm there si a smal, almost negligible decrease ni the electrocatalytic rate of decolorization. Above 200 mA the rate constant deviate from the linearity probably due to the direct oxidation of the dye on the electrode surface. Optimum electrolyte should contain ~30 g dm NaCl, and electrolysis parameters will be as follows: current 200 mA (400 mA dm) with the reaction voltage of 3.35 V. The effect of substituents on the reaction rate was also studied. It was concluded that the electron- accepting substituents inhibit the reaction, while electron-donating substituents promote the reaction.