Decoloration Mechanism of Dyes by Bleaching Agents. II.

Analysis of Reaction Kinetics for Dyes having Double Phenolic Hydroxyl Groups

Abstract

A pseudo-first-order reaction constants (k_obs) of dyes that having hydroxy groups in the molecule (Acid Orange 6, Mordant Black 17, Alizarin Red S) by various bleaching agents were measured as a function of pH. Used bleaching agents were hydrogen peroxide, peracetic acid, potassium permonosulfate, and sodium hypochlorite. The k_obs of each dye showed maximum at intermediate pH between an acid dissociation constant of bleaching agent (pK_a) and one of the acid dissociation constants of dye (pK_d1 or pK_d2). Distribution curves of k_obs on pH showed unsymmetrical distribution, because k_obs was affected by both acid dissociation constants (pK_d1and pK_d2) of dye. These results were analized with reaction kinetics by assumming the ionic reaction among protonated and deprotonated form of the bleaching agents and the dyes. It was proved that the distribution curves of k_obs on pH follow the following equation.
k_obs= [A] _t/1+ [H⁺] /K_a· (k_1a/1+K_d1/ [H⁺] K_d1K_d2/ [H⁺] ²+k_1b/1+K_d2/ [H⁺] + [H⁺] /K_d1)
where [A] _t is the total bleaching agent concentration, k_1a and k_1b are second-order reaction con-stant by assuming the reaction between protonated dye ; [DyeH₂] and deprotonated bleaching agent ; [A^-], and monodeprotonated dye ; [DyeH^-] and deprotonated bleaching agent ; [A^-]. The calculated values obtained by above equation were in good agreement with the observed values. This result demonstrated the aditivity of the reaction rate on the decoloration in each case. Therefore, it was concluded that the decoloration mechanism of the dyes by bleaching agents conforms to proceed according to the ionic reaction, and each deprotonated form of dye participate in the decoloration reaction.