Kinetics and mechanism of the reduction of some azo-dyes by inorganic oxysulfur compounds

The kinetics of the reduction of Sunset Yellow (E-110, C.I. 15985), SY, and Ponceau-4R (E-124, C.I. 16255), PR, dyes by dithinoite and bisulfite was studied using stopped-flow and conventional UV/vis spectrophotometer under pseudo- first order conditions. The reaction followed a total one and half-order kinetics, first order with respect to dye and half-order with respect to dithionite, while with bisulfite overall second-order kinetics has been revealed. The reduction using dithionite has higher rate constant than bisulfite. With dithionite, PR has higher rate constant compared with SY, while the reverse has been observed with bisulfite. The direct involvement of SO2- radical in the reduction mechanism has been established. The formation of carbon-4 sulfonate adducts was concluded in the case of using bisulfite. Addition of beta-cyclodextrin showed an enhancement in the reduction rate of SY, while no detectable effect was noticed with PR. This increase in the rate constant is attributed to the inclusion of p-sulfophenylazo side of SY into beta-cyclodextrine cavity. Addition of cationic surfactant, CTAB, retarded the rate for both dyes due to the squeezed of dye molecules in the micellar core. The reaction was found to be an enthalpy controlled. Finally the reduction mechanism was proposed. (C) 2002 Elsevier Science Ltd. All rights reserved.