2-STEP REDUCTION OF INDIGO CARMINE BY DITHIONITE - A STOPPED-FLOW STUDY

A water-soluble indigo, Indigo Carmine, has been studied in order to d etermine the dynamics and mechanism of reduction by dithionite by a st opped-flow method. Dithionite is known to reduce substrates in two for ms, SO2.- and S2O4(2-). SO2.- is a stronger reducing agent than S2O4(2 -) and is the predominant reducing species at low concentrations of di thionite, whereas at high concentrations of dithionite direct reductio n by S2O4(2-) is more predominant. The decrease in the absorption of t he dye is monitored at its absorption maximum, 610 nm. The reduction o f Indigo Carmine by dithionite takes place in two steps and direct inv olvement of S2O4(2-) is observed in both steps. Electrochemical invest igation reveals that the reduction potential of Indigo Carmine depends on the pH of solution and the variation in the rate constant with pH is attributed to a protonation reaction following charge transfer.