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

A water-soluble indigo, Indigo Carmine, has been studied in order to determine the dynamics and mechanism of reduction by dithionite by a stopped-flow method. Dithionite is known to reduce substrates in two forms, SO2.- and S2O4(2-). SO2.- is a stronger reducing agent than S2O4(2-) and is the predominant reducing species at low concentrations of dithionite, whereas at high concentrations of dithionite direct reduction by S2O4(2-) is more predominant. The decrease in the absorption of the dye is monitored at its absorption maximum, 610 nm. The reduction of Indigo Carmine by dithionite takes place in two steps and direct involvement of S2O4(2-) is observed in both steps. Electrochemical investigation 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.