Two-step reduction of indigo carmine by dithionite: a stopped-flow study

Abstract

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.