Kinetics of the Reduction of p–Phenylazobenzenesulphonic Acid by Sodium Dithionite in Alkaline Solution

The reduction of p–phenylazobenzenesulphonic acid with excess of sodium dithionite in alkaline solutions 0–006–0030 M in dithionite ion is first order with respect to p–phenylazobenzenesulphonate ion concentration and one–half order with respect to dithionite ion concentration. The SO2 radical ion is a reaction intermediate in mechanisms proposed for the reduction.