Some mechanistic insights into a model bleaching process of quinones by bisulfite and dithionite: an ESR-CIDEP study

The free radical mechanisms of a model bleaching system involving quinones and bisulfite and dithionite in solutions were investigated by time-resolved ESR techniques. The major role of the radical anion, SO2.-, is established with a number of different parent ions, depending upon the environment. The reduced colorless products are mainly the hydroquinone dianions which can effectively be photoionized into semiquinone radical anions (thus the color reversal) and solvated electrons. The strongly polarized semiquinone radical anion and the solvated electron confirm the mechanisms of the photoionization. Addition of calcium carbonate to the system helps to stabilize the hydroquinone dianions by the formation of the Ca++/Q2- ion pairs and thus reduces the effects of photoreversion.