EPR study of Fremy's salt nitroxide reduction by ascorbic acid: influence of the bulk pH values

EPR and UV spectroscopy were used to investigate the efficiency of ascorbic acid in reducing Fremy's salt. Our data indicates that the first proton electron transfer from ascorbate occurs within the mixing time. Even after the disappearance of the UV signal of the ascorbate, EPR measurements showed that the reaction goes forward, indicating a biphasic redox process. The slower time-course of this second phase was related to the initial concentrations of the reductant. Experiments performed at four different pH values demonstrated that the reduction was a function of the bulk solution pH. At the lower pH, after a fast initial reduction, the Fremy's salt EPR signal remained constant, while at physiological or higher pH a further reduction was found. The reaction rates demonstrate that the reducing power of ascorbic acid towards Fremy's salt strongly depends on its dissociation state.