Reducing Radicals in Nitrate in Solutions. The NO32- System Revisited

The one-electron reduction product of nitrate, the NO32- radical, was reinvestigated because of the contemporary interest in its reactivity in high-level liquid radioactive systems. Indirect observations suggest that the same dianion is produced by the precursor to the solvated electron. The redox potential of the couple NO3-/NO32- was calculated, using several ab initio approaches combined with semiempirical solvation models, to be Eo ∼ -1.1 V vs NHE. Using the theoretical prediction to guide the experiment, we find a value of Eo = -0.89 V at zero ionic strength. Kinetic and free-energy-relationship arguments are used to question the acid-base equilibria and reaction mode that converts the NO32- to the oxidizing NO2 radical. It is proposed that the reaction of the dianion with various general acids is an O2- (i.e., water or OH-) transfer to the acid and not proton transfer to NO32-. Implications of the highly negative redox potential of the dianion and the existence of the protonated forms to practical systems are discussed.