Relative Standard Electrode Potentials of I3−/I−, I2/I3−, and I2/I− Redox Couples and the Related Formation Constants of I3− in Some Pure and Mixed Dipolar Aprotic Solvents

Abstract Standard potentials of I3−/I− and I2/I3− redox couples relative to that of aqueous saturated calomel reference electrode SCE (W), ESCE(w) have been determined in propylene carbonate (PC), N,N-dimethylformamide (DMF), acetonitrile (ACN), and their binary mixtures at 25 °C from potentiometric measurements. The results helped evaluate the standard potentials of the redox couple I2/I− relative to ESCE(w) as well as the related formation constant (Kf) of I3− in these solvent systems. The solvent effects on the formation equilibrium I2+I−\ightleftharpoonsI3− were analyzed in the light of the transfer free energies of I2, ΔGt°(I2), as obtained from solubility measurements of I2 in the solvents, that of I−, ΔGt°(I−), as obtained earlier by use of the widely used TATB reference electrolyte assumption and that of I3−, ΔGt°(I3−), as obtained by subtracting these two contributions from the respective transfer free energies of formation equilibrium in the mixed solvents relative to a chosen reference solvent in each solvent system. The solvation behavior of these individual iodine species viz. I2, I−, I3− was interpreted in terms of involved interaction viz. Born, ion-dipole, ion-induced dipole, dispersion type soft–soft interactions, CTTS complexation and also the cavity effect as computed by the scaled particle theory (SPT).