HIGH-TEMPERATURE SPECTROPHOTOMETRIC AND ELECTRON-SPIN-RESONANCE SPECTROSCOPIC INVESTIGATIONS OF VANADIUM COMPLEXES IN THE MOLTEN-SALT GAS SYSTEM V2O5-K2S2O7/SO2-SO3-N2

Electronic absorption (UV/VIS) and ESR spectra have been obtained in the temperature ranges 430-480-degrees-C and 355-478-degrees-C from V2O5-K2S2O7 molten mixtures in the composition range 0.1-0.5 mol dm-3 V2O5 in contact with SO2 and SO3 (P(SO2)/P(SO3) = 0.1-5, P(SO2) + P(SO3) = 0.1 atm, P(N2) almost-equal-to 0.9 atm). The data obtained by both spectrophotometric and ESR methods indicate that the interaction of SO2 and SO3 with the melts results in a redox equilibrium between monomeric or dimeric V(V) and monomeric V(IV) complexes. An absorption band centred at 730 nm due to the V(IV) complexes was observed and the molar absorption coefficients at 730 nm due to the V(IV) and V(V) complexes (tail of V(V) charge transfer band) were found to be epsilon(V)IV,730 = 18.91 +/- 0.08 and epsilon(V)V,730 = -2.7 + 9.48 x 10(-3) T dm3 mol 1 cm-1 in the temperature range 430 < T < 480-degrees-C. The characteristic eight-line feature of the ESR spectra with g = 1.979 +/- 0.005 and ABAR = 111 +/- 5 G confirms the presence of V(IV) in monomeric complexes, presumably [VO(SO4)2]2-, in the melts. From the temperature-dependence studies of the electronic absorption spectra the equilibrium constants and enthalpies for a number of possible equilibria have been calculated.