COMPLEX-FORMATION IN PYROSULFATE MELTS .4. DENSITY, POTENTIOMETRY, CALORIMETRY, AND CONDUCTIVITY OF THE SYSTEMS CS2S2O7-V2O5, CS2S2O7-CS2SO4, AND CS2S2O7-CS2SO4-V2O5 IN THE TEMPERATURE-RANGE 340-550-DEGREES-C

Four different methods of investigation have been applied at 340-550-d egrees-C to solutions Of V2O5 in CS2S207 or in the CS2S207-CS2SO4 molt en system. Fundamental physico-chemical data and evidence of complex f ormation in these catalytically important melts have been obtained in the temperature range of working industrial catalysts. From the potent iometric measurements the mole fractions Of CS2SO4 in molten CS2S207 s aturated with CS2SO4 at 470, 480, 490, and 500-degrees-C were found to be 0.06844, 0.07550, 0.08244, and 0.09000, respectively. From these v alues the solubility products, K(s)(T), for CS2SO4 have been obtained and DELTAH-degrees = 42.4(4), i.e. 42.4 +/- 0.4 kJ/mol, for the dissol ution process at 470-500-degrees-C could be calculated. The potentiome tric measurements of dilute solutions of V2O5, i.e. in the mole fracti on range X(v2O5) = 0.007 25-0.007 55, in the CS2S207-CS2SO4 molten sys tem at 470-500 -degrees-C, could best be explained by the reaction (VO 2)2(SO4)2S2O74- (or possibly formulated as (VO)20(SO4)44-)+ SO42- half arrow right over half arrow left (VO2)2(SO4)34- + S2O72-, where the e quilibrium constant at 470-degrees-C was found to be 46.4. Including m ore concentrated solutions Of V2O5 in CS2S207, i.e. X(V2O5) = 0-0.5, t he measured densities and the previously measured conductivities have been fitted to the general polynomial rho = SIGMA0nA(n)X(n) + SIGMA1mB (m)(t - 500)m and kappa = SIGMA0nA(n)Z(n) + SIGMA1mB(m)(t-450)m + SIGM A1(l)C(l)RX(l)(t-450), respectively. The calculated excess molar volum es and molar conductivities exhibit a large deviation from ideality. T ogether with the observed strongly exothermic enthalpy of liquid-liqui d mixing found by calorimetry, this shows that a pronounced interactio n between the components in the molten binary CS2S207-V2O5 system take s place, probably leading to the formation of dimeric V(V) complexes s uch as (VO2)2(SO4)2S2O74-, (or(VO)20(SO4)44-) as in the dilute solutio ns and polymeric complexes like (VO4SO4)n(n-).