Solubility of vanadyl sulfate in concentrated sulfuric acid solutions

The specific energy of the vanadium redox battery is determined by the solubility of the four vanadium oxidation states in sulfuric acid. While recent studies have shown that a higher vanadium concentration than that initially proposed might be feasible, further reliable solubility data for the various vanadium ions is required if the electrolyte composition is to be properly optimized. This study describes the results of a solubility study of vanadyl sulfate in sulfuric acid. VOSO4 is the species which exists in the discharged positive half-cell of the vanadium redox cell. The solubility data have been generated in sulfuric acid concentrations that range from 0 to 9 mol/l and at temperatures between 10 and 50°C. The solubility of VOSO4 is found to decrease continuously with increasing H2SO4 concentration and decreasing temperature. At 20°C, the solubility of VOSO4 in distilled water is 3.280 mol/l whereas in 9 M H2SO4 it is 0.260 mol/l. The drop in solubility with increasing H2SO4 concentration is significant and is more pronounced at lower concentrations. A multivariable solubility prediction model has been developed as a function of temperature and total sulfate/bisulfate (SO42− and HSO4−) concentration using the extended Debye–Huckel functional form. The average absolute deviation of the predicted solubility values from experimental data is 4.5% with a maximum deviation of about 12% over the abovementioned temperature and sulfuric acid concentration range. When solubility data in the more useful H2SO4 concentration range of 3–7 M is considered, the solubility correlation improved with an average absolute deviation of only 3.0% and a maximum deviation of about 7%.