UV-Vis Spectrophotometric Indirect Determination of Sulfite in Vanadium Energy-Storage Medium

A self-designed SO₂-distillation apparatus was fabricated. A definite volume of the vanadium energy-storage medium was transferred quantitatively into the 250 mL distillation flask of the apparatus and water was added to make its volume to ca. 125 mL. Under the condition of passing Ar-gas through the solution at a flow-rate of 200 mL·min^-1, the distillation of SO₂ was performed by heating the distillation flask in a oil-bath at a constant temperature of 140℃ for 20 min. Before the passing of Ar-gas and introduction of sample solution, 10 mL of 2.00 g·L^-1 anhydrous lead acetate solution were added into the absorption tube of the apparatus preliminarily. During the course of distillation, SO₂ in the distillate reacts with Pb²⁺ to form PbSO₃ precipitate in the absorption tube. At the end of distillation, 20 mL of sat'd solution of iodine were added to the tube to react with PbSO₃ for 3 min, giving PbSO₄ and I₃^- quantitatively due to oxidation by iodine. After reaction for 3 min, the solution was transferred into a brown-glass 100 mL volumetric flask and after adding 200 μL conc. HCl, the solution was diluted to 100.0 mL with water, mixed thoroughly, and absorbance of the solution at 287 nm due to the presence of I₃^- was measured, and mass concentration of SO₂ was obtained indirectly on the base of the absorbance value obtained. The action of conc. HCl added to the solution was to eliminate interference of precipitate and to make the absorbance of the solution stable. Working curve was prepared with a set of solutions containing standard Na₂SO₃ solution in various concentrations and treated in the same procedure by adding Pb(Ac)₂ solution and sat'd I₂ solution in succession and etc, but without distillation. It was found that linearity range for SO₂ was found with in 3.0 mg·L^-1. Two substantial samples (A and B) were analyzed by this method giving SO₂^- contents of 1.60 mg·L^-1 and 0.49 mg·L^-1, and values of RSDs (n=10) 2.5% and 4.2% respectively. Recovery was tested by using sample A as matrix and addition of standard solution at 3 concentration levels (0.85, 1.69 and 2.54 mg·L^-1), giving results of recovery 89.4%, 93.5% and 102% respectively.