Na₂S₂O₃-Titrimetric Determination of Selenium in Crude Selenium with Preseparation by Reduction

Na₂S₂O₃ titrimetry with preseparation of selenium by reduction was applied to the determination of Se in crude selenium. The sample (0.100 0 g) was dissolved in HNO₃ and fumed with 3 mL of H₂SO₄ until the SO₃ fume just evolve. The salts were dissolved by heating with 100 mL of HCl (1+1) and 1 to 2 g of tartaric acid. Hydroxylamine hydrochloride (4 to 6 g) was added to the solution, which was preserved at 80 to 90℃ for 1.5 to 2 h to reduce Se to its elementary state. The mixture was filtered and the elementary Se was dissolved by heating with 10 mL of HCl and 0.5 mL of HNO₃, and then 100 mL of H₂O and 2 g of urea were added, and the solution was boiled for 3 min. After cooling, selenium was determined by titration with Na₂S₂O₃ standard solution in the presence of KI using starch solution as indicator. Test for precision was made by analyzing 2 samples of crude selenium, giving values of RSD's (n=7) of 0.22% and 0.23%. Test for recovery was made by standard addition method with 2 crude selenium samples as matrix, values of recovery found were 99.8% and 100%.