Determination of Chromium in Beer Yeast by Color-Fading Spectrophotometry with Wright's Stain

The beer yeast sample (0.200 0 g) was digested with HNO₃ and H₂O₂ in microwave digestor. The clear solution obtained was evaporated to near dryness. The residue was taken up with water and its acidity was adjusted to pH 12. Solution of KMnO₄ was added dropwise to oxide Cr(Ⅲ) to Cr(Ⅵ) and excess of KMnO₄ was de-colorized by addition of few drops of C₂H₅OH. The solution was then neutralized by dilute HNO₃, filtered and its volume was made up to 50 mL. 2.0 mL of this sample solution were transfered to a 25 mL volumetric flask, and 2 mL of 50 g·L^-1 thiourea solution, 1.0 mL of 5 mol·L^-1 H₃PO₄ solution, 4.0 mL of 0.2 mol·L^-1 KI solution, 3.5 mL of 10 g·L^-1 PVA solution and 4.0 mL of 0.5 g·L^-1 Wright's stain solution were added in succession, and its volume was made up to 25 mL with water with thorough mixing. Simultanuously, a blank solution was prepared in the same way but without the addition of sample solution. The 2 solutions were set for 20 min in darkness. Due to the action of Cr(Ⅵ) in the reaction system, color-fading of Wright's stain was observed. Absorbance of sample solution (A) and blank solution (A₀) were measured at the wavelength of 662 nm, and ΔA were calculated by A₀-A. It was shown that linear relationship between values of ΔA and mass concentration of Cr(Ⅵ) was kept within 0.40 mg·L^-1, with detection limit (3s/k) of 0.005 5 mg·L^-1. In analysis of some substantial sample by this method, the results obtained were in consistency with those obtained by AAS. Values of RSDs (n=5) found were in the range of 1.4% to 2.3%. Test for recovery was made by the standard addition method, giving values of recovery in the range of 99.1% to 102%.