Determination of Cr(Ⅵ) by Spectrophotometry Combined with Ionic Liquid Aqueous Two-Phase Extraction

The water sample (2.0 mL) was diluted to 20.0 mL with water, then 1.0 mL of sulfuric acid (1+7) solution and 1.0 mL of 2.5 g·L^-1 diphenylcarbazide solution were added and mixed immediately. After 10.0 min, 1.0 mL of 24.0 g·L^-1 1-butyl-3-methylimidazolium bromide solution, 24.0 g dipotassium hydrogen phosphate were added in sequence, and then mixed well for aqueous two-phase extraction. Then the mixture was centrifuged at 3 500 r·min^-1 for 2.0 min. The upper ionic phase was taken, and made up to 3.0 mL with water. The absorbance at 533 nm was measured with blank sample solution as reference. Linear relationship was found between the absorbance and the mass concentration of Cr(Ⅵ) in the range of 0.015-0.300 mg·L^-1, with detection limit (3s/k) of 7.8×10^-4mg·L^-1 and enrichment factor of 6.67 times. The method was applied to determination of Cr(Ⅵ) in actual water samples, giving values of recovery obtained by standard addition method in the range of 95.3%-111%.