Spectrophotometric determination of ultra trace uranium(VI) in seawater after extractive preconcentration with ionic liquid and dimethylphenylazosalicylfluorone

Abstract

A simple and effective method is presented for extractive preconcentration of uranium(VI) in water, in which the ionic liquid, 1-octyl-3-methylimidazolium hexafluorophosphate, abbreviated as [C₈mim][PF₆], was used as novel medium for liquid/liquid extraction of uranium(VI) and new reagent, dimethylphenylazosalicylfluorone, abbreviated as DMPASF, was employed to form neutral uranium(VI)-DMPASF complex. DMPASF reacted with uranium(VI) rapidly to form a stable red complex, the complex was then extracted into [C₈mim][PF₆] phase, and the uranium(VI) in [C₈mim][PF₆] was back-extracted into aqueous phase again with 3.0 mol L⁻¹ hydrochloric acid. The extraction efficiency, back-extraction efficiency and preconcentration factor were 98.9%, 96.5% and 200 times for 100 µg of standard uranium(VI) in 1000 mL of water samples, respectively. The preconcentration coupled with spectrophotometry with chlorophosphonazo III, abbreviated as CPAIII, was developed for determination of ultra trace uranium(VI) in natural water. The apparent molar absorptivity of the uranium(VI)-CPAIII complex and the detection limit which was calculated using three times the standard error of estimate of the calibration graph was found to be 3.17 × 10⁶ l mol⁻¹ cm⁻¹ and 0.22 ng mL⁻¹ of uranium(VI), respectively. The absorbance of the uranium(VI)-CPAIII complex at 670 nm increase linearly with the concentration of uranium(VI) up to 45 µg of uranium(VI) in 1000 mL of aqueous solution. The interference study show that the determination of uranium(VI) is free from interferences of almost all positive and negative ions found in water samples. The relative standard deviation (N = 10) was 1.3% for uranium for 10 replicate determinations in the solution containing 1.0 µg L⁻¹ of uranium(VI). The proposed method has been successively applied to determination of ultra traces of uranium(VI) in seawater samples.

Keywords:

• ionic liquid
• uranium(VI)
• solvent extraction
• dimethylphenylazosalicylfluorone
• spectrophotometry

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant 20676052).