Trace determination of Cr(III) and Cr(VI) species in water samples via dispersive liquid-liquid microextraction and microvolume UV-Vis spectrometry. Thermodynamics, speciation study

A simple procedure for dispersive liquid-liquid microextraction (DLLME) of Cr(III) and Cr(VI) species has been established. The ion associate represented by halochromate (CrO3Cl−) anion and rhodamine 6G hydrochloride dye (RG+) cation is selectively extracted via DLLME following micro-volume UV-Vis spectrometric detection. The reaction conditions as pH adjustment, acidity, concentration of reagent and electrolyte were optimized. At appropriate conditions, the proposed method exhibited a two order of magnitude wide linear range of 25-950 μg L− 1 with a detection limit of 7.48 μg L− 1 for Cr(VI). The relative standard deviation of 2.16 (n = 5) at 90 μg L− 1 of Cr(VI) was evaluated. The ions that commonly occur in real water samples were also investigated and pose no interference to Cr(VI) determination. Regarding Cr(III) species, it was evaluated after oxidation to Cr(VI) with H2O2 in KOH media. The method was validated by determination of Cr(III) and Cr(VI) in spiked water samples (tap and sea water). The results were also compared successfully with the data obtained by inductively coupled plasma-optical emission spectrometry (ICP-OES). The difference in accuracy and precision (Student t- and F- tests) between the two methods was insignificant at 95% confidence. Thermodynamic parameters (ΔH, ΔS, and ΔG) of the produced complex ion associate were determined.