2007 Volume 56 Issue 9 Pages 737-743
A sensitive and selective method for the determination of sub μg/L levels of inorganic chromium species in river water is described. The proposed method is based on the cloud point extraction (CPE) with non-ionic surfactant and graphite furnace atomic absorption spectrometry. Hydrophobic chromium chelates in a 30 mL of water sample were extracted into a surfactant-rich phase of polyoxyethylene (8)octylphenyl ether (Triton X-114) after heating over the cloud point temperature (kept in a thermostated water bath at 40°C for 15 min). Separation of the aqueous and surfactant-rich phases was accomplished by centrifugation and cooling in an ice bath. The surfactant-rich phase was dehydrated in a drying oven, and the residual extract was dissolved in a 1.0 mL of 0.1 M nitric acid/methanol solution to decrease the viscosity of the extract. A 20 μL aliquot of the resulting solution was injected into a graphite furnace. In this study, we used ammonium pyrrolidinedithiocarbamate (APDC) as a chelating agent which reacts with both Cr(III) and Cr(VI). The CPE behavior of Cr(III) and Cr(VI) according to the variation of pH and APDC concentration in sample solution was investigated. The results indicated that the major factor of CPE is the concentration of dissociated PDC− and the chemical species of chromium, Cr(OH2)5(OH)2+ and HCrO4−. Cr(VI) was selectively extracted at pH 3.0, and total Cr {Cr(III) + Cr(VI)} was extracted at pH 5.0 without an oxidation process. The optimum concentration of APDC was 0.08% for Cr(VI) and 0.10% for total Cr. The linear calibration curves of Cr(VI) and total Cr for ca. 30-fold preconcentration were obtained over the range of 0 to 0.5 μg/L in the initial solution. The detection limit (3σ) was 2.5 ng/L for Cr(VI) and 3.7 ng/L for total Cr. The proposed method was successfully applied to river water and certified reference material.
