Ultra-trace determination of Hg(ii) in drinking water and local Thai liquors using homogeneous liquid–liquid extraction followed by fluorescence quenching of its ternary complex

Abstract

The method for preconcentration and trace determination of Hg(II) based on the total fluorescence quenching using a 1,10-phenanthroline (1,10-phen) and dichlorofluorescein (DCF) ternary complex after homogeneous liquid–liquid extraction of the metal complex was developed. Hg(II) ions were dissolved in acid and the sample was complexed with an excess amount of diethyldithiocarbamate (DDTC). The complex of Hg(II)–DDTC formed in an aqueous phase and was extracted into a layer of perfluorooctanoic acid dissolved in lithium hydroxide solution, resulting in ∼100 μL of the sediment liquid phase prior to analysis of the ternary complex by spectrofluorophotometry. The optimization conditions were investigated in detail including pH and buffer solution, type of ligand, ionic strength, counter ion and micellar medium. Under the optimized conditions with a preconcentration factor between 15 and 20 using a mixture of 0.2 mol L⁻¹ acetate buffer pH 4.5, 2.5 × 10⁻³ mol L⁻¹ 1,10-phen, 1% (v/v) Triton X-100 and 2.9 × 10⁻⁶ mol L⁻¹ DCF in 15 mL final volume, the decrease in fluorescence intensity of the ternary complex was measured against the reagent blank, in the absence of Hg(II) ions, at the excitation and emission wavelengths of 504.0 and 525.0 nm, respectively. The calibration curve was widely linear over the range of 4.0 μg L⁻¹ to 2.0 mg L⁻¹ with a correlation coefficient greater than 0.997. The method recovery of Hg(II) was about 76.9 to 98.2% at a concentration of 250 μg L⁻¹. The relative standard deviation (RSD) was below 5.5% with a detection limit of 1.0 μg L⁻¹. The proposed method was successfully applied for the determination of Hg(II) in drinking water, distilled spirit and fruit wine samples. The results obtained were in agreement with those of FI-HGAAS.