Chemical equilibria and sequential extractive spectrophotometric determination of selenium (IV) and (VI) using the chromogenic reagent 4, 4′-dichlorodithizone

The chromogenic reagent 4,4′-dichloro(3-mercapto-1,5-diphenylformazan), Cl₂H₂D _Z , forms a yellow–red-coloured complex with selenium(IV). The produced complex species was extracted quantitatively into n-hexane, and its absorbance was measured at 416 nm. The chemical composition of the extracted selenium(IV)-Cl₂H₂D _Z chelate and the molar absorptivity at 416 nm were found to be [SeO (Cl₂HD _Z )₂] and 9 × 10⁴ L mol⁻¹ cm⁻¹, respectively. The values of the extraction constants (K _D, K _ex, β) enable a convenient application of the proposed system for the liquid–liquid extraction procedure and sequential spectrophotometric determination of traces of inorganic selenium(IV) and/or selenium(VI) after reduction of the later to selenium(IV) with HCl (6 M). Beer_s law and Ringbom_s plots were obeyed in the concentration range 0.01–20 and 0.5–19 µg mL⁻¹ of selenium(IV), respectively, with a relative standard deviation of 2.2%. The proposed method has been successfully applied to the determination of selenium(IV) or (VI) and total inorganic selenium(IV) and (VI) in tap and freshwater samples.