Selective photometric determination of low conentrations of selenium(IV) and selenium(VI) in bottled drinking water

A procedure is developed for the selective photometric determination of selenium(IV) in bottled drinking water by the oxidation of Methylene Blue in 1 M HCl to colorless decomposition products and of selenium(VI) by its interaction with the specified reagent at pH 5–6 with the formation of a colored ion pair. The limits of detection are 1 and 0.8 µg/L, respectively. At the concentration of selenium(IV) 2 µg/L, the admissible weight ratios are: SeO₄^2-, Br₃^- (1: 20); Br^– (1: 60); I^–, IO₃^- and IO₄^- (1: 100). At equal concentration of selenium(VI), the following species: SeO₄^2-(1: 20); Br₃^-, Br^–, I^–, IO₃^-, and IO₄^- (1: 100) do not interfere with the determination. Other anions and cations present in highly mineralized waters do not interfere with the determination. The relative error of determination is 8–10% in the concentration range 2–10 µg/L of selenium(IV) and selenium(VI) and does not exceed 5% in their concentration range of 10–100 µg/L.