Speciation and separation of Cr(VI) and Cr(III) using coprecipitation with Ni²⁺/2-Nitroso-1-naphthol-4-sulfonic acid and determination by FAAS in water and food samples

A speciation procedure for chromium(III) and chromium(VI) in the water and food samples has been established in the presented work. The procedure presented based on quantitative recoveries of Cr(III) > 95% using coprecipitation of Ni²⁺/2-Nitroso-1-naphthol-4-sulfonic acid. The total chromium was measured after reduction of Cr(VI) by concentrated H₂SO₄ and ethanol. The Cr(VI) was calculated as the difference between the total Cr content and the Cr(III) contents. The analytical variables, pH, amounts of precipitate, sample volume for optimum recoveries of Cr(III) were investigated. The influences of the some alkaline and earth alkaline ions and some transition metals on the recoveries of analyte ions were also investigated. The preconcentration factor was 50. The detection limit (3σ) of the reagent blank for chromium(III) was 1.33 μg L^-1. The relative standard deviations for determinations were found to be <7%, and % of relative error was calculated <6%. The method was validated by certified reference materials of tea (GBW 07605) and bush branches and leaves (GBW 0703). The procedure was successfully applied to the speciation of chromium in natural water and food samples (fish, white cheese, cow's meat, black tea, boiled wheat).