Highly Sensitive Spectrophotometric Reaction Rate Method for the Determination of Selenium Based on the Catalytic Reduction of Sulfonazo by Sulfide

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A sensitive and rapid kinetic spectrophotometric method was developed for the determination of ultratrace amounts of selenium [Se (VI), Se (IV), and total selenium]. The method is based on the catalytic effect of Se (IV) on the reduction of sulfonazo by sodium sulfide. The reaction was followed spectrophotometrically by measuring the decrease in absorbance at 570 nm for the reaction mixture or by measuring the increase in absorbance of the reaction mixture at 680 nm and at 25°C, by the fixed‐time method. Selenium could quantitatively be determined in the range of 0.5–180 and 50–2300 ng mL−1 at 680 and 570 nm, respectively. The limit of detection is 0.3 ng mL−1 Se (IV) at 680 nm. All of the variables that affect the sensitivity in the two wavelengths were investigated, and the optimum conditions were established. The interference effect of various cations and anions on the Se (IV) determination was also studied. The selectivity of the selenium determination was greatly improved with the use of the cation exchange resin. The method was used for the determination of Se (IV) in a food sample, natural water, and synthetic samples with satisfactory results.