An optical sensor for selenite determination in aqueous samples

Abstract

Background: In the present work, an optical sensor has been developed for the determination of selenium (IV) in aqueous samples. The optical sensor is a cellulose triacetate plasticized membrane with tri-octyl methyl ammonium chloride (Al-336) immobilized in it as an ion exchanger. Methods: The sensor works in two steps. In the first step, a 1 × 2 cm piece of the membrane is allowed to equilibrate with a known amount of selenium (IV) solution and potassium iodide in acidic medium. During this equilibration, the iodometric reaction takes place and, I3− (tri-iodide anion) is formed which gets sorbed into the membrane due to which the colorless film changes to yellow color showing an absorption maxima at 355 nm which is a characteristic of I3−. In the second step the I3− loaded membrane is allowed to equilibrate with a solution of variamine blue dye in presence of sodium acetate. In this step, oxidation of the dye takes place and the membrane shows a color change from yellow to violet and a shift in the absorption maxima from 355 to 550 nm. Results: The method is used to study the quantitative determination of selenium in aqueous samples, where the amount of selenium is varied and the corresponding absorbance at 550 nm is measured. A good linear curve is obtained in the concentration range of 0.2 to 3 μg mL−1 of selenium. Conclusions: The proposed method can be successfully applied to determination of selenium (IV) in various pharmaceutical and environmental samples.