Photometric control of oxsoforms of selenium (IV) and selenium (VI) of different toxicity and biological activity in bottled drinking water of natural sources
Determination of low selenium concentrations in the drinking water, showing at different concentrations and the degree of oxidation both biogenic and toxic properties, is an important ecological and analytical task. The purpose of the study was to develop a highly sensitive and selective technique of photometric control of migratory forms of selenium in drinking bottled water. Reactivity of selenit- and selenat- ions with respect to the photometric indicator methylene blue (MG) has been theoretically grounded and quantified. It was established that a 1M solution HCl Se (IV) oxidizes a reduced form of MG, followed by destruction of oxidized indicator particles, which are converted to unstained products. In weakly acidic media (pH 5-6) containing MG and Se (VI), is formed a resistant ionic associate, absorbs at 605 nm. In case of determination of Se (IV) based on the measured optical density (670 nm), which regularly decreases with the increase of concentration of analyte, the proportion of destructively oxidized MG was calculated, and then according to equation of calibration curve the content of Se (IV) was calculated. In determining Se (VI) the proportion of the resulting ion associate, which is proportional to the concentration of selenate ions was calculated. The technique and results of separate photometric control of Se (IV) and Se (VI) in bottled water of some natural sources of the European region of Russia and the Caucasian Mineral Waters are presented. The main advantage of the suggested methods are their competitiveness with other more costly methods of instrumental analysis and reliable results on the content of selenium in varying degrees of oxidation, without the need for expensive equipment and deficit reagents.