Spectrophotometric determination of silicate with molybdate and Malachite Green by flotation-dissolution and filtration-dissolution methods

Abstract

Two sensitive spectrophotometric methods for the determination of silicate are proposed. The first is a flotation-dissolution method and the second a filtration-dissolution method. In an acidic medium, silicate reacts with molybdate to form molybdosilicate, which then reacts with a cationic dye to form a coloured ion associate. Malachite Green (MG), Crystal Violet, Ethyl Violet and Methylene Blue were examined as possible counter ions; MG was selected. The ion associate with MG was floated on the interface between an aqueous phase and an organic phase [isobutyl methyl ketone-cyclohexane (1 + 8)], then dissolved in acetone and the absorbance measured at 620 nm; the molar absorptivity and the absorbance of the reagent blank were 4.2 × 10⁵ l mol^–1 cm^–1 and 0.023, respectively. The detection limit was 1 × 10^–8M of silicate. In the filtration-dissolution method, the ion associate was filtered through a membrane filter, then dissolved together with the membrane filter in Methyl Cellosolve and the absorbance measured at 627 nm; the molar absorptivity and the absorbance of the reagent blank were 4.0 × 10⁵ l mol^–1 cm^–1 and 0.075, respectively. The detection limit was 1.5 × 10^–8M of silicate. Using the proposed methods, trace amounts of silicate in purified waters were determined.