Flow injection spectrophotometric determination of silicate based on the formation of the ion associate between molybdosilicate and Malachite Green

Abstract

A flow injection method is proposed for the determination of silicate. The method is based on the formation of an ion associate between molybdosilicate and Malachite Green. A three-channel manifold was used in the development of the reaction. The sample injected into a water carrier reacts with a molybdate solution and finally the ion associate is formed from merging molybdosilicate and Malachite Green solutions. The influence of chemical, physical and flow variables on the absorbance was studied spectrophotometrically at 645 nm. Experimental conditions to avoid the adsorption of the ion-associate complex on the surfaces of the flow system were also investigated. The precision of the method is better than 2.0% and the detection limit is 14 ng of SiO₂ per millilitre. The sample throughput is 70 samples h^–1. The method proposed has been applied to the determination of silicate in natural waters. Results are in a good agreement (within 2%) with those obtained by the standard spectrophotometric method.