Determination of micro amounts of phosphorus with Malachite Green using a filtration–dissolution preconcentration method and flow injection–spectrophotometric detection

Abstract

A sensitive spectrophotometric method for the determination of micro-amounts of phosphorus existing as orthophosphate is proposed. The method is based on a filtration–dissolution preconcentration and flow injection with spectrophotometric detection. In an acidic medium orthophosphate reacts with molybdate to form molybdophosphate, which then reacts with a cationic dye, Malachite Green (MG), to form a coloured ion associate. The ion associate is filtered through a membrane filter (cellulose nitrate; pore size 1.0 µm, diameter 9 mm), then dissolved together with the membrane filter in methyl Cellosolve and their absorbance measured at 627 nm with a flow injection system equipped with an autosampler. For the filtration of the ion associate, a filtration apparatus was designed. The laboratory-made filter holders were made of poly(chlorotrifluoroethylene) and poly(tetrafluoroethylene)(PTFE) and were equipped with a 50 cm³ separating funnel. The filter holders were connected to an aspirator. For the measurement of absorbances of the methyl Cellosolve solutions, a flow injection system was assembled by using a double-plunger micropump, an autosampler, spectrophotometric detector with a 10 mm flow-through cell and a recorder. All of the flow lines were made of 0.5 mm i.d. PTFE tubing. The calibration graph was linear up to 18 ng cm^–3 of phosphorus using a 10 cm^–3 sample, and the limit of detection was 60 ppt of phosphorus. Using the proposed method, phosphorus in sea-water was determined.