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 pm, 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(chlorotrifluoroethy1ene) and poly(tetrafluoroethy1ene) (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 em-3 of phosphorus using a 10 cm3 sample, and the limit of detection was 60 ppt of phosphorus. Using the proposed method, phosphorus in sea-water was determined.

Determination of phosphate using a highly sensitive paired emitter–detector diode photometric flow detector

“…Spectrophotometric procedures for monitoring orthophosphate include the molybdenum blue method [1,[3][4][5][6][7], the yellow vanadomolybdate complex method [8][9][10][11] and the malachite green method [12][13][14][15][16][17]. The malachite green method has been shown to enhance sensitivity by approximately 4 times when compared to the aforementioned methods [12,18].…”

Trace and Ultratrace Analysis of Purified Water Samples and Hydrogen Peroxide Solutions for Phosphorus by Flow-Injection Method

“…Malachite Green has found the widest application. [9][10][11][12][13][14][15] Motomizu et al [16][17][18] have reported a highly sensitive spectrophotometric method for determining phosphorus at sub-ppb and ppt levels using relatively small volumes of the sample solutions, an organic solvent (methyl cellosolve: MC), and a tiny membrane filter for collecting the ion associate of molybdophosphate with Malachite Green. The absorbance of the collected ion associate dissolved in MC was measured by an off-line flow injection-spectrophotometric detection method, which is relatively simple and shows excellent sensitivity.…”

Novel flow injection-fluorometric method for the determination of trace silicate and its application to ultrapurified water analysis

“…Motomizu et al developed several spectrophotometric methods for highly sensitive determination of silicate based on the formation of an ion associate between molybdosilicate and Malachite Green (MG) by flotation-extraction method [1], flotation-dissolution and filtration-dissolution method [2], flotation-exchange-extraction method [3] and filtration-concentration of the ion associate on a tiny membrane filter [4]. The filtration-concentration method with tiny membrane filter was also applied to the trace determination of phosphorus [5,6]. Takaku et al [7] reported High Resolution ICP-MS (HR-ICP-MS) method for the determination of silicate in ultra-high-purity water.…”