Application of organic solvent-soluble membrane filters in the preconcentration and determination of trace elements: spectrophotometric determination of phosphorus as phosphomolybdenum blue.

A simple and rapid preconcentration technique, based on collecting trace elements on a membrane filter and dissolving the membrane filter in an organic solvent, has been applied to the spectrophotometric determination of phosphorus in water. Phosphorus, 0.5-7 mug in 50-500 ml of water sample, is collected as phosphomolybdenum blue on a nitrocellulose or acetylcellulose membrane in the presence of n-dodecyltrimethylammonium bromide, the membrane is dissolved in 5 ml of dimethyl-sulphoxide (DMSO), and the absorbance of the DMSO solution is measured at 710 nm against a reagent blank. Moderate concentrations of silicate, anionic and non-ionic surfactants and high concentrations of sodium chloride do not interfere. Interference from arsenate can be eliminated by reducing the arsenate to arsenite. Condensed and organic phosphates can be determined if they are first converted into orthophosphoric acid by digestion with persulphate. The limit of determination is 0.002 mug of phosphorus in 100 ml of sample.