Elimination of the Schlieren effect in the determination of reactive phosphorus in estuarine waters by flow-injection analysis

Conventional flow-injection manifolds with sample injection for the determination of reactive phosphorus in estuarine waters are limited by the Schlieren or refractive index (RI) effect which can cause major errors in quantification. A simple flow-injection (FI) manifold which obviates this RI error in reactive phosphorus measurement is reported. It involves the injection of acidic molybdophosphate reagent into a carrier stream of sodium chloride solution of similar refractive index, which is then sequentially merged with a sample (the salinity of which may vary widely from sample to sample) and a reductant. Despite the occurrence of sizeable blank signals, reactive phosphorus has been successfully measured in samples with salinities ranging from 0 to 34%. using calibration standards prepared in deionised water, with a detection limit of 6 μg l−1 P.