Solvent Extraction-Spectrophotometric Determination of Trace Orthophosphate in Water by Stopped-Flow Time Difference Analysis Technique

Abstract

For the determination of phosphate from μg l-1 to sub-μg l-1 level in water, the time difference analysis technique (a sort of end-point assay using the stopped-flow method) was used by combining the Molybdenum Blue method and the pre-concentrating solvent extraction of 12-molybdophosphate. Isobutyl alcohol and stannous chloride were used as the solvent and the reductant, respectively. The reducing reaction was fast and the colored complex formed by the reaction was unstable. Phosphate concentration was easily determined by analyzing the reaction curves recorded for only 0.5s. The apparent molar absorptivity of phosphate was about 2×105M-1 cm-1 at 730nm, and the detection limit was 0.3μg l-1. The calibration curve was linear from zero to 5μg l-1. The method was applied to the determination of phosphate in tap water samples; the relative standard deviation was less than 3% and the recovery of 20μg l-1 phosphate was 100±6%.