The Spectrophotometric Determination of Phosphate Following the Collection of Ion Pair of Molybdophosphate with Bis[2-(5-chloro-2-pyridylazo)-5-diethylaminophenolato]cobalt(III) on an Organic Solvent-Soluble Membrane Filter

A rapid and sensitive method of determining phosphate was described. The ion pair of molybdophosphate with bis[2-(5-chloro-2-pyridylazo)-5-diethylaminophenolato]cobalt(III) (Co-5-Cl-PADAP) formed in 0.075 mol dm−3 sulfuric acid, 0.225 mmol dm−3 ammonium molybdate, and 25 μmol dm−3 Co-5-Cl-PADAP was collected on an organic solvent-soluble membrane filter made of cellulose nitrate. The membrane filter with the ion pair was then dissolved into N,N-dimethylformamide, and the absorbance of the solution was measured for the determination of phosphate. The calibration curve was linear up to 0.4 μg as phosphorus, with the correlation coefficient of 0.999, and the apparent molar absorption coefficient of phosphate was 2.9×105 dm3 mol−1 cm−1. The relative standard deviation was 1.7% for 0.4 μg of phosphorus. Although dodecyl sulfate existing in an amount ten times that of phosphorus caused a positive error, no ions commonly existing in natural waters interfere with the determination of phosphate. The proposed method was applied to the determination of μg dm−3 levels of phosphate-phosphorus in natural waters, and satisfactory results were obtained.