Flotation-Spectrophotometric Determination of Phosphate with Bis[2-(5-chloro-2-pyridylazo)-5-diethylaminophenolato]-cobalt(III) Chloride

Abstract

A sensitive and accurate method is described for the determination of phosphate with bis[2-(5-chloro-2-pyridylozo)-5-diethylaminophenolato]cobalt(III) chloride (Co-5-CI-PADAP). The ion pair of Co-5-CI-PADAP with molybdophos-phate was floated at the phase boundary between aqueous phase and butyl acetate phase when the aqueous phase was adjusted to 0.125 mol dm^-3 sulfuric acid, 0.225 mmol dm^-3 ammonium molybdate and 25μmol dm^-3 Co-5-CI-PADAP. After the aqueous phase was discarded, the ion pair which had been floated was dissolved by addition of methanol. The absorbance of the organic phase was measured at 560 nm. The calibration curve was linear up to 0.4 μg phosphorus (r=0.999). The molar absorption coefficient was 3.4×10⁵ dm³ mol^-1 cm^-1 and the relative standard deviation was 1.45% for 0.4 μg phosphorus (6 runs). Arsenic(V) and dodecylsulfate interfered. The proposed method was applied to the determination of phosphate in river water.