Determination of phosphate as aggregates of ion associates by light-scattering detection and application to flow injection

Abstract

Trace amounts of phosphate were determined by detecting the light-scattering intensity with a spectrofluorimeter. The ion associate of molybdophosphate with a chloro derivative of Malachite Green (Cl-MG) or Rhodamine B (RB) formed aggregates in an acidic solution before precipitating. The formation rates of molybdophosphate and its ion associate were rapid, but that of the aggregate was relatively slow. To stabilize the aggregates and to prevent their precipitation in an aqueous medium, poly(vinyl alcohol) solution was added after the formation of the ion associate. The light-scattering intensity of the ion associate with Cl-MG, which did not emit fluorescence, could be detected. The intensities were obtained at the same wavelength of emission as that of excitation: 460 nm for Cl-MG and 460 and 580 nm for RB. The diameter of the aggregate was determined to be mainly 650 nm by using a laser light-scattering detector. The number of ion associates in the aggregate was roughly estimated to be 4.3 × 10⁷. The calibration graph was rectilinear up to 10^–6 mol dm^–3 phosphate in the batchwise method. The method was applied to flow injection, where the calibration graph was rectilinear from 2 × 10^–7 to 1 × 10^–6 mol dm^–3 and the detection limit was 6 × 10^–8 mol dm^–3. The average peak height for 8.0 × 10^–7 mol dm^–3 phosphate was 9.6 cm and the RSD of five measurements was 1.1%. The sample throughput was 15 h^–1. Interferences from bulky and hydrophobic anions were slight.