Determination of soluble phosphates in water samples using ytterbium(III) and dynamic measurements of light scattering intensity at long wavelength

A selective and fast method has been developed for the determination of phosphates by measuring the formation of ytterbium(III) phosphate through the variation of the light scattering intensity with time. The low solubility of this compound causes an efficient dispersion of the radiation at 490 nm, which is measured at 980 nm using the second-order grating effect. This approach minimizes potential background signals from the sample matrix. The initial rate of the system is automatically obtained in only 0.5 s by stopped-flow mixing technique. The variable optimization study has been carried out using univariate and multivariate methods. The dynamic range of the calibration graph is 0.09–7.9 mmol L⁻¹ (Pearson's correlation coefficient = 0.9999) and the detection limit is 0.03 mmol L⁻¹. The precision of the method, expressed as relative standard deviation, is 2.3%. The study of the potential interference of different inorganic anions showed that arsenate is the main interferent, although it is tolerated in a molar ratio of 5.5. The method has been satisfactorily applied to the determination of soluble phosphates in tap, ground and river water using a previous preconcentration step with a Dowex 1 (1 × 4–400) anionic resin. A 500-fold concentration has been achieved, which has allowed to decrease the detection limit up to 60 mmol L⁻¹. The recovery range is 97.5–102.5%. The results obtained are consistent with those obtained with the standard molybdenum blue method.