Simultaneous determination of orthophosphate and silicate in brackish water

A method has been developed for the simultaneous determination of phosphate and silicate in brackish water samples. Molybdate/antimony, ascorbic acid and oxalic acid reagents are added to the samples and after a total reaction time of 30 min spectra are recorded in the wavelength range 410-820 nm. The developed and utilised reagent compositions lead to almost equal formation rates of the two molybdate complexes. Multivariate calibration is performed by partial least squares regression enabled by the spectral differences between the two complexes. Typical calibration ranges are 0-20 mu mol/l for phosphate and 0-40 mu mol/l for silicate which cover the natural variations in most brackish water samples. The root mean square error of prediction was ca. 2 and 4 mu mol/l, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.