Spectrophotometric determination of phosphate and silicate by sequential injection using molybdenum blue chemistry

This paper presents a spectrophotometric sequential injection (SI) determin ation of phosphate and silicate in environmental samples and cell cultivati on medium using the molybdenum blue reaction. The interference of silicate in the determination of phosphate was eliminated by using a reagent compose d of 5 mmol l(-1) ammonium molybdate in 0.2 mol l(-1) nitric acid, containi ng 0.25% (w/v) oxalic acid to avoid the formation of molybdosilicic acid. T he interference of phosphate in the determination of silicate was avoided a dding a 10% (w/v) oxalic acid solution to the reaction zone where the molyb dophosphoric and molybdosilicic acids were previously formed, in order to d estroy the molybdophosphoric acid. To perform this task in the single Line SI system, obtaining a total sample and reagent zones penetration, it was u sed as a combination of sandwiching the sample zone between reagent zones a nd flow reversal through an auxiliary reaction coil. The method has a phosp hate sampling frequency of 75 h(-1), with a linear dynamic range between 0. 2 and 7 mg l(-1) and a detection limit of 0.1 mg l(-1) PO43 = P. For silica te, the sampling frequency is 40 h(-1) with a linear dynamic range between 5 and 50 mg l(-1) and a Limit of detection of 1 mg l(-1) SiO32 = Si. (C) 20 00 Elsevier Science B.V. All rights reserved.