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

This paper presents a spectrophotometric sequential injection (SI) determination of phosphate and silicate in environmental samples and cell cultivation medium using the molybdenum blue reaction. The interference of silicate in the determination of phosphate was eliminated by using a reagent composed of 5 mmol l−1 ammonium molybdate in 0.2 mol l−1 nitric acid, containing 0.25% (w/v) oxalic acid to avoid the formation of molybdosilicic acid. The interference of phosphate in the determination of silicate was avoided adding a 10% (w/v) oxalic acid solution to the reaction zone where the molybdophosphoric and molybdosilicic acids were previously formed, in order to destroy the molybdophosphoric acid. To perform this task in the single line SI system, obtaining a total sample and reagent zones penetration, it was used as a combination of sandwiching the sample zone between reagent zones and flow reversal through an auxiliary reaction coil. The method has a phosphate 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 silicate, 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.