Simultaneous spectrophotometric flow injection determination of phosphate and silicate.

A method for the simultaneous determination of phosphate and silicate based on spectrophotometric measurement at 385 nm of a single peak using a flow injection system with two component calibration is described. In the system, a stream of sample containing both analytes is merged with a stream of ammonium molybdate to form (at 1<pH<2) molybdophosphoric and molybdosilicic acids. Total absorbance of the compounds is registered in a form of a constant signal. Simultaneously, oxalic acid solution is injected into a carrier stream (H2SO4) and then merged with the stream of sample containing the mixture of heteropolyacids. A characteristic peak is registered as a result of selective decomposition of molybdophosphoric acid by oxalic acid. The area (or the absorbance measured at the constant signal) and the absorbance measured at the minimum of the peak can be used as measures corresponding to the phosphate and silicate concentrations, respectively. The time of the peak registration is about 3 min. Two-component calibration with the use of four standard solutions of the phosphate/silicate concentrations established in accordance with 2(2) factorial design was applied. Phosphate and silicate can be determined within the concentration ranges of 0.20-15.00 and 0.20-20.00 mg L(-1), with precision less than 2.7 and 0.9% (RSD), respectively and accuracy better than 6.2% (RE). The detection limit is 0.054 and 0.092 mg L(-1) for phosphate and silicate, respectively. The method was applied to determination of the analytes in certified reference materials of groundwater, wastewater, and river water giving results consistent with the certified values.