Simultaneous determination of iron(II) and iron(III) by flow injection analysis - A mathematical simulation of the detector response

A mathematical model for the description of the detector signal obtained in flow injection asynchronous merging zone technique (FIA-AMZ) is proposed. FIA-AMZ is based on the separate injection of a sample and an appropriate reagent in such a way that both injected solutions are covered only partly. The resulted detector signal consists of two consecutive peaks whose heights are proportional to the concentration of the detectable component present in the sample and another component which becomes detectable after the reagent has been added. The detector signals (the concentration profiles of the analytes) are described by a modified Gauss curve, which proved to be better than a combination of Gauss-Lorentz curves. The conversion of undetectable to detectable form of the analyte is described by a sigmoidal function the slope of which corresponds to the frontal dispersion of the added reagent and to the reaction kinetics. The model is tested on the system for the simultaneous spectrophotometric determination Fe2+ and Fe3+. It is shown that fitting of the experimental curves with a model function enables the optimisation of the experimental conditions at applied FIA manifold, and consequently the simultaneous determination of concentrations of Fe2+ and Fe3+ ions in a single measurement.