Selective determination of protolytes by flow injection analysis. A guide for the rational selection of reagent composition

Systematic methodology for selecting a reagent mixture for performing selective spectrophotometric determinations of protolytes by flow injection analysis (FIA) is proposed. A theoretical study is performed of the instantaneous equilibria prevailing at the peak maximum when two protolytes, injected into a carrier, merge and react with a reagent (buffer/indicator) stream. Based on this study, the conditions to be fulfilled by the buffer (strength and concentration) to neutralize the stronger component without reacting with the weaker one are deduced, for a fixed percentage of allowed error. Furthermore, consistent guidelines are given for the selection of a suitable indicator to combine with a preselected buffer in order to obtain sensitive signals. Examples of selective determinations of acidic and basic components in mixtures with different reagent systems are shown. Results obtained with synthetic and real samples by FIA are compared with those obtained by potentiometric titrations.