Determination of formation quotients by a flow injection procedure

Abstract

A method is described for determining equilibrium constants for reactions of 1:1 stoichiometry in which a flow injection procedure is used to generate absorbance–time data that are analysed by two iterative computational procedures. A two-line manifold was used with a well stirred mixing chamber downstream of the confluence point. The physical dispersion of the system was characterized first by the passage of an absorbing solution through the manifold in the absence of chemical reaction. Three chemical systems were then studied at controlled pH and ionic strength in which a metal ion was injected into a carrier and merged with a stream of ligand. These systems were the reaction of iron (III) with salicylic acid, the reaction of iron(III) with thiocyanate and the reaction of lanthanum(III) with Methyl Thymol Blue. Absorbance–time data were taken from the trailing edge of the peak profile between dispersion coefficient values of 5 and 25. Results for the formation quotients in agreement with previously reported values were obtained, except that iron(III) thiocyanate would appear to be more stable when formed in a flow injection (FI) manifold than when formed in a static batch procedure. The flow injection method greatly simplifies the experimental procedure compared with that of Job's method or the method of continuous variation, and the iterative computational methods account for absorbance by the ligand at the wavelength monitored. The educational aspects of this approach are critically evaluated and it is proposed that the FI method would form the basis of a set of teaching experiments.