DETERMINATION OF FORMATION QUOTIENTS BY A FLOW-INJECTION PROCEDURE

A method is described for determining equilibrium constants for reacti ons of 1:1 stoichiometry in which a flow injection procedure is used t o generate absorbance-time data that are analysed by two iterative com putational procedures. A two-line manifold was used with a well stirre d mixing chamber downstream of the confluence point. The physical disp ersion of the system was characterized first by the passage of an abso rbing solution through the manifold in the absence of chemical reactio n. 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 th e reaction of lanthanum(III) with Methyl Thymol Blue. Absorbance-time data were taken from the trailing edge of the peak profile between dis persion coefficient values of 5 and 25. Results for the formation quot ients in agreement with previously reported values were obtained, exce pt that iron(III) thiocyanate would appear to be more stable when form ed in a flow injection (FT) manifold than when formed in a static batc h procedure. The flow injection method greatly simplifies the experime ntal procedure compared with that of Job's method or the method of con tinuous 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 experimen ts.