COMPARISON OF MIXING DEVICES FOR FLOW-INJECTION DETERMINATIONS BASED ON DOUBLET PEAK FORMATION

The well-stirred tank model accurately describes the separation (Delta t) between flow-injection doublet peaks and has been found to be appl icable to a variety of mixing devices that do not contain moving parts such as a magnetic follower. The reaction between lanthanum(III) and methyl thymol blue was used as a model reaction for a comparison study of mixing devices. Column and open-tubular reactors were included in the study. Mixing devices were compared on the basis of the straight l ine fit of Delta t versus the natural logarithm of the concentration o f injected La(III). The linearity of the Delta t-ln[La(III)] plots was equivalent for several reactors. A mixing device composed of a column of alternating helices was selected as the best alternative mixer to the well-stirred tank when the magnitude of the slope of the plot and practicality were considered. Experiments showed that the well-stirred tank model qualitatively describes the behavior of these alternating helical reactors (AHR) in experiments designed to produce doublet peak s. The AHR was used as the mixing device in flow-injection determinati ons, based on doublet peaks, of zinc, hydroxide ion and of water hardn ess. A paired t-test showed that over the 16 determinations performed there was no significant bias at the 95% confidence level. Factors aff ecting the relative standard deviation of the concentrations measured are discussed.