Stopped-flow continuous mixing methods

This chapter deals with a variety of automatic methods with very peculiar features that distinguish them from those described In the preceding chapters and make them applicable to particular problems In areas such as completely continuous flow analysis (e.g. waste water analysis) and clinical analysis. These methods can be classified Into three general groups, namely continuous mixing methods, stopped-flow continuous mixing methods and continuous-flow titrations. 

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Methods of Measurement.—Spectrophotometric methods, both conventional and stopped-flow, continue to dominate the scene. An interesting variation is being pursued by Tockstem and Skopal, who have used stopped-flow mixing methods with electrode-potential measurements to follow the rate of reaction. Data for the Fe i -h and reactions are in satisfactory agreement with earlier work. ... 

The dead time is typically 3-5 ms. so stopped flow is not quite as fast as continuous flow, but it requires less than a milliliter of each solution per run. Methods have been described for measuring the dead time " " these are based upon standard reactions whose kinetic behavior is well known. The error introduced by collecting data before mixing is complete can be corrected." ... 

Advantages. Polarization measurements permit continuous binding analysis with subsecond resolution if required. When applied in stop-flow mixing conditions the technique has the best time resolution of the methods presently available. 

Measurements of kinetic parameters of liquid-phase reactions can be performed in apparata without phase transition (rapid-mixing method [66], stopped-flow method [67], etc.) or in apparata with phase transition of the gaseous components (laminar jet absorber [68], stirred cell reactor [69], etc.). In experiments without phase transition, the studied gas is dissolved physically in a liquid and subsequently mixed with the liquid absorbent to be examined, in a way that ensures a perfect mixing. Afterwards, the reaction conversion is determined via the temperature evolution in the reactor (rapid mixing) or with an indicator (stopped flow). The reaction kinetics can then be deduced from the conversion. In experiments with phase transition, additionally, the phase equilibrium and mass transport must be taken into account as the gaseous component must penetrate into the liquid phase before it reacts. In the laminar jet absorber, a liquid jet of a very small diameter passes continuously through a chamber filled with the gas to be examined. In order to determine the reaction rate constant at a certain temperature, the jet length and diameter as well as the amount of gas absorbed per time unit must be known. 

Stopped flow and continuous flow methods [11] have been used to follow proton transfer reactions with half-lives in the millisecond range. The stopped flow method which is more popular is essentially a device for mixing the reactants rapidly (typically in one millisecond) together with some means of observing the fast reaction which follows. Proton transfer from p-nitrobenzyl cyanide to ethoxide ion in ethanol/ether mixtures at —77 °C was studied in this way [12]. The reaction was followed spectrophotometrically. The most rapid reaction occurred with ti/2 ca. 2 x 10 2 sec although the equipment was suitable for following reactions with f1/2 ca. 2 x 10 3 sec. A similar method has been used to measure rates of proton transfer between weak carbon acids (for example, triphenylmethane) and bases (for example, alkoxide ions) in dimethyl sulphoxide [13], A continuous flow apparatus with spectrophotometric detection was used [14] to measure rates of ionization for substituted azulenes in aqueous solution (4), reactions for which half-lives between 2 and 70 msec were observed. 

This is a continuous flow mode [27] based on an adaptation of the stopped-flow assemblies originally designed to study and use reactions with fast kinetics. The essential modification with respect to the methods described above is the incorporation of a storage coil between the mixing unit and the flowcell to prevent the reacting mixture from reaching the detector in too short a time, which allows this mode to be applied to reactions attaining equilibrium in times of the order of a few seconds (or minutes). 

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