Rapid mixing methods continuous flow

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. 

The most widely used fast mixing method is the continuous-flow method. The reactants flow in separate continuous streams that meet in a mixing chamber and then pass along an observation tube or chamber with detection devices at appropriate points along its length (see Fig. 18.2). The detection devices, which measure the composition of the flowing sample, may be optical, thermal, chemical, electrical, or any other method applicable to a rapidly moving sample. Reactions with halftimes of the order of 10" sec can be observed by this method. 

It seems that most of the potential of the continuous-flow methods has already been realized. One limitation arises from the mixing process itself. If two liquids are to be mixed rapidly they must be broken up into tiny packets moving fast. They will be slowed by viscosity, and the smaller the packets, the more effective viscosity will be. The effect is that more work must be done on the liquids both to break them up... 

Dixon and Norman developed a rapid-mixing continuous flow method to observe... 

There are two common flow methods that can be used to speed up the mixing of liquids or gases. In the continuous-flow method, two fluids are forcibly pumped into a chamber where they are rapidly mixed. The newly mixed fluid passes into a transparent tube of uniform diameter. The flow rates into the mixing chamber are kept constant so that the distance along the tube is proportional to the elapsed time after mixing. The concentration of a reactant or product is determined spectrophotometrically as a function of position along the tube, using the tube as a spectrophotometer cell. 

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. 

The objective of rapidly mixing reactant solutions for kinetics measurements was first reported about 80 years ago. The method required continuous flowing of solutions, and the term continuous flow was assigned to the method.56 Reactant solutions were flowed along tubes and mixed at a Y-type junction. Whereupon... 

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