Control Over Reaction Conditions

Advantages Precise control over reaction conditions... 

An additional problem with the thin-disk method is that control over reaction conditions within the thin disk is not usuaUy possible. Although pH and other solution composition variables can be controlled in the influent solution, once the solution contacts the thin disk, direct control with feedback is no longer possible. 

The 3.5-L reaction chamber of the ultraCLAVE enables processing of several samples (rotors with as many as 50 positions are available) or a single large reaction mixture. Reactions can be scaled from microliters to 3 L under identical conditions using the same reaction system. Continuous, unpulsed delivery of micro-wave power enables the most precise possible control over reaction conditions at millisecond frequencies. 

There are practical factors that limit the accuracy predicted by Equation 8.2. For example, velocity distributions in microchannels can cause dispersion, that is, the spreading of reactant bands, which would limit the time resolution and contribute to reduce control over reaction conditions. Factors contributing to dispersions include... 

Gas contact is typically carried out in absorption towers over which the alkaline solutions are recirculated. Strict control over the conditions of absorption are required to efficiendy capture the NO and convert it predominantly to sodium nitrite according to the following reaction, thereby minimizing the formation of by-product sodium nitrate. Excessive amounts of nitrate can impede the separation of pure sodium nitrite from the process. 

In principle, these approaches are very attractive because they probe multiple pathways in the critical regions where the pathways are separated, but in practice these are extremely challenging experiments to conduct, and the interpretation of results is often quite difficult. Furthermore, these experiments are difficult to apply to bimolecular collisions because of the difficulty of initiating the reaction with sufficient time resolution and control over initial conditions. 

We are all familiar with use of microwave radiation in cooking food where it increases the speed of reaction. Recently this method has been used to synthesize solid state materials such as mixed oxides. The first solid state reaction experiments were performed in modified domestic ovens, and these are still used, but more specialised (and expensive ) ovens have also been developed to give more control over the conditions. We shall briefly consider how microwaves heat solids and liquids because this gives us insight into which reactions will be good candidates for this method. 

Rigidity of Control of Reaction Conditions. The logarithmic-extrapolation method has the advantage over the other two methods with respect to control of reaction conditions, because the rate constants need not be known. It is necessary to control conditions strictly during any single determination (a complete kinetic run), but it is not necessary to reproduce these conditions exactly from one analysis to the next, as is the case with the other two methods. For the same reason, the logarithmic-extrapolation method is more applicable if varying amounts of a catalyst are to be present in the system. 

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