Practical Measurements of Reaction Rates

Many teehniques have been employed in kinetie studies to determine reaetion rate eonstants and reaetion orders from either reaetant or produet eoneentrations at known times. The most desuable analytieal methods allow eontinuous and rapid measurement of the eoneentration of a partieular eomponent. Any of the methods used to monitor the eourse of reaetion must satisfy the following eriteria  

Analyses of kinetie data are based on identifying the eonstants of a rate equation involving the law of mass aetion and some transfer phenomena. The law of mass aetion is expressed in terms of eoneentrations of the speeies. Therefore, the ehemieal eomposition is required as a funetion of time. Laboratory teehniques are used to determine the ehemieal eomposition using an instrument tliat is suitably ealibrated to give the required data. The teehniques used are elassified into two eategories, namely ehemieal and physieal methods. 

Chemical methods involve removing a portion of the reacting system, quenching of the reaction, inhibition of the reaction that occurs within the sample, and direct determination of concentration using standard analytical techniques—a spectroscopic metliod. These methods provide absolute values of the concentration of the various species that are present in the reaction mixture. However, it is difficult to automate chemical mediods, as the sampling procedure does not provide a continuous record of tlie reaction progress. They are also not applicable to very fast reaction techniques. 

Physical techniques can be used to investigate first order reactions because the absolute concentrations of the reactants or products are not required. Dixon et. al [3] studied the base hydrolysis of cobalt complex, [Co(NH3)5L]3+, where L = (CH3)2SO, (NH2)2C = O, (CH3)03P = O in glycine buffers. 

Substituting Equation 3-197 into the integrated first order reaetion Equation 3-33 gives the eorresponding equations expressed in terms of the solution absorbanee  

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Mechanisms of retention are similar in either situation, but expression of the mechanism will vary. Practically, measurement of reaction rates in soil systems is usually limited to observing changes in reactant. This means that observations will be related to the most rate-limiting step, since this will control the amount of reactant observed at any given time. Unfortunately, diffusion rates are often the rate-limiting step, so observation of reaction kinetics often depends on the ability to negate or interpret diffusion processes. This ability differs, depending on the experimental technique chosen. 

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