Chemical kinetics methods

Despite the variety of methods that had been developed, by 1960 kinetic methods were no longer in common use. The principal limitation to a broader acceptance of chemical kinetic methods was their greater susceptibility to errors from uncontrolled or poorly controlled variables, such as temperature and pH, and the presence of interferents that activate or inhibit catalytic reactions. Many of these limitations, however, were overcome during the 1960s, 1970s, and 1980s with the development of improved instrumentation and data analysis methods compensating for these errors. ... 

Every chemical reaction occurs at a finite rate and, therefore, can potentially serve as the basis for a chemical kinetic method of analysis. To be effective, however, the chemical reaction must meet three conditions. First, the rate of the chemical reaction must be fast enough that the analysis can be conducted in a reasonable time, but slow enough that the reaction does not approach its equilibrium position while the reagents are mixing. As a practical limit, reactions reaching equilibrium within 1 s are not easily studied without the aid of specialized equipment allowing for the rapid mixing of reactants. 

A final requirement for a chemical kinetic method of analysis is that it must be possible to monitor the reaction s progress by following the change in concentration for one of the reactants or products as a function of time. Which species is used is not important thus, in a quantitative analysis the rate can be measured by monitoring the analyte, a reagent reacting with the analyte, or a product. For example, the concentration of phosphate can be determined by monitoring its reaction with Mo(VI) to form 12-molybdophosphoric acid (12-MPA). 

Representative Method Although each chemical kinetic method has its own unique considerations, the determination of creatinine in urine based on the kinetics of its reaction with picrate provides an instructive example of a typical procedure. 

Chemical kinetic methods of analysis continue to find use for the analysis of a variety of analytes, most notably in clinical laboratories, where automated methods aid in handling a large volume of samples. In this section several general quantitative applications are considered. 

Chemical kinetic methods have been applied to the quantitative analysis of a number of enzymes and substrates.One example, is the determination of glucose based on its oxidation by the enzyme glucose oxidase. ... 

Plot of equation 13.18 showing limits for which a chemical kinetic method of analysis can be used to determine the concentration of a catalyst or a substrate. 

Noncatalytic Reactions Chemical kinetic methods are not as common for the quantitative analysis of analytes in noncatalytic reactions. Because they lack the enhancement of reaction rate obtained when using a catalyst, noncatalytic methods generally are not used for the determination of analytes at low concentrations. Noncatalytic methods for analyzing inorganic analytes are usually based on a com-plexation reaction. One example was outlined in Example 13.4, in which the concentration of aluminum in serum was determined by the initial rate of formation of its complex with 2-hydroxy-1-naphthaldehyde p-methoxybenzoyl-hydrazone. ° The greatest number of noncatalytic methods, however, are for the quantitative analysis of organic analytes. For example, the insecticide methyl parathion has been determined by measuring its rate of hydrolysis in alkaline solutions. 

Chemical kinetic methods also find use in determining rate constants and elucidating reaction mechanisms. These applications are illustrated by two examples from the chemical kinetic analysis of enzymes. 

Selectivity The analysis of closely related compounds, as we have seen in earlier chapters, is often complicated by their tendency to interfere with one another. To overcome this problem, the analyte and interferent must first be separated. An advantage of chemical kinetic methods is that conditions can often be adjusted so that the analyte and interferent have different reaction rates. If the difference in rates is large enough, one species may react completely before the other species has a chance to react. For example, many enzymes selectively cat-... 

Time, Cost, and Equipment Automated chemical kinetic methods of analysis provide a rapid means for analyzing samples, with throughputs ranging from several hundred to several thousand determinations per hour. The initial start-up costs, however, may be fairly high because an automated analysis requires a dedicated instrument designed to meet the specific needs of the analysis. When handled manually, chemical kinetic methods can be accomplished using equipment and instrumentation routinely available in most laboratories. Sample throughput, however, is much lower than with automated methods. 

Although similar to chemical kinetic methods of analysis, radiochemical methods are best classified as nuclear kinetic methods. In this section we review the kinetics of radioactive decay and examine several quantitative and characterization applications. 

In general, the sensitivity of FIA is less than that for conventional methods of analysis for two principal reasons. First, as with chemical kinetic methods, measurements in FIA are made under nonequilibrium conditions when the signal has yet to reach its maximum value. Second, dispersion of the sample as it progresses through the system results in its dilution. As discussed earlier, however, the variables that influence sensitivity are known. As a result the FIA manifold can be designed to optimize the sensitivity of the analysis. 

Kinetic methods of analysis are based on the rate at which a chemical or physical process involving the analyte occurs. Three types of kinetic methods are discussed in this chapter chemical kinetic methods, radiochemical methods, and flow injection analysis. 

Chemical kinetic methods are particularly useful for reactions that are too slow for a convenient analysis by other analytical methods. In addition, chemical kinetic methods are often easily adapted to an automated analysis. For reactions with fast kinetics, automation allows hundreds (or more) of samples to be analyzed per hour. Another important application of chemical kinetic... 

A brief history of chemical kinetic methods of analysis is found in the following text. 

The following are useful resources for further information regarding chemical kinetic methods of analysis. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

A significant simplification of the algorithm is associated with applying chemical kinetic methods taken from the graphs theory. A graph is a geometrical scheme consisting of a set of points connected by lines. It can be a complex electric scheme, a railway network, a plan of constructional works or finally, a complex chemical reaction. 

The features of environmental samples make them especially suitable for automated kinetic analyses. The determination of metal ions in environmental samples is of great ecological significance as these species cannot be degraded biologically or chemically. Kinetic methods are useful tools for determining heavy metals in environmental water and air, where toxic species occur at low concentration levels. Table 6 collates some relevant applications, among which those concerned with water analyses are prominent. 

Design and operation of chemical reactors in a chemical industry profoundly influence the impact that the industry may have on the surrounding environment. Understanding different types of reactions and characterising their kinetic behaviour are important for optimal design and operation of chemical reactors. This chapter outlines the basic principles of chemical kinetics, methods of obtaining rate equations for different types of reactions, principles of catalysis and kinetics of catalytic reactions. A brief introduction on the types and classification of reactors is presented in this chapter. 

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