Kinetic methods of analysis

the use of kinetic methods was limited before the development of such instrumentation. By then analysts had to rediscover the power of kinetic methods. A full flowering of kinetic methods occurred in the 1960s coinciding with the development of automated instrumentation which not only rendered such methods more convenient but more reproducible as well. 

In this chapter, ftmdamental principles and calculations of chemical kinetics and its application to anal) ical chemistry are described. We will try to demonstrate that the use of spreadsheet techniques makes a dramatic difference in the convenience and reliability of currently used kinetic 

The rate or velocity of a reaction is the change in reaction coordinate, i.e., the concentration of reactant or product, per unit of time. The reaction rate, v, which can be expressed either as the rate of disappearance of the reactant or that of the formation of the product is  

Measurement of the reaction rate requires that the concentration of a reactant or product be measured as a function of time at frequent time intervals or, preferably, continuously with time. 

In general, in a reaction in solution between substances A and B to form C, the rate expression from the Law of Mass Action can be written as 

amount of,catalyst can be determined by measuring how much the rate of,at. chemicai reaction.is affec.ted. Catalytic methods, which are among the most sensitive of all analytical methods, are used for trace analysis of metals in the environmehtit orga.nics in a va rtety of samples, and enzymes in biological systems. 

In kinetic methods of analysis, measurements are made while net changes are occurring in the extent of the reaction. In equilibrium methods of analysis, measurements are made under conditions of equilibrium or steady state. 

J inetic methods of analysis differ in a fundamental way from the equilibrium, or thermodynamic, methods we have dealt with in previous chapters. In kinetic methods of analysis, measurements are made under dynamic conditions in which the concentrations of reactants and products are changing as a function of time. In contrast, thermodynamic methods are performed on systems that have come to equilibrium or steady state, so that concentrations are static. 

Selectivity in kinetic methods is achieved by choosing reagents and conditions that produce differences in the rates at which the analyte and potential interferences react. Selectivity in thermodynamic methods is realized by choosing reagents and conditions that create differences in equilibrium constants. 

Kinetic methods greatly extend the number of chemical reactions that can be used for analytical purposes because they permit the use of reactions that are too slow or too incomplete for thermodynamic-based procedures. Kinetic methods can be based on complexation reactions, acid-base reactions, redox reactions, and others. Many kinetic methods are based on catalyzed reactions. In one type of catalytic method, the analyte is the catalyst and is determined from its effect on an 

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There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

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). 

A kinetic method of analysis designed to rapidly mix samples and reagents when using reactions with very fast kinetics. 

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. 

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. 

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. 

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. 

The following experiments may he used to illustrate the application of kinetic methods of analysis. Experiments are divided into two groups those based on chemical kinetics and those using flow injection analysis. Each suggested experiment includes a brief description. 

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. 

PMSA is easily synthesized reagent and its diluted aqueous solutions (10 -10 M) ai e stable in time. PMSA is stronger oxidizer than both widely used in the kinetic methods of analysis and K S O. ... 

DETERMINATION OF ORGANIC COMPOUNDS BY RADICAL INDICATOR REACTIONS IN KINETIC METHODS OF ANALYSIS... 

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