Noncatalytic kinetic methods

Noncatalytic reactions are less frequently used in kinetic-based determinations than are those involving a catalytic effect. However, recent advances in instrumentation mean that noncatalytic kinetic methods are powerful alternatives to equilibrium (nonkinetic) methods. This type of reaction is of especial relevance to the analysis of mixtures of closely related compounds, for which a munber of differential reaction rate methods have been developed. Whether for individual or joint determinations of species, the main field of application of noncatalytic reactions is organic analysis, unlike catalytic reactions, where a metal ion usually acts as the catalyst this has also contributed to their current wide acceptance. 

The use of kinetic methods of analysis based on noncatalytic reactions has leveled off in the last few years, especially as regards practical applications in any case some of them are the best choices available to analytical chemists for individual and multicomponent determinations of species in a wide variety of real samples. This growing use of noncatalytic kinetic methods can largely be ascribed to the increased automation of reaction rate-based determinations, especially in organic analysis. Applications of these methods to real samples lie in various areas those of environmental, clinical, pharmaceutical, and nutritional interest are discussed in some detail below on account of their great significance. 

Noncatalytic kinetic methods have been widely applied in this area for several reasons, namely... 

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. 

The precision of both noncatalytic and catalytic kinetic methods depends on such experimental conditions as pH, ionic strength, and temperature. With careful control of these variables, relative standard deviations of 1% to 10% are typical. Automation of kinetic methods and computerized data analysis can often improve the relative precision to 1% or less. 

Kinetic methods have been classified according to a number of criteria. One classification distinguishes between catalytic and noncatalytic methods (see Table 1). The former are further divided according to the type of reaction involved, while the latter are categorized according to whether they are used to determine a single species or several components in mixtures (differential reaction-rate methods)... 

The earliest examples of analytical methods based on chemical kinetics, which date from the late nineteenth century, took advantage of the catalytic activity of enzymes. Typically, the enzyme was added to a solution containing a suitable substrate, and the reaction between the two was monitored for a fixed time. The enzyme s activity was determined by measuring the amount of substrate that had reacted. Enzymes also were used in procedures for the quantitative analysis of hydrogen peroxide and carbohydrates. The application of catalytic reactions continued in the first half of the twentieth century, and developments included the use of nonenzymatic catalysts, noncatalytic reactions, and differences in reaction rates when analyzing samples with several analytes. 

The kinetics of the noncatalytic reaction were studied by the method of Bawn and Williamson (4). They found 3.3-3.7 moles/liter for the equilibrium constant for the formation of the intermediate acetaldehyde monoperacetate (AMP) and a first-order rate constant for the decomposition of this intermediate to acetic acid of 0.015 min."1 at 25°C. We found difficulty in reproducing our results probably caused mainly by the high values of the blanks in the iodometric methods used. However, as an average of four determinations we obtained 0.03 min."1 at 30°C. 

For the very restricted conditions where Eq. (5.2) provides a rigorous description of the reaction kinetics, the activation energy, E, is a constant independent of conversion. But in most cases it is found that E is indeed a function of conversion, E (x). This is usually attributed to the presence of two or more mechanisms to obtain the reaction products e.g., a catalytic and a noncatalytic mechanism. However, the problem is in general associated to the fact that the statement in which the isoconversional method is based, the validity of Eq. (5.1), is not true. Therefore, isoconversional methods must be only used to infer the validity of Eq. (5.2) to provide a rigorous description of the polymerization kinetics. If a unique value of the activation energy is found for all the conversion range, Eq. (5.2) may be considered valid. If this is not true, a different set of rate equations must be selected. 

For this particular case, both a, and a2 are unique functions of conversion, meaning that dx/dt depends only on conversion and temperature i.e., the polymerization kinetics may be described by the phenomenological Eq. (5.1). Moreover, if one of the mechanisms (e.g., the catalytic) predominates over the other one (e.g., the noncatalytic), Eq (5.2) may be used to correlate experimental results and the activation energy may be obtained using isoconversional methods. 

The underlying concept of this method for the synthesis of filamentous carbonaceous nanomaterials is fairly simple. As the temperature rises above a certain limit, which depends on the thermodynamic and kinetic para meters of carbon containing compounds, such as hydrocarbons, such compounds tend to pyrolyze in the air free conditions to form free carbon. For example, the noncatalytic pyrolysis of methane can be achieved at ambient pressure and at temperatures above 900—1000 K to produce soot (near spherical nanosized carbon particles) and hydrogen ... 

An identical mathematical description of the kinetics of curing of reactants different in chemical nature and that obtained on the basis of fundamentally different experimental methods allows us to assume that this apparent selfacceleration course of some rheokinetic parameters is common to the processes of formation of materials with a crosslinked structure. It should be emphasized once more that the self-acceleration" effect must not be identified with the self-catalysis of the reaction of interaction between epoxy monomers and diamines which is studied in detail on model compounds [116, 117]. For each particular curing process the self-acceleration effect is influenced by the mechanism of network formatic, namely, chemical self catalysis [118], the appearance of local inhomogeneities [120], the manifestation of gel eff t [78], parallel course of catalytic and noncatalytic reactions [68]. It is probably true that the phenomena listed above may in one form or another show up in specific processes and make their contribution into self-acceleration of a curing reaction. 

Analytical methods based on noncatalytic reactions are useful for both the kinetic determination of a... 

Table 1 Analytical methods used in kinetic determinations based on noncatalytic reactions...

Flow systems, both continuous and discrete, are used in kinetic-based determinations for monitoring fast reactions mainly. To this end (1) the dead time in the mixing system should be several orders of magnitude lower than the half-life of the reaction concerned and (2) nearly the whole kinetic curve must be recorded in order to implement reaction rate-based determinations and perform fundamental kinetic studies (e.g., the determination of reaction orders and rate constants). The advent of stopped-flow mixing and the continuous-addition-of-reagent technique has made noncatalytic reactions competitive with equilibrium methods in practical terms. 

Kinetic catalytic methods for determination of species can be classified in a manner similar to that of the kinetic noncatalytic methods described elsewhere in this encyclopedia (Table 1). Methods commonly used to measure induction periods are commented on in dealing with Landolt reactions below. 

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