Kinetic methods, advantages

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. 

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-... 

In our studies we mainly used the kinetic method of detection which was initially proposed in paper [75]. The advantages of this method if compared to the method of measurements of the stationary electric conductivity deal (apart from fast availability of the data) with the fact that in several cases it enables one to obtain information concerning concentration of detective particles even during development of certain surface reactions in which these particles directly participate. The most simple example is provided by (lie surface recombination whose study... 

The very slow dissociation rates for tight binding inhibitors offer some potential clinical advantages for such compounds, as described in detail in Chapter 6. Experimental determination of the value of k, can be quite challenging for these inhibitors. We have detailed in Chapters 5 and 6 several kinetic methods for estimating the value of the dissociation rate constant. When the value of kofS is extremely low, however, alternative methods may be required to estimate this kinetic constant. For example, equilibrium dialysis over the course of hours, or even days, may be required to achieve sufficient inhibitor release from the El complex for measurement. A significant issue with approaches like this is that the enzyme may not remain stable over the extended time course of such experiments. In some cases of extremely slow inhibitor dissociation, the limits of enzyme stability will preclude accurate determination of koff the best that one can do in these cases is to provide an upper limit on the value of this rate constant. 

Similar estimations have been made (Laidler et al., 13) by means of calculations involving the transition states. This method offers the advantage of using more exact data for the available degrees of freedom in the activated state of the reactant. Beyond this, it does not differ essentially from the kinetic method. The vibration in the direction that leads from the former to the new atomic arrangement is replaced by a translation, and the frequency v by a transition probability kT/h, giving... 

One of the most important aspects of a kinetic study on soil constituents is the method one uses to measure rate coefficients and other kinetic parameters. At the outset, it should be realized that no kinetic method currently available is perfect. Each has its own advantages and disadvantages, and these must be assessed carefully before using. 

The objectives of this chapter are to discuss kinetic methods that are available for studying reactions on soil constituents, how one can analyze the data from these techniques, and the advantages and disadvantages of each method. 

It is even more difficult to predict the future of kinetic methods. There is scope here for considerable development and they have many advantages they can be used for difficult mixtures, they are well suited for automation and kinetics is an integral part of every course in chemistry, so its principles are well understood. However, as yet, these methods have not become as widespread as one might expect.1... 

A kinetic method for pK determination in aqueous DMSO has recently been developed (Earls et al., 1974 Cockerill et al., 1974). This is based on rates of isotopic exchange and is applicable to the p a range ca. 15-25. The method has the advantage that it does not depend on spectral changes occurring on ionization. However, the majority of p/sfa values of weak acids have been obtained by the equilibrium technique, using acidity functions (AF) (Hammett, 1940 Bowden, 1966 Rochester, 1970). 

Photosynthetic reaction centers offer an inherent advantage in the fact that the electron-transfer reactions are activated by light. Thus, very fast laser-based kinetic methods, called laser flash photolysis, can be applied to follow the time course of the reactions. The initial electron transfer step in these systems occurs in approximately 2ps and is followed by several subsequent steps that are progressively slower. Numerous investigators have focused their attention on these reactions, which are described elsewhere see Photosynthesis). 

For all production methods and especially for direct photolytic methods, care needs to be taken so that the radicals are generated, or rapidly converted, into a thermal distribution as relaxation of electronic or vibrational states into the monitored ground state would complicate the kinetics. The advantages of pulsed lasers as light sources are well documented [9]. 

These have also been applied to the analysis of drug substances because they have advantages over equilibrium techniques, especially when mixtures of closely related compounds, compounds that react slowly, or catalytically acting compounds are to be analyzed. The selectivity and sensitivity of kinetic methods of analysis combined with the selectivity and sensitivity of ion-selective electrodes provide a versatile combination that may lead to new analytical schemes. ... 

One salient advantage of fixed-time kinetic methods is significantly increased selectivity. Tolerated concentrations of interfering metal ions are often reported to be one to two orders of magnitude higher than in equilibrium methods. The increased tolerance to interferences of kinetic methods may be the result of the following facts ... 

A major advantage of kinetic methods is their immunity to errors resulting from long-term drift of the measurement system. 

The use of Equation 29-29 has a fundamental advantage common to most kinetic methods—that is, the absolute determination of concentration or of a variable proportional to concentration is unnecessary. The difference between two concentrations is proportional to the initial concentration of the analyte. 

The reactions used in kinetic methods fall into two categories catalyzed or uncatalyzed. As noted earlier, catalyzed reactions are the most widely used because of their superior sensitivity and selectivity. Uncatalyzed reactions are used to advantage when high-speed, automated measurements are required or when the sensitivity of the detection method is great. ... 

List three advantages of kinetic methods. Can you think of two possible limitations of kinetic methods when compared with equilibrium methods ... 

Using a new method, the catalytic kinetic method of analysis, Beklemishev et al. (1997) measured the concentrations of manganese in tap and river water. Their analytical method relies on an indicator reaction that is catalyzed by Mn(Il) (the oxidation of 3,3, 5,5 -tetramethylbenzidine [TMB] by potassium periodate [KIO4]) and is carried out on the surface of a paper-based sorbent. The advantages of this new technique are that it has a much lower detection limit (0.005 pg/L) than do established methods and is transportable, allowing it to be used for rapid tests in the field (i.e., spot tests and similar procedures). 

In contrast to the classical kinetic method, the KGCM can be used not only for the solution of quantitative problems but also for qualitative analysis, as the relative rate constant is a characteristic of the sample substance, similar to the partition coefficient on the differences in which identification of chromatographic zones is based. An important advantage of the relative kinetic reaction rate constant is its strong dependence on the nature of the substance (see, for example. Tables 2.1 and 2.2). The relative constant can be defined, for example, from the equation... 

Perhaps the most widely used kinetics method is that developed by Freeman and Carroll (83) in 1958. The advantage of this method is that considerably fewer experimental data are required than in the isothermal method and that the kinetics can be obtained over an entire temperature range in a continuous manner without any missing regions. In addition, where a sample undergoes considerable reaction in being raised to the temperature of interest, the results obtained by an isothermal method are often questionable. The order of reaction, nt and the activation energy of the reaction are calculated from the equation... 

The development of kinetic methods (both stopped-flow and differential) by FIA offers substantial advantages over conventional methodologies. 

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