Kinetic methods determination

Table 3.21 Phem y l isocyanate kinetic method - determination of polyols ...

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 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. 

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. 

This experiment describes a kinetic method for determining the concentrations of Mn04 and Gt207 in a mixture. The analysis is based on a difference in the rate at... 

In addition to ready thermal decomposition, 1,2-dioxetanes are also rapidly decomposed by transition metals (39), amines, and electron-donor olefins (10). However, these catalytic reactions are not chemiluminescent as determined by the temperature drop kinetic method. 

A kinetic method for the determination of 2,4-dinitrophenol is proposed. The method is based on the inhibiting effect of 2,4-dinib ophenol on the Mn(II) catalysis of the oxidation of malachite green with potassium periodate. The reaction was followed spectrophotometrically at 615 nm. The optimal experimental conditions for the determination of 2,4-dinitrophenol were established under the optimal reaction conditions ... 

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

Following this procedure urea can be determined with a linear calibration graph from 0.143 p.g-ml To 1.43 p.g-ml and a detection limit of 0.04 p.g-ml based on 3o criterion. Results show precision, as well as a satisfactory analytical recovery. The selectivity of the kinetic method itself is improved due to the great specificity that urease has for urea. There were no significant interferences in urea determination among the various substances tested. Method was applied for the determination of urea in semm. 

The kinetic method of determining relative acidity suffers from one serious complication, however. This complication has to do with the fate of the ion pair that is formed immediately on removal of the proton. If the ion pair separates and difiuses into the solution rapidly, so that each deprotonation results in exchange, the exchange rate is an accurate measure of the rate of deprotonation. Under many conditions of solvent and base, however, an ion pair may return to reactants at a rate exceeding protonation of the carbanion by the solvent. This phenomenon is called internal return ... 

Kinetic methods. These methods of quantitative analysis are based upon the fact that the speed of a given chemical reaction may frequently be increased by the addition of a small amount of a catalyst, and within limits, the rate of the catalysed reaction will be governed by the amount of catalyst present. If a calibration curve is prepared showing variation of reaction rate with amount of catalyst used, then measurement of reaction rate will make it possible to determine how much catalyst has been added in a certain instance. This provides a sensitive method for determining sub-microgram amounts of appropriate substances. 

NMR line broadening is a suitable kinetic method for determining activation parameters for Co—C bond homolysis, and gave A//- values in the range 18-22.5 keal mol for a selection of Co(Por)R complexes containing secondary or tertiary alkyl groups.Bond dissociation enthalpies and entropies for several... 

The kinetic method, developed by Cooks et al., is commonly nsed to measnre acidities for systems that readily form proton-bound clnster ions. For example, acidities are determined from the branching ratios for dissociation of proton-bonnd cluster ions of anions and reference. In the simplest form, there is assnmed to be a semi-logarithmic relationship between the CID branching ratio, r, and the difference in the free energies for the two processes (Eq. 5.2). 

Wenthold and co-workers have used this approach to generate the negative ion of 5-dehydro-m-xylylene (Eq. 5.19). ° Using bracketing experiments to determine the gas-phase acidity at the 5-position of m-xylylene, and the kinetic method to determine the triradical electron affinity, the BDE at the 5-position of m-xylylene was measured (Eq. 5.20). 

This closure property is also inherent to a set of differential equations for arbitrary sequences Uk in macromolecules of linear copolymers as well as for analogous fragments in branched polymers. Hence, in principle, the kinetic method enables the determination of statistical characteristics of the chemical structure of noncyclic polymers, provided the Flory principle holds for all the chemical reactions involved in their synthesis. It is essential here that the Flory principle is meant not in its original version but in the extended one [2]. Hence under mathematical modeling the employment of the kinetic models of macro-molecular reactions where the violation of ideality is connected only with the short-range effects will not create new fundamental problems as compared with ideal models. 

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. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

More refined kinetic methods, and the ability to determine very precisely the relative proportions of o-, m- and p-isomers formed—by, for example, spectroscopic methods rather than by isolation as in the past—now allow of a much more quantitative approach to aromatic substitution. One very useful concept here is that of partial rate factors the rate at which one position, e.g. the p-, in C6HSY is attacked compared with the rate of attack on one position in benzene it is written... 

Determined by spectrophotometric methods (Cramer et al., 1967). Determined by kinetic methods (VanEtten et al., 1967a). 

The association of kinetics and stereochemistry is particularly useful for obtaining data on the structure, open or bridged depending on the substituents, of bromination intermediates. These short-lived reactive intermediates cannot be observed under the reaction conditions, but indirect kinetic methods enable us to determine the magnitude of bromine bridging on which the selectivity of product formation depends. 

Glew, D.N., Roberson, R.E. (1956) The spectrophotometric determination of the solubility of cumene in water by a kinetic method. J. Phys. Chem. 60, 332-337. 

Other methods employ a microplate format followed by fast HPLC. Some researchers approach the determination from a different perspective. For example, an alternative method for ionizable substances is the pSol determination based on an acid-base titration.25 26 Kinetic solubility determinations involve determining the concentration of the compound in the buffer of interest when an induced precipitate first appears. 

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