Variable-ionic-strength kinetic

Variable-Ionic-Strength Kinetic Experiments Conclusions... 

Ahbrandi, G., Coppolino, S., D Aliberti, S., Ficarra, P., Micali, N., and Villari, A. (2003), Variable-ionic strength kinetic experiments to study drug stabihty, J. Pharm. Sci., 92, 1730-1733. 

In the process of establishing the kinetic scheme, the rate studies determine the effects of several possible variables, which may include the temperature, pressure, reactant concentrations, ionic strength, solvent, and surface effects. This part of the kinetic investigation constitutes the phenomenological description of the system. 

The variable factor in reaction series usually was a substituent change, although solvent variation also has been given special attention (39-44). Variations of catalyst (4, 5, 23-25, 45-49), ionic strength (50), or pressure (51, 52) also have been studied. In exceptional cases, temperature can become the variable parameter if the kinetics has been followed over a broad temperature range and the activation parameters are treated as variable (53), or temperature as well as structural parameters can be changed (6). Most of the work done concerns kinetics, but isoequilibrium relationships also have been observed (2, 54-58), particularly with ionization equilibria (59-82). 

In order to elucidate organometallic reaction mechanisms it is necessary to investigate the effect of as many chemical (concentration, pH, solvent, ionic strength) and physical (temperature, pressure) variables as possible on the observed kinetic behaviour, prior to suggesting possible mechanisms. High-resolution NMR can be used nowadays for determining rates of reactions and the effect of pressure on these rates. The pressure dependence of a rate up to a few kilobars can be used to calculate the volume of activation AF, according to equation (1). A process that is accelerated by pressure exhibits... 

Although the rate of the reaction is the parameter in kinetic studies which provides the link between the experimental investigation and the theoretical interpretation, it is seldom measured directly. In the usual closed or static experimental system, the standard procedure is to follow the change with time of the concentrations of reactants and products in two distinct series of experiments. In the first series, the initial concentrations of the reactants and products are varied with the other reaction variables held constant, the object being to discover the exact relationship between rate and concentration. In the second series, the experiments are repeated at different values of the other reaction variables so that the dependence of the various rate coefficients on temperature, pressure, ionic strength etc., can be found. It is with the methods of examining concentration—time data obtained in closed systems in order to deduce these relationships that we shall be concerned in this chapter. However, before embarking on a description of these... 

Figure 4. Effect of the ionic strength on oxidation kinetics of Am (III) at 25° C (1) 0.2N HC Oh, 0.1 M Na2S2Os, 0.0IM AgNOs,0.001M Am (III), NaClO,t variable (2) same as 1 + 0.03M H3POh.

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. 

Some publications report a single data point, representing the distribution of the adsorbate of interest between the solid and the liquid at certain conditions (initial concentration of adsorbate, solid to liquid ratio, equilibration time, temperature, pH, ionic strength, etc.), which are more or less precisely described. The disadvantage of such an approach is that the result is only valid for these particular experimental conditions, and a change in any of the above variables can lead to a completely different result. Therefore systematic studies are preferred in which one or a few parameters vary while the other parameters are kept constant. Such results are often presented in graphical form. The kinetic studies in which the equilibration time is the independent variable and the other parameters are kept constant are discussed separately in Section VII. 

A kinetic study is not complete unless the effects of various experimental variables on the experimental rate functions are determined. By systematically changing experimental variables and determining the effect on the rate function, valuable clues are obtained that will aid in deducing mechanisms to explain the observed rate function. The most commonly manipulated variables in soil kinetic studies include reactant concentrations (both solute and solid phase), temperature, pH, ionic strength, and solution composition (other than pH, ionic strength, and solute concentration). 

Nonspecific interference can be encountered as a result of changes in temperature, ionic strength, and pH, or as a result of the presence of hemolysis or excessive quantities of bilirubin, heparin, and urea. Any of these factors can alter the composition of the incubation medium and affect the kinetics or equilibrium of the antigen-antibody reaction. Nonspecific interference contributes to assay variability and results in a decrease in sensitivity. This is particularly prevalent in early enzyme IA applications. Assay sensitivity can be greatly improved with increased assay specificity. 

Temperature and pH are the most relevant environmental variables affecting enzyme behavior. However, there are other variables, like ionic strength, that may also have a significant effect on enzyme kinetics. Ionic strength is defined as ... 

The effect of ionic strength in enzyme kinetics is seldom made explicit though special precautions are usually taken to obtain kinetic parameters under constant ionic strength. In fact, ionic strength has been considered as a neglected variable in enzyme kinetics, but it has been proven that it affect enzyme behavior at values over 100 mM (Dale and White 1982). Enzymes are polyionic polymers and may have charged amino acid residues at the active site moreover, substrates may also contain... 

The elucidation of a substitution reaction mechanism depends on reliable kinetic and thermodynamic data obtained by measuring changes in the reaction rate as a function of a chemical property (e.g., concentration, pH, ionic strength, solvent polarity) or physical quantity (e.g., temperature). The determination of an empirical rate law, the observation of steric or electronic effects induced by the entering, spectator, or leaving groups, and the estimation of activation parameters from variable-temperature experiments (i.e.. A// and Aj ) contribute to the adjudication of a plausible mechanism for a given reaction. 

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