The kinetic salt effect

Many biochemical reactions in solution are between ions to treat them, we need to 

The thermodynamic version of transition state theory simplifies the discussion of reactions in solution, particularly those involving ions. For instance, the kinetic salt effect is the effect on the rate of a reaction of adding an inert salt to the reaction mixture. The physical origin of the effect is the difference in stabilization of the reactant ions and the activated complex by the ionic atmosphere (Section 5.1) formed around each of them by the added ions. Thus, in a reaction in which the activated complex forms in the pre-equilibrium 

Consider a reaction with the mechanism A + B C — products, where A and B are charged species, and the rate of formation of products is 

Our goal is to write an expression for the rate constant k,. We begin by writing the thermodynamic equilibrium constant in terms of activities a and activity coefficients y  

If we let k be the rate constant for formation of products from the activated complex C, then we may also write 

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In preformulation one task is to establish the stability of the drug substance in both solid and dissolved state. In the latter case it is important, with small samples of drug substance, to assess (a) the effect of buffer type, (b) the effect of buffer concentration, (c) the effect of pH in a practical range, (d) the effect of temperature, and (e) the kinetic salt effect. 

On the experimental side, evidence was accumulating that there is more than one kind of reducing species, based on the anomalies of rate constant ratios and yields of products (Hayon and Weiss, 1958 Baxendale and Hughes, 1958 Barr and Allen, 1959). The second reducing species, because of its uncertain nature, was sometimes denoted by H. The definite chemical identification of H with the hydrated electron was made by Czapski and Schwarz (1962) in an experiment concerning the kinetic salt effect on reaction rates. They considered four... 

ANALYSIS OF THE KINETIC SALT EFFECT OF TETRA-fl-BUTYLAMMONIUM PERCHLORATE ON THE REACTION OF TETRAETHYLTIN WITH MERCURIC CHLORIDE30... 

The interpretation of the kinetic salt effect in aqueous solutions is based on the theory of strong electrolytes, but the latter can only be applied to ammonia solutions of uni-univalent salts at concentrations lower by a factor of 106 than the concentrations employed in the above experiments (Pleskov, 1937). It follows that the existing quantitative theory cannot be used to interpret the results obtained. 

D24.4 In the kinetic salt effect, the rate of a reaction in solution is changed by modification of the ionic strength of... 

The principal reducing species in the radiolysis of neutral water was shown to have unit negative charge from studies of the kinetic salt effect on its reactions [8, 9]. The dependence of the rate coefficient on ionic strength, /u, for a reaction in water at 298°K is given by the Bronsted— Bjerrum equation... 

This indicates that the rate constant for reactions in solution depends on the ionic strength of the solution, behavior referred to as the kinetic salt effect. Thus, addition of inert ions will increase the rate of reaction between ions of like charge and vice versa. 

The magnitude of the kinetic salt effect for a bimolecular reaction depends strongly on the charges on the reacting species. Equation 2.30, derived from the Debye-Hiickel theory, has been shown to represent most experimental data well (Atkins, 1998, p. 836). 

Fig. 7.18 An illustration of the kinetic salt effect. If the reactants have opposite charges, then the rate decreases as the ionic strength, I, is increased. However, if the charges of the reactant ions have the same sign, then the rate increases when a salt is added.

Discuss the physical origin of the kinetic salt effect. 

In alkaline solution the activation parameters are A/f = 22.5 kcal mol- and A5 = —19 cal deg mol, determined from ki values [equation (6)]. The activation parameters for exchange with DgO are the same, within experimental uncertainty. The kinetic salt effect was also studied for this bromate-water oxygen exchange. The experiments in alkaline solution were not successful, but sufficient results were obtained to yield further information germane to the ko and k paths of equation (6). The effects of added cations (Li+, Na+) on oxygen-exchange rates between bromate and water have been discussed in terms of ion association. ... 

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