Strength Primary Salt Effects

For drug degradation involving reactions with or between ionic species, the rate is affected by the presence of other ionic species such as salts like sodium chloride. Ionic strength affects the observed degradation rate constant, k, by its effect on the activity coefficients,/, in Eq. (2.6). Ionic strength, g, is described by 

Consequently, arate constant for a reaction between ionic species depends on ionic strength according to Eq. (2.108)  

As shown in Fig. 82, the effect of ionic strength on the degradation of thiamine hydrochloride is described by Eq. (2.109) rather than by Eq. (2.108).394 

The use of ionic strength changes to probe mechanisms has its limitations. For example, it is necessary to consider changes in the pA) values of ionic species with changes in ionic 

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An effect of ionic strength on as a consequence of effects on the activity coefficient ratio is called a primary salt effect. We will, in Section 8.3, consider this effect quantitatively. 

In many drug solutions, it is necessary to use buffer salts in order to maintain the formulation at the optimum pH. These buffer salts can affect the rate of drug degradation in a number of ways. First, a primary salt effect results because of the effect salts have on the activity coefficient of the reactants. At relatively low ionic strengths, the rate constant, k, is related to the ionic strength, p, according to... 

Equation 7.1.2 characterizes what is known as the primary salt effect (i.e. the influence of ionic strength on the reaction rate through the activity coefficients of the reactants and the activated complex). Much early work on ionic reactions is relatively useless because this effect was not understood. Now it is common practice in studies of ionic reactions to add a considerable... 

In primary salt effect, addition of an electrolyte (salt) or variation of ionic strength affects the activity coefficients and hence the rate of reaction. However, in a reaction where H+ or OH ions produced from a weak acid or weak base act as catalyting agent, the addition of salt influences the concentration of H+ or OH ions. Since the rate of reaction depends upon the concentration of H+ or OH, it will be affected by the salt concentration. This phenomenon is known as secondary salt effect. 

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

Primary Salt Effect It is defined as the effect of ionic strength on the velocity of the ionic reaction. This effect is involved in non-catalytic reactions. 

Throughout this chapter we have formulated rate laws in terms of concentrations and ignored activity corrections, as is almost always done in environmental chemistry. However, where ionic strength, I, varies and both reactants are charged, a substantial "primary salt effect" can be expected (167). The effect is described by... 

Kinetic results which apparently do not fit the above treatment of the primary salt effect do so when the observed rates are correlated with the actual ionic strengths rather than the stoichiometric values. The actual concentrations in the reaction solution are calculated using the known value of the equilibrium constant describing the ion pair. This is discussed in Problem 7.5. 

Reaction involves an ion and a molecule and could show a primary salt effect. The equilibrium constant describing the formation of the ion pairs also depends on ionic strength. 

Bronsted-Bjerrum equation — The rate constant k of a chemical reaction involving ionic species A and B may be influenced by other ionic species in solution not directly participating in this reaction (i.e., a dissolved salt, thus the associated observation is called primary salt effect). The change of the rate as a function of the ionic charge of the involved species and the - ionic strength of the solution is given by the Bronsted-Bjerrum equation... 

In comparisons of rate coefficients measured at different pH values, it is necessary to keep a constant ionic strength by addition of a neutral salt such as NaCl, KC1, NaNOs, or NaC104, as the pH of a buffer system may be altered by a change of ionic strength (secondary salt effect [1]). Furthermore, the catalytic rate coefficient of a hydrogen ion or hydroxide ion catalyzed reaction is dependent on the ionic strength also (primary salt effect [1], see also Vol. 2, p. 337). 

Transition-state theory (Gardiner, 1969) Lasaga, 1981) predicts that rate coefficients for second-order reactions in solution depend on the activity coefficients of the reactants and activated complex and therefore vary with ionic strength (the primary salt effect), and this has been found to be the case. However, the dependence of rate coefficients of kinetic reactions in soils on ionic strength has apparently not been studied. 

These catalytic species are often referred to as general acid-base catalysts, in contrast to specific acid-base catalysts. The effects of these species are also sometimes referred to as secondary salt effects in contrast to the primary salt effect of ionic strength described in Section 2.2.7. 

If the ions have like signs, ZaZb is positive and the rate constant increases with increase in ionic strength. If the ions are oppositely charged, the rate constant decreases with increase in ionic strength. Equation (33.56) is a description of the primary kinetic salt effect or, more simply, the primary salt effect. Figure 33.3 shows a verification of this equation by... 

This secondary salt effect can cause either an increase or decrease in the rate constant and can be either larger or smaller than a primary salt effect. (If the rate expression is written in terms of the true hydrogen-ion concentration, a secondary salt effect obviously does not exist.) In some instances, both the primary and secondary salt effects occur, in which case cancellation of effects may cause the reaction rate to appear to be independent of ionic strength. 

Equation (7.1.2) characterizes what is known as the primary salt effect (i.e., the influence of ionic strength on... 

Fig. 1. The primary salt effect-variations of reaction rates with ionic strength. The circles are observed values the straight lines are theoretical (Equation (2)).

The effects of temperature and ionic strength on the oxidation of co-ordinated thiocyanate in cis- and /m j-[Co(en)2(NH3)NCS]2+ by peroxodisulphate have been investigated. A rate equation, first-order in both reactants, was found and rate constants of 1,44 x 10 and 5.12 x 10 1 mol s were evaluated for the cis- and fra/is-complexes respectively at 45 °C (extrapolated to /=0). Careful analysis of the activation parameters reveals that although the effect of ionic strength on the rate is consistent with a primary salt effect, the theory does not extend to the derived activation parameters. 

A. Reaction rate and ionic strength, also known as the primary salt effect. For reactions between ions in solution, the presence of other ions influences the reaction rate. The Debye-Hiickel theory provides an explanation for both the direction and the magnitude of the effect. It stems from the stabilization of an ion by a cloud of oppositely charged ions. The stabilization of an ion of charge z scales as where /u. is the ionic strength, /u = (1 /2) c z due to the other... 

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