Inert Salts concentration effect

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

The original ion-exchange treatment was developed for competition between reactive and inert monoanions, but Chaimovich, Quina and their coworkers have extended it to competition between mono and dianions (Cuccovia et al., 1982a Abuin el al., 1983a). The ion-exchange constant for exchange between thiosulfate dianion and bromide monoanion is not dimensionless as in (7) but depends on salt concentration, and the formalism was developed for analysing micellar effects upon reaction of dianionic nucleophiles, e.g. thiosulfate ion. 

E> can be measured directly by membrane or vapor pressure osmometry. The application of an alternative method was described recently [64, 65]. It is based on an analysis of the sedimentation equilibrium in an analytical ul-tracentrifuge, where the solution contains the polyelectrolyte as well as a small concentration of an inert salt. In sedimentation equilibrium, the concentration gradients of both components are coupled via a Donnan-type equilibrium, which is governed by the effective charge number zeff of the polyion. Both concentration gradients can be determined in one experiment, when the polyion and the coion of the salt have sufficiently separated absorption bands in the UV or visible range. 

Discussing interface inhibition one finds that the pure squeezing out effect (salting out effect), which may concentrate inhibiting neutral molecules at the metal electrolyte interface, will be rather rare. However, it is possible that the activity of ions or molecules taking place in the corrosion reaction is decreased simply by the accumulation of neutral molecules in the vicinity of the metal surface. Such substances could be alcohol, water soluble inert solids in general, or inert ions. 

In systems where only water molecules are coordinated to the metal ion (but not the organic component), the cause of the higher complex stability is the lower stability of the aquo complex, which is due to the reduced water activity resulting from dilution by the organic solvent [Be 70]. A similar effect may be achieved if the water activity of an aqueous solution is diminished by the dissolution of an inert salt, as shown by the complex stability measurements of Burger et al, [Bu 68] in concentrated alkali metal perchlorate solutions. 

At low pH values, when additional protons are present, the separation step becomes reversible and one observes homogeneous proton recombination. The reaction under these conditions undergoes a transition from unimolecular (correlated pairs) to a bimolecular (or pseudo-unimolecular) reaction. The rate of this recombination reaction is expected to diminish with increasing concentration of inert salt, which screens the Coulombic attraction between the proton and the anion. In fact, the classical Bronsted-Bjerrum theory of salt effects puts all of the effect in the recombination reaction while predicting zero salt effect on the dissociation direction [7]. 

From the relations 2.12 and 2.13, the pH value of a buffer will change with its dilution, because of changes in the ionic strength. Table 2.2 shows the magnitude of the effect of diluting an equimolar solution of a HA/A buffer (total molar concentration stated) with an equal volume of water. The quantity ApHv is defined as the increase in pH of a solution when it is diluted in this way. Dilution of acidic buffers increases the pH with bases there is a decrease. Conversely, the addition of an inert salt such as NaCl... 

The above discussion shows that the dependence of the reaction rate upon the pH contains very important information on the reaction mechanism. Each rate must be measured at constant pH, which usually involves measuring it in a buffer solution. In addition, usually an inert salt is added to maintain ionic strength constant to avoid the salt effects discussed in Chapter 9. In fact, experimentally, the rates are measured at different buffer concentrations, keeping the pH and the ionic strength constant. Under these conditions, and for a constant substrate concentration, there is a linear dependence between the rate and the buffer concentration, as illustrated in Figure 13.3. Extrapolating to zero buffer concentration, one obtains the rate for a constant pH. When general acid-base catalysis is present. 

Effects of the inert inorganic salts on the rate constants (k) for the reactions involving ionic reactants are generally explained in terms of the Debye-Huckel or extended Debye-Huckel theory. In actuality, the extended Debye-Huckel theory involves an empirical term, which makes the theory a semiempirical theory. However, there are many reports in which the effects of salts on k of such ionic reactions cannot be explained by the Debye-Huckel theory. For instance, pseudo-first-order rate constants (k bs) for the reaction of HO with acetyl salicylate ion (aspirin anion) show a fast increase at low salt concentration followed by a slow increase at high concentration of several salts. But the lowest salt concentration for each salt remains much higher than the limiting concentration (0.01 M for salts such as M+X ) above which the Debye-Huckel theory is no longer valid. These k bs values fit reasonably well to Equation 7.48... 

Here (log cmc) is tire log cmc in tire absence of added electrolyte, is related to tire degree of counterion binding and electrostatic screening and c- is tire ionic strengtli (concentration) of inert electrolyte. Effects of added salt on cmc are illustrated in table C2.3.7. 

Ionic reactions are usually studied in the presence of an inert electrolyte so as to avoid salt effects. The investigator decides on one ionic strength and then adjusts the concentration of the electrolyte from one experiment to the next as the reactant... 

It is generally observed that the rate of reaction can be altered by the presence of non-reacting or inert ionic species in the solution. This effect is especially great for reactions between ions, where rate of reaction is effected even at low concentrations. The influence of a charged species on the rate of reaction is known as salt effect. The effects are classified as primary and secondary salt effects. The primary salt effect is the influence of electrolyte concentration on the activity coefficient and rate of reaction, whereas the secondary salt effect is the actual change in the concentration of the reacting ions resulting from the addition of electrolytes. Both effects are important in the study of ionic reactions in solutions. The primary salt effect is involved in non-catalytic reactions and has been considered here. The deviation from ideal behaviour can be expressed in terms of Bronsted-Bjerrum equation. 

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