Equilibrium, salt effect

If the rate equation contains the concentration of a species involved in a preequilibrium step (often an acid-base species), then this concentration may be a function of ionic strength via the ionic strength dependence of the equilibrium constant controlling the concentration. Therefore, the rate constant may vary with ionic strength through this dependence this is called a secondary salt effect. This effect is an artifact in a sense, because its source is independent of the rate process, and it can be completely accounted for by evaluating the rate constant on the basis of the actual species concentration, calculated by means of the equilibrium constant appropriate to the ionic strength in the rate study. 

Equivalent mechanisms. Consider two schemes for the first term in Eq. (9-47). One involves a prior equilibrium to form FeOH2+, the other SnOH+. Calculate the salt effects at each step and for the overall composite that gives ki. 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

Ciparis, J. N., "Data of Salt Effect in Vapor-Liquid Equilibrium," Lithuanian Agricultural Academy, Kaunas, Lithuania, USSR, 1966. 

Salt effects in kinetics are usually classified as primary or secondary, but there is much more to the subject than these special effects. The theoretical treatment of the primary salt effect leans heavily upon the transition state theory and the Debye-Hii ckel limiting law for activity coefficients. For a thermodynamic equilibrium constant one should strictly use activities a instead of concentrations (indicated by brackets). 

Al-Sahhaf, T. and Kapetanovic, E. Salt effects of lithium chloride, sodium bromide, or potassium iodide on liquid-liquid equilibrium in the system water + 1-butanol, J. Chem. Eng. Data, 42(l) 74-77, 1997. 

The decomposition of the salt [(cp)Fe(CgHg) Na+] displaces the equilibrium with the participation of this salt to the right. Hence, the difference in stability of the (20e) anion (cp)Fe(CgHg) depending on the cation nature is the major factor responsible for the salt effect. The NaPFg salt can induce electron transfer between neutral organometallic species very efficiently. 

The same problems encountered in mineral-water isotopic thermometry could be present in T estimates based on isotopic equilibrium between brine and dissolved species, such as SO ". Isotope geo thermometers based on mineral couples or gaseous couples (e.g., CH4-CO2), however, are not affected by isotopic salt effect in brines (Horita et al., 1993b). 

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... 

For comparison with these data, the types of salt effects that have been observed for model protein and polypeptide systems that achieve true thermodynamic equilibrium in solution can be summarized into three classes ... 

The topic covered in the 10 papers of the first section is commonly referred to as salt effect in vapor-liquid equilibrium and is potentially of great industrial importance. This salt effect leads to extractive distillation processes in which a dissolved salt replaces a liquid additive as the separating agent the replacement often results in a greatly improved separating ability and reduced energy requirements. Two papers in this volume, those by Sloan and by Vaillancourt, illustrate the use of such processing to concentrate nitric acid from its aqueous azeotrope. Nevertheless, the effect has not been exploited by industry to nearly the extent that would seem to be merited by its scientific promise. 

Correlation and Prediction of Salt Effect in Vapor-Liquid Equilibrium... 

A review is presented of techniques for the correlation and prediction of vapor-liquid equilibrium data in systems consisting of two volatile components and a salt dissolved in the liquid phase, and for the testing of such data for thermodynamic consistency. The complex interactions comprising salt effect in systems which in effect consist of a concentrated electrolyte in a mixed solvent composed of two liquid components, one or both of which may be polar, are discussed. The difficulties inherent in their characterization and quantitative treatment are described. Attempts to correlate, predict, and test data for thermodynamic consistency in such systems are reviewed under the following headings correlation at fixed liquid composition, extension to entire liquid composition range, prediction from pure-component properties, use of correlations based on the Gibbs-Duhem equation, and the recent special binary approach. 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

Such complexities tend to explain why progress has been relatively slow, at least until recently, in the formulation of effective relations and techniques for the representation of salt effect in vapor-liquid equilibrium. 

The original equation for salt effect in vapor-liquid equilibrium, proposed by Furter (7) and employed subsequently by Johnson and Furter (8), described the effect of salt concentration on equilibrium vapor composition under the condition of a fixed ratio of the two volatile components in the liquid phase. The equation, derived from the difference in effects of the salt on the chemical potentials of the two volatile components, with simplifying approximations reduces to the form... 

Pure-component properties from which prediction of salt effect in vapor-liquid equilibrium might be sought, include vapor pressure lowering, salt solubility, degree of dissociation and ionic properties (charges and radii) of the salt, polarity, structural geometry, and perhaps others. 

It has been generally held, at least until recently, that a salt dissolved in the liquid phase would enrich the equilibrium vapor in the component in which it was less soluble and impoverish it in the component in which it was more soluble. It was also assumed that the magnitude of the effect on vapor composition depended not only on how much salt was present but also on the degree of difference between the solubilities of the salt in the two liquid components taken separately. Various investigators, including Tursi and Thompson (28) and Fogg (29), have tried to relate the salt effect to this solubility difference alone, but their success... 

Over the years, various other theories and models have been proposed for predicting salt effect in vapor-liquid equilibrium, including ones based on hydration, internal pressure, electrostatic interaction, and van der Waals forces. These have been reviewed in detail by Long and McDevit (25), Prausnitz and Targovnik (31), Furter (7), Johnson and Furter (8), and Furter and Cook (I). Although the electrostatic theory as modified for mixed solvents has had limited success, no single theory has yet been able to account for or to predict salt effect on equilibrium vapor composition from pure-component properties alone. 

The research programs on extractive distillation by salt effect and on salt effect in vapor-liquid equilibrium at the Royal Military College of Canada are supported by the Defence Research Board of Canada, Grant No. 9530-142. 

A method of prediction of the salt effect of vapor-liquid equilibrium relationships in the methanol-ethyl acetate-calcium chloride system at atmospheric pressure is described. From the determined solubilities it is assumed that methanol forms a preferential solvate of CaCl296CH OH. The preferential solvation number was calculated from the observed values of the salt effect in 14 systems, as a result of which the solvation number showed a linear relationship with respect to the concentration of solvent. With the use of the linear relation the salt effect can be determined from the solvation number of pure solvent and the vapor-liquid equilibrium relations obtained without adding a salt. 

Then, we can obtain the preferential solvation number from the observed values of the salt effect. As the concentration of solvent is decreased by the number of solvated molecules, the actual solvent composition participating in the vapor-liquid equilibrium is changed. Assuming that a salt forms the solvate with the first component, the actual composition X a is given by... 

The establishment of the method of prediction has been attempted by the reverse calculation of the preferential solvation number from measured values, using Equations 4 and 7 which are based on the assumption that the salt effect in the vapor-liquid equilibrium is caused by the preferential solvation formed between a volatile component and a salt. The observed values were selected from Ciparis s data book (4), Hashitani s data (5-8), and the author s data (9-15). S was calculated by Equation 7 when the relative volatility as in the vapor-liquid equilibrium with salt is increased with respect to the relative volatility a in the vapor-liquid equilibrium with salt, but by Equation 4 when as is decreased. The results are shown in Figures 5-12. From these figures, it will be seen that the following three relations exist ... 

Prediction of salt effect. The procedure for calculation of the preferential solvation number S has been described above. By reversing this procedure, that is, by determining xia from S, we can estimate the salt effect using the vapor-liquid equilibrium without a salt. When the salt concentration is below saturation, the preferential solvation number S can be expressed as follows in cases where the solvation is formed with the first component. 

The salt effect is attributable to the formation of preferential solvation from the standpoint of molecular structure. In other words, when calcium chloride, which dissolves readily in methanol but very little in ethyl acetate, was added to the methanol-ethyl acetate system to saturation, calcium chloride formed with methanol the preferential solvate which may be written CaCl2 6CH30H. It was also shown from the observation of solubility that the solvated methanol molecules did not participate in the vapor-liquid equilibrium. 

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