Activity Coefficients of Acids, Bases, and Salts

ACTIVITY COEFFICIENTS OF ACIDS, BASES, AND SALTS (continued)... 

Standard KCl Solutions for Calibrating Conductivity Cells Molar Conductivity of Aqueous HF, HCl, HBr, and HI Equivalent Conductivity of Electrolytes in Aqueous Solution Ionic Conductivity and Diffusion at Infinite Dilution Activity Coefficients of Acids, Bases, and Salts... 

The mass action law formalism, through its equilibrium constants, takes into account the interactions of the solvent with the various acids, bases, and salts these certainly are the dominant effects, comparable to Kepler s law in the above analogy. However, the formalism of the mass action law does not explicitly consider the mutual interaction of the solute particles, nor the effect of these solutes on the concentration of the solvent. Activity coefficients /have therefore been introduced in order to incorporate such secondary effects they are individual correction factors that multiply... 

M.K. Chantooni, Jr. and I.M. Kolthoff, Acid-base equilibria in methanol, acetonitrile, and dimethyl sulfoxide in acids and salts of oxalic acid and homologs, fumaric and o-phthalic acids. Transfer activity coefficients of acids and ions, J. Phys. Chem. 79 (1975), pp. 1176-1182. 

Ionic equilibria in solvents of low relative permittivity have been described in the literature and equations for calculating titration curves have also been proposed. The equations involve the partial dissociation of the acids, bases, and salts in these media, and the effect of the activity coefficients. The equations can be applied to pH and titration curve computation in solvents such as t-butyl and isopropyl alcohols, ethylene diamine, pyridine, or tetrahydrofuran. 

For symmetric electrolytes i=l for 1 2 electrolytes (e.g., Na2S04), 1 3 electrolytes (AICI3), and 2 3 electrolytes ([Al2(S04)3], the corresponding valnes of A, are 1.587, 2.280, and 2.551. Mean ionic activity coefficients of many salts, acids, and bases in binary aqneons solutions are reported for wide concentration ranges in special handbooks. 

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... 

So far, we have focused on how differences in molecular structure affect the solubilities and activity coefficients of organic compounds in pure water at 25°C. The next step is to evaluate the influence of some important environmental factors on these properties. In the following we consider three such factors temperature, ionic strength (i.e., dissolved salts), and organic cosolutes. The influence of pH of the aqueous solution, which is most important for acids and bases, will be discussed in Chapter 8. 

Other than specific effects that result from conventional chemical interactions (such as acid-base or complex formation), the main factors to be considered are hydration of ions, electrostatic effects, and change in dielectric constant of the solvent. For example, hydration of ions of added salt effectively removes some of the free solvent, so that less is available for solution of the nonelectrolyte. The Setschenow equation probably best represents the activity coefficient of dilute solutions (less than 0.1 M) of nonelectrolytes in aqueous solutions of salts up to relatively high concentrations (about 5 M) ... 

A more quantitative prediction of activity coefficients can be done for the simplest cases [18]. However, for most electrolytes, beyond salt concentrations of 0.1 M, predictions are a tedious task and often still impossible, although numerous attempts have been made over the past decades [19-21]. This is true all the more when more than one salt is involved, as it is usually the case for practical applications. Ternary salt systems or even multicomponent systems with several salts, other solutes, and solvents are still out of the scope of present theory, at least, when true predictions without adjusted parameters are required. Only data fittings are possible with plausible models and with a certain number of adjustable parameters that do not always have a real physical sense [1, 5, 22-27]. It is also very difficult to calculate the activity coefficients of an electrolyte in the presence of other electrolytes and solutes. Even the definition is difficult, because electrolyte usually dissociate, so that extrathermodynamical ion activity coefficients must be defined. The problem is even more complex when salts are only partially dissociated or when complex equilibriums of gases, solutes, and salts are involved, for example, in the case of CO2 with acids and bases [28, 29]. 

In acid-base catalysis it is convenient to distinguish between a primary salt effect and a secondary salt effect. The primary effect is related to the dependence of the rate constant on the activity coefficients of the species entering the rate law... 

Yi is the individual activity coefficient of one ionic species. As one has always cations and anions together within an electrolyte, one measures experimentally an average activity coefficient y+, which is closely related to the individual activity coefficients Y. For a simple electrolyte composition containing only one salt, acid, or base of stoichiometry. Cl Ay", Y is defined by Equation 1.42. 

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