Equilibrium activity coefficients, ionic media

In this work the concepts of ionic medium, effective ionic strength and free versus total activity coefficients are examined. Then they are applied to the study of permissible and incorrect translations of equilibrium constants from one medium to another. 

One method takes into account the individual characteristics of the ionic media by using a medium-dependent expression for the activity coefficients of the species involved in the equilibrium reactions. The medium dependence is described by virial or ion interaction coefficients as used in the Pitzer equations and in the specific ion interaction model. 

The specific ion interaction approach is simple to use and gives a fairly good estimate of activity factors. By using size/charge correlations, it seems possible to estimate unknown ion interaction coefficients. The specific ion interaction model has therefore been adopted as a standard procedure in the NEA Thermochemical Data Base review for the extrapolation and correction of equilibrium data to the infinite dilution standard state. For more details on methods for calculating activity coefficients and the ionic medium/ ionic strength dependence of equilibrium constants, the reader is referred to Ref. 40, Chapter IX. 

Experience shows that the activity coefficients on this scale stay near unity (usually within experimental error) as long as the concentrations of the reactants are kept low, say less than 10% of the concentrations of the medium ions. The activity ( concentration) of several ions, notably H+, can be determined conveniently and accurately by means of e.m.f. methods, either with or without a liquid junction. In the latter case the liquid junction potential is small (mainly a function of [H+] ) and easily corrected for (3). The equilibrium constant for any reaction, on the ionic medium scale, may then be defined as the limiting value for the concentration quotient ... 

Since an equilibrium is assumed between the transition state and the reactant(s), and because the corresponding equilibrium constant can be expressed in terms of activity coefficients and concentrations to account for the non-ideality of the medium, it follows that there should be an activity coefficient effect upon reaction rates. This is observed as a dependence of the rate constant upon ionic strength - the kinetic electrolyte effect [2]. Thus, for a bimolecular reaction,... 

Because of difficulties in precisely calculating the total ion activity coefficient (y) of calcium and carbonate ions in seawater, and the effects of temperature and pressure on the activity coefficients, a semi-empirical approach has been generally adopted by chemical oceanographers for calculating saturation states. This approach utilizes the apparent (stoichiometric) solubility constant (K ), which is the equilibrium ion molal (m) product. Values of K are directly determined in seawater (as ionic medium) at various temperatures, pressures and salinities. In this approach ... 

The equilibrium between H3O+ and OH ions will exist in pure water and in all aqueous solutions if the ionic strength of the medium is low, the ionic activity coefficients may be taken as unity, and hence the ionic product of water, now represented by is given by... 

The majority of equilibrium constants (including those given in Tables I-III) have been determined in solutions containing high (usually >1 M) and constant concentration of an inert electrolyte (e.g., alkali perchlorate). In this way the variation of the activity coefficients of the studied species (kept below 0.1 M) is so small that no correction factors have to be applied. The equilibrium constants, however, are strictly valid only in the ionic medium in which they have been determined. In order to avoid the burden of experimental determination of equilibrium constants in each ionic medium encountered, semi-empirical methods have been developed to recalculate the constants from one ionic medium to another. One such method is the specific interaction theory (SIT), developed by Guggenheim il55) and Scatchard (156,157) on the basis... 

The fluoride complexes were also investigated in a nitrate ionic medium and the authors note that the distribution coefficients are lower than in a perchlorate medium, suggesting formation of a ternary complex ThF(N03). This review suggests that a more probable explanation is that the effect is due to changes in activity coefficients between the two media and therefore does not accept the suggestion of formation of ternary complexes. No equilibrium constant was determined for acetate and formate complexes because of indications of coextraction of a Th-TTA-acetate/formate complex, (so-called synergistic extraction). 

The dissociation constants for tri- di- and mono-chloroacetic acids are 0.2, 0.05 and 0.0014 M, respectively. The experimental procedures and data analysis used in this study are satisfactoiy and the numerical values of the proposed equilibrium constants are therefore considered to be reliable, but are not selected since data on organic ligands are not included in the present review. However, the equilibrium constants for the weak complexes require extensive changes in the ionic medium and the observed variation in distribution coefficients could therefore also be a result of activity coefficient variations. 

The reliability of the values presented in literature raises no doubts. To characterize the Cu Cu(II) P-alanine system, we selected from [16] such data, which best matched the ionic strength of the solutions log = 7.34, log P2 = 12.8, log = 10.2, log = 13.84. When calculating equilibrium potentials, apart from these values, the values of standard potentials and the activity coefficients of Cu and Cu " ions equal to 0.6 and 0.07 [18], respectively, were used. The latter value is rather low in the sulfate medium (solutions containing 0.3 M K2SO4) because Cu " ions tend to form associates with SO ". 

For a neutral complex such as ThF4 or UF4, the concentration in solution in equilibrium with a solid phase of the same stoichiometry has to be constant, independent of the concentration of ligand present. Nor does the solubility depend much on I, as the activity coefficient of a neutral species does not change very much with the ionic medium. 

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