Ion interaction approach

Pitzer, K. S., 1979, Theory ion interaction approach. In R. M. Pytkowitz (ed.), Activity Coefficients in Electrolyte Solutions, vol. 1. CRC Press, Boca Raton, pp. 157-208. 

Note that in all ion interaction approaches, the equation for mean activity coefficients can be split up to give equations for conventional single ion activity coefficients in mixtures, e.g., Eq. (6.1). The latter are strictly valid only when used in combinations that yield electroneutrality. Thus, while estimating medium effects on standard potentials, a combination of redox equilibria with H " + e 5112(g) is necessary (see Example 3). 

By plotting logio7 , hci + D vs. much a straight line with the slope (H+,cr) is obtained. The degree of linearity should in itself indicate the range of validity of the specific ion interaction approach. Osmotic coefficient data can be treated in an analogous way. 

From Tables 6.3 and 6.4 it seems that the size and charge correlations can be extended to complex ions. This observation is very important because it indicates a possibility to estimate the ion interaction coefficients for complexes by using such correlations. It is, of course, always preferable to use experimental ion interaction coefficient data. However, the efforts needed to obtain these data for complexes will be so great that it is unlikely that they will be available for more than a few complex species. It is even less likely that one will have data for the Pitzer parameters for these species. Hence, the specific ion interaction approach may have a practical advantage over the inherently more precise Pitzer approach. 

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. 

Equations used to calculate L and 4>CP are taken from K. S. Pitzer, Ion interaction approach theory and data correlation , Chapter 3 in Activity Coefficients in Electrolyte Solutions, 2nd Edition, K. S. Pitzer, Editor, CRC Press, Boca Raton, Florida, 1991. Equations for calculating L, L2, Ju and J2 are summarized in K. S. Pitzer, J. C. Peiper, and R. H. Busey, Thermodynamic properties of aqueous sodium chloride solutions , J. Phys. Chem. Ref Data, 13, 1-102 (1984). 

Krumgalz BS, Pogorelsky R, Pitzer KS (1995) Ion interaction approach to calculations of volumetric properties of aqueous multiple-solute electrolyte solutions. J Soln Chem 24 1025-1038... 

Krumgalz BS, Starinsky A, Pitzer KS (1999) Ion-interaction approach pressure effect on the solubility of some minerals in submarine brines and seawater. J Soln Chem 28 667-692... 

Pitzer KS (1991) Ion interaction approach theory and data correlation. In Pitzer KS (ed) Activity coefficients in electrolyte solutions, 2nd edn. CRC Press, Boca Raton, FL, pp 75-153... 

The third mechanistic jq>proach has been proposed by Bidlingmeyer et al. and Deming et al. (B6-B8, K24). In the ion-interaction approach i i pairs do not form in the mobile phase rather it is assumed that there is a dynamic equilibrium of lipophilic ions. This results in the formation of an electrical double layer on the hydrocarbonaceous stationary phase. Retention is based, therefore, upon an electrostatic attraction due to surface charge density of the ion-pairing ions and from a sorption effect onto the nonpolar stationary phase. 

Alternatively, water activities can be taken from Table B-1. These have been calculated for the most common ionic media at various concentrations applying Pitzer s ion interaction approach and the interaction parameters given in [91 PIT]. Data in italics have been calculated for concentrations beyond the validity of the parameter set applied. These data are therefore extrapolations and should be used with care. 

Pitzer, K. S., Ion interaction approach theory and data correlation,... 

Ion Interaction. Ion-interaction theory has been the single most noteworthy modification to the computational scheme of chemical models over the past decade this option uses a virial coefficient expansion of the Debye-Huckel equation to compute activities of species in high ionic strength solutions. This phenomenological approach was initially presented by Pitzer ( ) followed by numerous papers with co-workers, and was developed primarily for laboratory systems it was first applied to natural systems by Harvie, Weare and co-workers (45-47). Several contributors to the symposium discussed the ion interaction approach, which is available in at least three of the more commonly used codes SOLMNEQ.88, PHRQPITZ, and EQ 3/6 (Figure 1). 

A controversy exists between the proponents of ion-association versus the ion-interaction approach. This controversy usually revolves around the issue of chemical realism since many known ion pairs such as CaSO ° were not explicitly defined by the ion-interaction approach. However, the impact of the strong iomc interaction can be reflected in the magnitude of the virial coefficient terms. More recently, this deficiency has been addressed for the carbonate system, however, questions still remain whether mked methodologies reflect the true solution chemistry or are simply forced fits of experimental data. 

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