Specific ion interaction

A more significant body of literature focuses on the use of protoplasts in understanding processes related to stress tolerance. The role of Ca in salt toleranee has been evaluated using maize root protoplasts. Exposure of the plasmalemma directly to external media revealed a non-specific replacement of Ca by salt. Sodium was found to replace Ca though this could be reversed by adding more Ca (Lynch, Cramer Lauchli, 1987). This approach assists in understanding the role of specific ion interaction in enhancing salt tolerance and is potentially applicable to studies on the molecular basis for ion specificity of plant membranes. 

The approach introduced by E. A. Guggenheim and employed by H. S. Harned, G. Akerlof, and other authors, especially for a mixture of two electrolytes, is based on the Br0nsted assumption of specific ion interactions in a dilute solution of two electrolytes with constant overall concentration, the interaction between ions with charges of the same sign is non-specific for the type of ion, while interaction between ions with opposite charges is specific. 

The method preferred in the NBA Thermochemical Data Base review is the specific ion interaction model in the form of the Bronsted-Guggenheim-Scatchard approach. 

The way in which the activity coelficient corrections are performed according to the specific ion interaction model is illustrated below for a general case of a complex formation reaction. Charges are omitted for brevity. 

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. 

Table 6.2 The Preparation of the Experimental Equilibrium Constants for the Extrapolation to 7=0 with the Specific Ion Interaction Method, According to Eq. (6.13)... 

Example 3 When using the specific ion interaction theory, the relationship between the normal potential of the redox couple in a medium... 

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. 

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. 

Tables 6.3-6.5 contain the selected specific ion interaction coefficients used in this review, according to the specific ion interaction model described in section 6.1. Table 6.3 contains cation interaction coefficients with Cl , CIO4, and NOs Table 6.4 anion interaction coefficients with Li with Na or NH4, and with K. The coefficients have the units of kg mol and are valid for 298.15 K. The species are ordered by charge and appear, within each charge class, in standard order of arrangement [33].

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. 

Table 18.2 Specific ion interaction constants for the Guggenheim equation...

In case of low charged species, and approximately below 3 mol kg 1 the Specific Ion interaction Theory (SIT) [29] can be applied for the calculations of activity coefficients. Data available on interaction coefficients are scarce. But, paradoxically for actinide ions such data are relatively well known. However, in certain cases, they can be estimated from the model developed by L. Ciaviatta [33,34],... 

The basis for this is the theory of specific ion interaction, proposed by J. N. Bronsted, J. Am. Chem. Soc., 44, 877 (1922), to the effect that ions will be uniformly influenced by ions of the same sign but specifically influenced by ions of opposite sign. This is quite reasonable in terms of the above equation, in which the uniform influence is included in the Debye-Huckel terms. The implication is that only ions of opposite sign will approach each other closely enough to produce specific interactions. What the nature of the interactions is, however, is not specified. 

Later modifications of this general approach became known as the specific-ion interaction theory (SIT) because of the explicit dependence... 

Pitzer (1973) re-examined the statistical mechanics of aqueous electrolytes in water and derived a different but semi-empirical method for activity coefficients, commonly termed the Pitzer specific-ion-interaction model. He fitted a slightly different function for behavior at low concentrations and used a virial coefficient formulation for high concentrations. The results have proved extremely fruitful for modeling activity coefficients over a very large range of molality. The general equation is... 

Figure 12 Pore-water chemistry and saturation indices versus depth at the tailings site of the Heath Steele mine. lA represents saturation indices calculated using an ion-association model, and SII represents saturation indices calculated using a specific ion-interaction model (after Ptacek and Blowes, 2000).

Measured in Seawater Calculated from Association Model Specific Ion Interaction ... 

Truesdell-Jones Equations, Specific Ion-Interaction Theory... 

Bronsted-Guggenheim-Scatchard specific ion interaction theory (SIT) (cf. Grenthe and Wanner 1989 Giridhar and Langmuir 1991 Nordstrom and Munoz 1994) is an ion- and electrolyte-specific approach to activity coefficients, which is, therefore, theoretically capable of greater accuracy than the Davies equation. The general SIT equation for a single ion, i, can be written... 

Figure 4.4 Comparison of the ion activity coefficient of Ca as computed using different approaches. These include I) the Davies equation (2) the mean salt approach using the Macinnes convention, and Truesdell-Jones equation (curve labeled Mean salt TJ) (3) the specific ion interaction (SIT) equation and (4) the extended Etebye-Hiickel equation.

The thermodynamic data compiled in the present review (see Chapters III and IV and Appendix E) refer to the reference temperature of 298.15 K and to standard conditions, cf. Section II.3. For the modelling of real systems it is, in general, necessary to recalculate the standard thermodynamic data to non-standard state conditions. For aqueous species a procedure for the calculation of the activity factors is thus required. This review uses the approximate specific ion interaction method (SIT) for the extrapolation of experimental data to the standard state in the data evaluation process, and in some cases this requires the re-evaluation of original experimental values (solubilities, emf data, etc.). For maximum consistency, this method, as described in Appendix B, should always be used in conjunction with the selected data presented in this review. However, some solubility data for highly soluble selenates were evaluated in the original papers by the Pitzer approach. No attempt was made to re-evaluate these data by the SIT method. 

Since a large part of the NEA-TDB project deals with the thermodynamics of aqueous solutions, the units describing the amount of dissolved substance are used very frequently. For convenience, this review uses M as an abbreviation of mol-dm for molarity, c, and, in Appendices B and C, m as an abbreviation of mol-kg for molality, m. It is often necessary to convert concentration data from molarity to molality and vice versa. This conversion is used for the correction and extrapolation of equilibrium data to zero ionic strength by the specific ion interaction theory, which works in molality units (c/ Appendix B). This conversion is made in the following way. Molality is defined as moles of substance B dissolved in 1 kilogram of pure water. Molarity is defined as Cg moles of substance B dissolved in (/ - c M) kilogram of pure water, where p is the density of the solution in kg-dm and the molar weight of the solute in kg-mof. ... 

The summation in Eq. (B.l) extends over all ions k present in solution. Their molality is denoted mk, and the specific ion interaction parameters, s(/, k, / ,), in general depend only slightly on the ionic strength. The concentrations of the ions of the ionic medium are often very much larger than those of the reacting species. Hence, the ionic medium ions will make the main contribution to the value of logm for the reacting... 

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