Specific Ion Interaction Theory

Equation (2.43) describes the effect of long-range forces and howtheycan be modified by short-range interactions between ions. In a solution, however, short-range interactions between ions and solvent molecules need to be considered, and it has been found that such reactions have an approximate variation which is proportional to the concentration of the ionic medium. Therefore, the expression for the activity coefficient can be extended to 

According to the specific ion interaction theory, the activity coefficient of an ion j of charge z in a solution of ionic strength / can be described by Eq. (2.56), when expressed in the form of Eq. (2.57)  

An individual activity coefficient cannot be measured directly by experimental methods. Therefore, a mean activity coefficient y of an electrolyte NX of concentration m dissociating into cations of valence Zj and V2 anions of valence Z2 is given by 

a plot of log + zj Z2. D versus should be linear, pass through the origin and have a slope of 2vj V2 (jv,x)/( i+V2) where D is given by 

With respect to the osmotic coefficient, described by Eq. (2.55), incorporation of the expression for log y as given in Eq. (2.58) into the integral used to derive Eq. (2.55) leads to an expanded equation for the osmotic coefficient, namely. 

---

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

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],... 

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

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... 

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. ... 

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 [73PIT] and in the specific ion interaction theory. 

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

When using the specific ion interaction theory, the relationship between the redox potential of the couple PuO /Pu " in a medium of ionic strength /, and the corresponding quantity at / = 0 should be calculated in the following way. The reaction in the galvanic cell ... 

Table B-4, Table B-5 and Table B-6 contain the selected specific ion interaction coefficients used in this review, according to the specific ion interaction theory described. Table B-4 contains cation interaction coefficients with Cl", CIO andNOj. Table B-5 anion interaction coefficients with Li, with Na or NH and with K. The coefficients have the units of kg-mol and are valid for 298.15 K and 1 bar. The species are ordered by charge and appear, within each charge class, in standard order of arrangement, cf. Section 11.1.8.

Figure V-16 Extrapolation to / = 0 of experimental results for the formation of Zrp using the specific ion interaction theory. The data refer to perchlorate media and are taken from [67NOR] (T), [49CON/MCV] ( ), [63AHR/KAR] ( ), [73NOR] (A) and [69NOR2] (o). The data from the first three of these studies have been recalculated in this review (see Appendix A and Table V-14). The dashed lines are back-propagated data using the 95% uncertainties in the stability constant and interaction coefficient to 1 = 5 mol kg". ...

Extrapolation to / = 0 of experimental data for the formation of ZrOH using the specific ion interaction theory.105... 

---