Ion interaction SIT coefficients

6 Summary of selected data for Th(IV) hydroxide complexes VII.3.6.1 Ion interaction (SIT) coefficients 

For polynuclear Th(IV) hydroxide complexes which are found to be major species or at least species with significant contributions in potentiometric studies, Ae(n,m) values are derived in Section VII.3.4.1 by hnear SIT regression from the conditional equilibrium constants logm ,(Th (OH) , 298.15 K) in perchlorate, nitrate and chloride media. The ion interaction coefficients for the Th(IV) hydroxide complexes 8(Th (0H) ,X ), withX = CIO, NOj and CT are calculated according to  

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Table A-81 Hydrolysis constants logm Pnm zero ionic strength and ion interaction (SIT) coefficients Sy (kg-mol ) selected in [2001NEC/KIM]. (In this paper the equilibrium constants logj have been reported for the reactions + OH ...

In this study ion interaction (SIT) coefficients for the Th" ion and trace activity coefficients in NaC104, NaNOs and NaCl solution were determined by solvent extraction with tri-n-butyl phosphate (TBP) at 22°C. The liquid-liquid phase equilibria between dilute to concentrated NaC104, NaNOs and NaCl solutions containing 0.01-0.02 M and (1 - 8) X 10 M Th" and organic phases consisting of 10-50 vol.% TBP in n-dodecane are given by ... 

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.

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],... 

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... 

SIT ion interaction coefficient between substance B and substance 62 stoichiometric coefficient of substance B (negative for reactants, positive for products) stoichiometric coefficient general equation for a chemical reaction equilibrium constant... 

Ion interaction coefficients, for the selenocyanate ion and its metal ion complexes are not available. Data for the thiocyanate ion have therefore been used as an approximation for extrapolation by SIT of selenocyanate equilibrium constants to standard conditions. 

The activity coefficients of Th" and the complexes ThX ate calculated with the specific ion interaction theoiy, SIT, assuming that logi y.j. 4+ and in... 

The SIT coefficients used by this review for the determination of standard state solubility constants and equilibrium constants for hydroxide, fluoride, and carbonate complexes from experimental data in chloride and nitrate media refer to the strict ion interaction approach. They imphcitly include the formation of weak Th(IV) complexes or ion pairs with nitrate and chloride ions. However, they must not be combined with... 

Throughout the present review the SIT is used for ionic strength corrections. However, numerous computer codes for geochemical model calculations, in particular for calculations in concentrated chloride solutions, are based on the ion interaction equations of Pitzer [1991PIT]. Pitzer parameters reported in the literature to calculate activity coefficients for the Th" ion in chloride solutions are briefly discussed and summarised in Table VI-2. 

As illustrated in Figure Vt-5, the different approaches and ion interaction coefficients can lead to appreciably discrepant activity coefficients for the Th" ioa For instance the values of in 0.1 or 0.5 m NaCl and hence the solubility constants calculated in [2000RAI/MOO] and [2003NEC/ALT] with the Pitzer model and SIT respectively for microcrystalline Th02(cr) at low pH, where hydrolysis is actually negligible, differ by about two orders of magnitude. 

The SIT coefficients selected in this table refer to a strict ion interaction approach where the effect of... 

The equilibrium constants and SIT coefficients selected in this table refer to a strict ion interaction approach where the effect of chloride or nitrate complexation is included in the interaction coefficients. Therefore these values must not be combined with the formation constants of chloride and nitrate complexes discussed in Sections VIII.2.2.1 and X. 1.3.3. 

Table VIII-19 SIT coefficients e(Th", Cl) based on a reanalysis of hteratuie data from [1950DAY/STO] and [1952WAG/STO] for the extraction of thorium from (H, Na ) (CT, CIO4) solutions using the strict ion interaction approach, i.e. without assuming the formation of chloride complexes. The calculations were performed with the NONLINT-SIT code and alternatively by hnear regression in plots of logiof) vs. m -see the Appendix A entry for [1950DAY/STO].

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