Solubility and Gibbs energy

VIII Thorium Group 17 (halogens) compounds and complexes 

Appendix A and for reasons given there they are not accepted by this review. The data from [1993FEL/RAI] were reinterpreted as described in the previous section using solubility data for ThF4(cr, hyd) and the NONLINT-SIT fitting code cf. Appendix D. 

These data, when combined with other auxiliary data, provide a log, K° value of -(36.829 + 0.790) based on the SIT model the value of logj K° = -33.898 found in [1993FEL/RA1] was based on the Pitzer model for Reaction (V111.6). 

Since both values are based on the same experimental data and the solubiUty study was conducted at relatively low ionic strengths where both the Pitzer and the SIT models should be in good agreement, the major difference in these values is as a result of the differences in values for equilibrium constants of aqueous complexes, particularly for ThFg, used in interpretation of data. 

As discussed earlier, the observed large difference in the standard state equilibrium constants based on the SIT and Pitzer models must result primarily from the differences in the standard state equilibrium constants for the major aqueous species used in these interpretations. Of course, the activity coefficients for the species involved, in particular that for the Th ion, are also different in the two models cf. Section VI.3.2). 

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Box 3.1 Relationship between Solubility and Gibb Energy Change for Solution... 

Although the difference in solubility (and Gibbs energy) may be rather large, it is often observed that some metastable solids show a high inertia in that their rate of conversion to stable solids may be extremely small. We shall return to this later. 

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