Solubilities some concluding remarks

Let us now return to equation 6.47, and relate the observed solubility of a salt to the magnitude of the difference between iatticeG° nd AjjydG (equation 6.49), and in particular to the sizes of the ions involved. 

Now consider the application of these two expressions to a series of salts of similar lattice type. For a series of MX salts where X is constant and M varies, if r S r, equation 6.55 shows that there will be little variation in Aiatuce -However, upon dissolution, if r S r, Ai,ydFf°(cation) will be much more negative than Ai,ydFf°(anion) for all values of 

Such a series is exemplified by the alkali metal hexa-chloroplatinates the hydrated sodium salt has a very high solubility, while the solubilities of K2[PtCl5], Rb2[PtCl5] and CszPtClfi] are 2.30 x 10- 2.44 x IQ- and 

04 X 10 0101 per 100 g of water (at 293 K). A similar trend is observed for alkali metal hexafluorophosphates (MPFg). 

Although the above, and similar, arguments are qualitative, they provide a helpful means of assessing the pattern in solubilities for series of ionic salts we stress ionic because equations 6.55 and 6.56 assume an electrostatic model. Our discussions in Section 5.15 and earlier in this section indicated how partial covalent character in silver halides affects solubility trends. 

Ion AhydT7 / kJ moP AhydSVjK- mor rAhydSVkJmor (for r = 298K) AhydG°/kJmol lon 1 

Now consider the application of these two expressions to a series of salts of similar lattice type. For a series of MX salts 

along a series of related salts with increasing r+, but with r 3 r+, Aiauice will remain nearly constant while Ahyd/T becomes less negative. Hence, Ajoi// (and thus AsoiG°) will become less negative (eq. 7.57) and solubility will decrease. 

---

We conclude this section by a few general remarks about extrathermodynamic approaches. These quantitative methods involve empirical approaches that cannot be derived strictly from thermodynamic theory. They are widely used to predict and/ or to evaluate partition constants and/or partition coefficients (see Box 3.2 for nomenclature) of organic compounds. There are many situations in which some of the data required to assess the partitioning behavior of a compound in the environment are not available, and, therefore, have to be estimated. For example, we may need to know the water solubility of a given compound, its partition coefficient between natural organic matter and water, or its adsorption constant from air to a natural surface. In all these, and in many more cases, we have to find means to predict these unknown entities from one or several known quantities. 

---