Substitutional Equilibria in Solution

We may characterize four types of purely substitutional equilibria of salts in organic solution with the following equations  

Equation (3) represents solvent-solvent competition for the nonanionic coordination positions around MAr In the first approximation, this is a competition between Lewis bases and is related to base strength. As widespread studies of acid-base reactions in general have shown, however, there is no universal ordering of bases, and the order of base strength in which electron sharing is involved is a function of the particular acid. More specifically, for the sort of reaction we have under consideration, it is influenced by specific bonding relations of the metal ion in question. An example is illustrated in Table IV, where the relative values of heat of solution in ethers and in alcohols, for uranyl nitrate and for cobaltous nitrate, show such a specifiicity. 

Detailed data on reaction (3) are not available and would be difficult to obtain. Certain qualitative relations can be seen, however. The effective retention of the full complement of water molecules by uranyl nitrate in extraction has been noted (4), indicating that, in systems in which the activity of water is reasonably high, ordinary oxygenated organic bases do not compete efficiently. In cobaltous nitrate solutions of low water content, addition of small amounts of water produces very marked effects, detectable spectrophotometrically (7). Similarly, for solutions of tctCoCl2 in acetone, 

In alcohols, water is found to be partially displaced, and it is possible to have the water/salt ratio in a saturated solution considerably less than that in the hydrated solid in equilibrium with it (4 10). As with pyridine, an alcohol in a heterogeneous system will sometimes displace water well enough to make the difference between extraction and nonextraction of a metal-ion value (29). Another way in which the differences between strong water-competitors such as alcohol and weaker competitors such as the ketones and ethers are manifested is when liquid such as CC14, with no solvent power of its own for the salt, is mixed with the oxygenated solvent. With thorium nitrate as the test salt, it is seen (Fig. 2) that, whereas addition 

Heats of Solution of Salts in Mixtures of Strong Bases and Acetone (kcal/mole) 

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