Scientia Sinica

No. | HUANG, YANG: ACTIVITY COEFFICIENTS OF NONELECTROLYTES ‘ 71

The results are contrary to our expectation. The chief reason why n-valeric acid is salted’ out by cobaltammines instead of being salted in is probably that the sizes of the complex cations are not sufficiently large. Before the experiment, we judged their sizes superficially by their chemical formulas. Actually they are not large. For example, from the crystal structure of [Co(NH;),] I; the “crystal radius” of [Co(NH;).]*** is only about 2.5 A.* The crystal structure of [Co(NH3)«] 3 is similar to the combined structure of rock salt and Seite” Each unit cell contains four molecules of [Co(NH;).] 1; four [Co(NH:),]*** form a face-centered cubic, eight I occupy the centres of eight small cubes, one [ the centre of the body, and three I the centres of twelve side lines. The side length of the unit cell is 10.89 A. Hence the nearest Co-I distance along the body diagonal is 10.89 x 3/4 or 4.71A. Since the “crystal radius” of Ir is 2.2A, that of [(Co(NH;).]*** is at most 2.5 A. Similarly, from the crystal structure of K;[ Co(NO,)<] and the side length of unit cell 10.44 A'!, we find that the “crystal radius” of [Co(NO;)6]7~~ is about 3.2A. Hence the crystal radii of the cations of our cobaltammines lie probably within the range of 2.5-3.2 A, and they are not very large. It is evident that the dispersion force between the ion and n-valeric acid cannot compete with the electrostatic force between the ion and water and the result is salting-out. It is significant that the cation of luteocobaltic chloride, i.e. [Co(NH;).]*** which has the smallest crystal radius, is salted out to the largest extent (see Table 2).

There is another possible factor which helps to bring about the saltingout of w-valeric acid. The carboxyl group of the acid is known to be an electron-attracting radical'*!. It probably repels the large cation and lowers down the dispersion effect between this ion and the acid.

SUMMARY

Mechanisms based on interaction by van der Waals-London dispersion force for explaining the salting-in effect of nonelectrolyte in water by large jons are proposed for the following cases; (1) ions of symmetrical structure and (2) ions of asymmetrical structure.

For the test of such mechanisms, we have measured the solubilities of n-valeric acid at 25°C in the aqueous solutions of the following five cobaltammines: (1) hexamminecobaltic chloride (luteo), (2) chloropentamminecobaltic chloride (purpuro), (3) nitritopentamminecobaltic chloride (xantho), (4) trans-dinitritotetramminecobaltic chloride, and (5) cis-dinitritotetramminecobaltic chloride. All these salts salt out #-valeric acid.

The activity coefficient, f, of n-valeric acid as determined by the ratio of its solubility in pure water, So, to that in a salt solution, S, is calculated for

*We are indebted to Prof. Y. C. Tang for this information.