Scientia Sinica

No. 1 HUANG, YANG: ACTIVITY COEFFICIENTS OF NONELECTROLYTES 65

In general, when the ionic radius is very large (e.g. iodate ion, picrate ion, or tetraalkylammonium -ion), very little or no solvation occurs!””!. Provided the nonelectrolyte molecules succeed in gathering around the ion, salting-in can still take place even if there is solvation. This is evident from the fact that the ratio of nonelectrolyte to water in the “free water” amounts only to a small fraction, and the removal of some nonelectrolyte molecules affects this ratio far more than the removal of the same number of water

molecules.

We have not considered the question of the breaking of water structure by the ion, since there is reason to believe that a very large ion has little effect on the structure of water!”!.

Our viewpoint is that provided there is no other disturbing factor, any kind of ion, (positive or negative), with a sufficiently large size will salt-in nonelectrolyte molecules of large sizes. Hence, the belief that the cation causes salting-out, and the anion, salting-in of the nonelectrolyte is incorrect.

We plan to test the mechanism proposed above by a series of experimental studies. We report here our first experimental study, namely the effect of five cobalt-ammines on the solubilities of 7-valeric acid in water. Since these complex salts possess large positive cations and as the molecules of n-valeric acid are also large, the London dispersion force between the two should be rather strong and we expect a salting-in of the acid. We choose n-valeric acid as a “‘nonelectrolyte” for the following reasons: (1) Being a weak acid (K,=10°), it can be determined readily by titration with standard alkali; (2) being a liquid, it reaches equilibrium easily with salt solution; (3) its solubility in water is neither too large nor too small, (37.43 g per litre at 25°C); (4) the cobalt complex is stable in acid solution”.

Literature reports only one work using cobalt complexes for the study

of salt effect, and that is the work of two Soviet chemists, Kozakevich and ©

Yankelevich@. They used phenol as the “nonelectrolyte”. Their complex ions contain NH;, H,O, C,H,(NH2)2, CoH;NH2, Cl” or C,0," as radicals.

EXxPERIMENTAL™

Preparation of reagents: The following reagents are prepared.

Cobalt-ammines: Five cobalt-ammines are prepared from the C. P. CoCl, 6H,O, or Co(NO;),:6H2O of Shika Company.

(1) Hexamminecobaltic chloride, [Co(NH;).]Cl, (luteo): This 1s prepared according to the method of Bjerrum and Reynold™!.

=This work began in the winter of 1949 and was interrupted by the illness of the junior author (Yang). It was resumed in the spring of 1954.