200 HYDROGEN ION CONCENTRATION 



It is a striking result that at the lower salt concentrations the 

 effect obtained approaches the calculated maximal effect, while 

 at the higher concentrations the observed values are increasingly- 

 low. The obtained values were temporarily completely reversible; 

 and in a series of experiments when ci was kept constant while C2 

 was varied in both directions, Beutner always obtained the same 

 E.M.F. values for the same values of C2. 



As the necessary condition for the maximal effect we specified 

 above that the concentration of the common ion should be the same 

 at the two sides of the oil phase, while it is unequal in the two aque- 

 ous phases. Let us now see how this condition is fulfilled in this 

 particular chain. Salicylic aldehyde is decidedly acid in character, 

 partly because of its phenolic OH-group and partly because it 

 always contains traces of salicylic acid. In contact with water 

 ahnost none of these acid groups pass into the aqueous phase, since 

 the distribution coefficient with respect to water is here extremely 

 small. But a small amount of KCl passes from the contiguous 

 aqueous phase into the oil phase. But, as we saw in the chapter 

 on dissociation in non-aqueous solvents, hydrochloric acid in oil 

 does not behave as a strong but as a weak acid, and hence we have 

 in the oil phase the double decomposition: 



KCl + salicylic acid ?=i K-salicylate + HCl 



a reaction which could not have occurred in aqueous solution. In 

 this way the KCl equilibrium is disturbed, the sahcyfic acid attract- 

 ing, to a certain extent, more K-ions into its solution. If in the 

 oil phase sahcyfic acid should be a stronger acid than HCl, then 

 the former will not remain in the free state, but it will attract enough 

 potassium to transform all of the acid in the oil into the K-salt, 

 independently of the KCl concentration in the aqueous phase. In 

 this case, the K+-concentration will be the same throughout the 

 oil phase. But should the saficylic acid in the oil solution not 

 behave as a stronger acid than HCl, but rather dissociate to the 

 same extent as HCl, even then the same result will ensue, with 

 greater or lesser completeness, depending upon the conditions. 

 When the KCl concentration in the water phase is very small, 

 then, after the establishment of equilibrium, the K+ will be present 

 in the oil almost entirely as K-salicylate. But when the KCl con- 

 centration in the water is very great, then it will also be greater in 



