120 J 



XI. 



THE INFLUENCE OF ELECTROLYTIC DISSOCIATION ON THE 

 DISTILLATION IN STEAM OF THE VOLATILE FATTY ACIDS. 



By JOSEPH EEILLY, M.A., D.Sc, F.K.C.Sc.L, 



AND 



WILFEED J. HICKINBOTTOM, B.Sc. 



[communicated by DK. F. E. HACKBTT, M.A.] 

 [Read April 26. Publislied Octoher 11, 1920.] 



Synopsis: — Distillation of dihUe solutions ; electrolytic dissociation. — An attempt 

 is made to explain the deviations from the distribution law observed when dilute 

 solutions of volatile fatty acids are distilled. The evidence indicates that it is 

 probably due to electrolytic dissociation, since the deviation increases with the 

 dilution, and is most marked with acids possessing a comparatively large dissociation 

 constant. The possibility of the occurrence of molecular complexes in the solution 

 or of hydration of the solute does not explain the experimental results. An 

 expression has been deduced to represent electrolytic dissociation during distillation, 

 and to correct the distillation constant. Applying the corrections for electrolytic 

 dissociation, it is found that the distillation constants are practically independent 

 of the concentration, provided that the solution is not highly concentrated. 



The addition of salts. — It is found that the addition of salts to solutions of acetic 

 and butyric acids increases the distillation constant in most cases. It seems pro- 

 bable that this cannot be explained by assuming a suppression of the ionisation of 

 the acid, or an increase in the boiling-point of the solution. 



In a previous publication the authors emphasized the fact that the distillation 

 constants of the lower saturated fatty acids were not constant, but varied 

 with the concentration.' An investigation was undertaken to determine 

 the nature of this disturbing influence. Since the effect appears to be 

 connected with the change in concentration, a method of distillation had 

 to be chosen in which there was a considerable change in the concentra- 

 tion of the solution. The distillation at varying volume fulfils these 

 conditions, and the experimental data was collected almost exclusively from 

 distillations with a progressively diminishing volume. Some experiments 

 were carried out using a progressively increasing volume of liquid in the 

 iiask, but the results were not so uniform as when the volume diminished. 



The apparatus employed consisted of a round-bottomed flask of about 

 400 c.c. capacity. A steam-jacket surrounded the flask in the manner already 

 described in a previous publication (loc. cit.). An electric hot-plate was used 



Sci. Proc. Roy. Dub. Soc, xv, 37, 513. 



