Reilly and HicKmBorTOM — Bis tillatton of Volatile Fatty Acids. 12.'1 



It is well known that in the liquid state formic and acetic acids are 

 associated, and also, to a less extent, the higher acids. It is probable that in 

 aqueous solution the molecular complexes are broken down into single 

 molecules, the dissociation increasing with the dilution.' If the dissociation 

 has not proceeded to completion at the dilution employed, the experimental 

 results will be further influenced by the breaking down of molecular com- 

 plexes into simpler aggregates. It might, however, be expected that in very 

 dilute solutions the dissociation would be complete, and, consequently, that 

 the constants would reach a limiting value. The experimental evidence 

 does not support this. 



II. C. Jones'- has suggested that in aqueous solution some part of the 

 water is combined with the dissolved substance to form a hydrate. The 

 amount of hydration increases with the dilution. This may also be a factor 

 which influences the distillation constant. 



In dilute aqueous solutions the lower fatty acids are partly dissociated 

 into ions, and there will be a progressive alteration in the dissociation on 

 increasing the dilution, if it be assumed that the ions take no part in the 

 distillation. It is seen that ionisation has the effect of reducing the actual 

 concentration of the undissociated acid. The values for the distillation 

 constant, calculated on the assumption that the congentration of the acid in 

 the solution is equal to that determined by titration, must be corrected 

 therefore for the ionisation. The values calculated directly from the 

 experimental results will become less with increasing dilution, if the ionisation 

 of the acid is not taken into account. 



The dissociation of a weak electrolyte in solution can be represented by the 

 following equation : — 



i - a 



where c = - 



V 



(v = volume in litres containing 1 gram raol.), 

 a = dissociation, 

 or a^ = k/c, where a is small. 



The equation 



dx 

 dv=^ 



a - X 



^ Cf. Murray, Amer. Chem. Journ., 1903, 30, 193. 

 2 Araer. Chem. Journ., 1900, 23, 103. 



