Reilly and Hickinbottom — Volatile Fatty Acids. 519 



Theory of distillation of a dilute solution of a volatile substance in water. 



According toNernst's law of distribution 



cone, in vapour phase 



: — t-. — — — : = constant. 



cone, m liquid phase 



The proportions of constituents in the distillate are assumed to be the 

 same as the ratio in the vapour phase in a state of equilibrium. 



This assumption is probably not strictly accurate, but approaches accuracy 

 if distillation is carried on slowly and regularly. 



Let x = amount of acid in distillate after volume has been distilled, 

 a = initial amount of acid in flask, 

 p = density of water vapour in flask, 

 <t = weight of water per unit volume of distillate ; 

 then if dx is the quantity of acid coming over in a quantity dv of distillate, 



C-— = concentration in vapour phase approximately, 



(TOV 



— y = „ „ liquid „ 



where V = constant volume of liquid in a flask ; then :- 



+ ft- 



pdx , a - x 



<t8v 



Sx ka 



^~ = + ~v r ( a ~ x ) = + \{a - x), 

 bv pV . x ■" 



which gives % = a (1 - e~ *») 



or a - x = ae~ Aw 



a 



= e M 



, a - x 



A = -log:, 



X 



writing A - - W in . 



6 v 5l ° a - x 



Then A is a constant, assuming - is constant. 



P 



This constant [A] Stein writes as - log — — , 



A - X 



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