200 



In neither case is the vapor saturated with the volatile acids of butter 

 fat during the period of distillation and the liquids in the still are made 

 up of water and the insoluble fatty acids. 



The vapor pressures of the. volatile acids differ and their solubility in 

 water and fatty acids influence the water of distillation. 



Of two acids having approximately the same vapor pressure, the one 

 which is least soluble in the mixed liquid will distill the faster. 



Combining the factors, solubility in water and in the fatty acids, a 

 mathematical expression for the rate of distillation becomes only ap- 

 proximately true. 



Theoretically,^ if we do not keep the volume constant as is the case in 



the R.-M. process of distillation, that is by making no addition to the liquid 



in the still during distillation, 



dy y 



The equation is — =r a - ii"^ integrating we get 



dx X 



Log y=a Log x+c or y=x''. 



y equals amount of volatile acids left in solution and x amount of 



liquid left in still, the original amount being taken as 1. 



On the other hand, if the volume is kept constant as is the case in 



steam distillation x becomes constant. 



In this case we consider the quantity of water removed to the quantity 



of volatile acids left in the still. 



dy 

 \\e then write equation = av Integrating we get the equation 



dx 



1 



—Log y=ax+c, or y= . 



ax 

 y = amount of volatile acide left in solution, original amount being 



taken as 1 ; x = amount of water and volatile acids distilled. 



The above equations do not take into account the condensation in the 



still. 



1 H. D. Richmond, Analyst, 1908. 

 S. Young, fractional distillation. 



