Separation of Three Liquids by Fractional Distillation. 9 



"When, however, we have to deal with a mixture of three 

 substances, the difficulty is greatly enhanced, and if the 

 boiling-points are not far apart it may be almost, if not quite 

 impossible to separate any quantity of the middle substance 

 in a state of purity by the ordinary methods of fractional 

 distillation. 



The variation in the composition of the distillate from a 

 mixture of two substances, the boiling-point of which rises con- 

 stantly during the distillation, has been carefully investigated 

 by F. D. Brown (Trans. Chem. Soc. 1879, p. 550 ; 1880, pp. 49 

 & 304 ; 1881, p. 517)*; and his results may be briefly stated 

 as follows : — Calling the relative weights of the two liquids 

 at any instant in the still W x and W 2 , the relative weights at 

 the same moment in the vapour (and therefore in the distil- 

 late at this instant coming over from the mixture W 1 + W 2 ) 

 x x and x 2 , and the vapour-pressures of the pure substances at 

 the boiling-point of the mixture, P x and P 2 , the composition 

 of the instantaneous distillate is given approximately by the 

 equation 



*i w x P, 



thus 





#2 " W 2 



1 2 





T"» 



but by substituting a 



constant, 



c, for the 



ratio 



Pi 



IV 





x 2 ~ 



W 1 

 'W 2 ' 





-*■ 2 



a still better result is 



obtained. 









* For an account of the experimental work and of the theoretical 

 conclusions relating to the distillation of pairs of liquid that are (a) non- 

 miscible, (b) miscible within limits, (c) miscible in all proportions, the 

 article by one of us on " Distillation " in Thorpe's ' Dictionary of Applied 

 Chemistry ' may be consulted. Pairs of liquids belonging to the first and 

 second classes boil at a lower temperature than even the more volatile 

 component when distilled alone, and no separation can be effected by 

 fractional distillation. Among those in the third class there are some 

 from which only one of the two substances can be so separated ; in each 

 case of this kind there is a particular mixture (the composition of which 

 varies to some extent with the pressure) which distils at a constant tem- 

 perature without change of composition, and it is this mixture that would 

 be separated by fractional distillation from that one of the pure substances 

 which is present in excess. In some cases — such as propyl alcohol and 

 water — the mixture of constant boiling-point and composition boils at a 

 lower temperature than either of the liquids when distilled alone; in 

 other cases — for example, formic acid and water — this mixture has a 

 higher boiling-point than either component. 



It is only when the boiling-point of every possible mixture lies between 

 those of the components that both liquids can be separated by fractional 

 distillation ; it is only to such cases that Brown's law is applicable, and, 

 in the case of three liquids, it is only such mixtures that are considered 

 in this paper. 



