54 MOLECULAR WEIGHT AND POLYMER1SM 



in hydroxyl-free solvents mostly occur with double mole- 

 cular magnitude (p. 60) an inconstant ratio of partition is 

 found, e. g. for acetic acid in water and benzene l : 



The fair agreement of the values of - =- as compared with 



^'2 

 j- 



the decided falling off in the values of agrees with the 



c i 

 conclusion arrived at also from other sources, that acetic 



acid in water exists as C. ? H 4 O 2 , but in benzene practically 

 as (C 2 H 4 Cg 2 . 



(b) The Cyclic Process cannot be carried out at Constant 

 Temperature. 



Whilst in the foregoing methods the underlying cyclic 

 process can be carried out at constant temperature, we 

 have now to consider methods in which that is not 

 the case. 



Molecular weight determination by lowering of the freez- 

 ing point. Separation of the solvent in the solid state, 

 i. e. freezing of the solution, affords, as is well known, the 

 oldest method for molecular weight measurement, which 

 was developed on an empirical basis by Raoult before the 

 theory of solutions was established. 



The cyclic process which again leads to the rule to be 

 applied, cannot be carried out at constant temperature, and 

 so involves the use of the second law of thermo-dynamics. 

 The solution is to be cooled by A t below the freezing point 

 of the solvent (T absolute), and at that temperature 

 a certain quantity of ice separated, a process which can be 



1 Nernst, Zeitschr.f. Phys. Chem. 6. 121. 



