32 BIOLOGICAL CHEMISTRY 



and raises the boiling point by 0-515 C. For acetic acid 

 the corresponding figures are 3*9 C. and 3-0 C. respectively. 



Let us take as an example an experiment in which a solution 

 in water was cooled to 1-4 C. below the freezing point of 

 water, the temperature rose quickly to 0-135 below the freez- 

 ing point, remained steady there for a short time, then began 

 to cool down slowly. The solution was supercooled 1-265 

 (1-4 0-135) below the observed freezing point. As ice was 

 separating heat was given off so that the solution warmed 

 up to the observed freezing point, then as more ice separated 

 the solution became more concentrated and the temperature 

 began to fall slowly. 



In order to obtain the true freezing point it is necessary 

 to correct the observed freezing point because of the concen- 

 tration brought about by the separation of ice. To do this 

 we make use of the latent heat of formation of ice, which is 

 80 calories per gram. Thus every degree of supercooling 

 causes the solution to be concentrated by /,, because i 

 per cubic centimetre is equal to i calorie, which is gV of 

 the heat given off when i gram of water is converted into 

 ice. 



In the example the supercooling was 1-265 so that the 

 observed freezing point was that of a solution concentrated 



by ^ 2 , i.e. to ? 735 O f j^ s or jgi na i volume and the observed 



oO oO 



freezing point was C , the value that it ought to have 



been. Therefore the true freezing point was ?~ X 



OO 



0-135 =0-133. 



As the depression of the freezing point of water for a mole- 

 cular solution is 1-86, the concentration of the solution used 



was -~= 0-071 M. 

 l-oo 



For biological purposes the boiling point method is not used 

 so much as the freezing point method, because proteins are 

 coagulated by heating, hence the conditions of the solution 

 are altered. Numerous precautions must be taken in measur- 

 ing the freezing and boiling points, but the details must be 

 sought in a book on methods of Physical Chemistry. 



In watery solutions a depression of the freezing point by 

 o-ooi of a degree Centigrade is equivalent to a pressure of 

 9 millimetres of mercury. The error of the freezing point 



