Crafts et al. 



92 — 



Water In Plants 



Mair, Glasgow, and Rossini (1941) successfully employed a platinum resistance 

 thermometer for measuring freezing points of hydrocarbons. Their paper should be 

 consulted for discussion of the theoretical aspects of freezing point measurements. 



Physical Principles : — The principle involved in the cryoscopic method 

 is that the presence of a solute lowers the vapor pressure (and diffusion 

 pressure) of a solvent. Since the vapor pressure of water is equal to that 

 of ice when both phases are in equilibrium, the addition of a solute to the 

 liquid lowers its vapor pressure. This causes the ice to melt, transforming 

 it to liquid. The melting process absorbs heat and the temperature drops 

 to a point at which the vapor pressures of the solid and liquid phases are 

 again equal. This is termed the freezing point of the solution. 



Fig. 29. — Construction of cryoscopic apparatus. The insulated vessel H is fitted 

 with a top D with holes drilled to receive a heavy wire stirrer A, an ordinary ther- 

 mometer B to record bath temperature, a glass air jacket K, and a small glass tube 

 I closed at one end, in which a piece of capillary tubing G containing water may be 

 frozen. Within the air jacket the sample tube I is held in place by a cork fitted with 

 holes to receive the freezing point thermometer C and a fine wire stirrer E, as well as 

 a small hole F for inserting the frozen capillary to initiate crystallization. 



Raoult's law states that the vapor pressure of water is lowered in pro- 

 portion to the mol fraction of solute added. 



Pq — p ^_ "2 ("f) 



p ~ ni + no 



where po and p are the vapor pressure of pure solvent and solution and 

 ni is the number of mols of solvent containing n2 mols of solute. 



