198 



SURFACES AND MEMBRANES 



that, for the same solvent, equimolecular amounts of different solute 

 depressed the freezing point to the same extent, so that the conclusion is 

 that equimolecular dilute solutions have the same freezing point or the 

 same A value. 



TABLE V-7 



Freezing Points of Some Common Salts of Physiological Importance, 



Compared with Sucrose 



Concentration in gram-molecular weights per 1000 grams water 



0.1 M is not considered a dilute solution. 



Glucose, M = 180.09, C = 0.1 M, A = 0.192. Morse, Frazer, and Lovelace [1907]. 



Osmotic-pressure experiments have furnished data showing that all 

 equimolecular dilute solutions of non-electrolytes have the same osmotic 

 pressure. Another conclusion is that dilute non-electrolyte solutions of 

 equimolecular concentrations have freezing points that are directly pro- 

 portional to their osmotic pressure. 



According to the van't Hoff's law, the osmotic pressure of a dilute 

 solution containing 0.01 gram molecule of non-electrolyte in 1000 grams 

 of water will develop an osmotic pressure of 0.224 atmosphere at 0° C. 

 It will freeze at —0.0186° C. Hence a depression of the freezing point 

 of 0.1° corresponds with an increase in osmotic pressure of 1.204 atmos- 

 pheres or 915 mm of mercury (1219.5 millibars) at 0° C. 



Freezing Point of a Dilute Solution of Electrolyte 



Equimolecular dilute solutions of electrolytes do not freeze at the same 

 temperature. This departure from the simple relations obtained for 

 non-electrolytes can be observed in Table V-7. Here are tabulated some 

 of the common salts of physiological importance which have freezing 

 points differing from the freezing point of sucrose by as much as a factor 

 of 2 or 3. 



Since the freezing point depends on the number of entities in solution, 

 it must follow that the molecule of a salt like sodium chloride, which 

 dissociates into a sodium ion and a chloride ion, would, for example, at 



