EXPERIMENTAL DETERMINATION OF THE FREEZING POINT 199 



0.001 M concentration produce a lowering of the freezing point equal to 

 twice that of a non-electrolyte, i.e., 2 X 0.00186° or 0.00372°. A similar 

 solution of barium chloride which dissociates into three ions should show 

 a freezing-point lowering equal to 3 X 0.00186°, or 0.00558°. The 

 corresponding experimental values shown in Table V-7 are 0.00366° 

 and 0.00530°. Since the experimental values are smaller than the 

 theoretical values, it follows that not all the molecules in the solvent are 

 dissociated into ions, so that at all concentrations we must allow for the 

 fraction of undissociated molecules found even in verjr dilute solutions. 

 In the classical theory of solution the undissociated entities are mole- 

 cules; in the Debye-Huckel theory they are supposed to be ions sur- 

 rounded by an atmosphere of oppositely charged ions, and under the 

 influence of an electric field it is this atmosphere that is constantly chang- 

 ing instead of the ionic entities. In either theory a knowledge of the 

 degree of dissociation is essential to determine the requisite freezing 

 point and from it to calculate the expected osmotic pressure. 



The degree of ionization of NaCl at 0.001 M concentration is 96 per 

 cent. This means that out of every 100 sodium chloride molecules in the 

 solvent 96 dissociate into sodium and chloride ions and 4 remain undisso- 

 ciated. It should follow that the expected freezing-point depression is 

 1.96 X 0.00186°, or 0.00365°, which is in close agreement with the 

 experimental value of 0.00366° of Table V-7. Since a depression of the 

 freezing point of 0.1° corresponds with an increase in osmotic pressure of 

 1.204 atmospheres, the above solution can develop an osmotic pressure 

 of 33.5 mm of mercury. How many millibars is this mercury pressure 

 equivalent to? 



Experimental Determination of the Freezing Point 



One of the most practical indirect methods for determining the osmotic 

 pressure, and one almost exclusively used by physiologists and in medical 

 practice, is the cryoscopic method. Since most physiological fluids are 

 in reality dilute solutions, as seen in Table V-8, they readily lend them- 

 selves to freezing-point determinations. An additional advantage is 

 founded upon the fact that it makes practically no difference whether 

 gross material is in suspension or whether more or less protein material is 

 present. The freezing point of blood, for instance, may be obtained by 

 using whole blood, blood plasma, or serum, since the corpuscles act as 

 particles in suspension. 



) Since a change in the freezing point of 0.01° corresponds with an 

 osmotic-pressure change of 91.5 mm of mercury, it is necessary to use a 

 thermometer calibrated to 0.01° so that an estimated change of plus or 



