56 DAVID R. BRIGGS 



osmotic pressure can be calculated, must be resorted to in such in- 

 stances. The more commonly measured colligative properties that 

 may be studied for this purpose are the depression of freezing point 

 and the depression of vapor pressure of the solvent in the solutions. 

 As has already been pointed out, these methods are not of sufficient 

 precision to detect the effects of most higher molecular weight sub- 

 stances upon the colligative properties of the solvent with the accu- 

 racy required for the calculation of the molecular weights of these sub- 

 stances. On the other hand, these methods are useful for body tissue 

 fluids where the degree of precision required in determining the abso- 

 lute effects upon the colligative properties of the solvent may not be 

 so exacting and where, because of the presence of low molecular 

 components, direct measurement of osmotic pressure is not feasible. 

 Determination of Freezing Point Depression. For general 

 purposes, a freezing point determination is made by placing the 

 sample, solvent or solution, in a tube surrounded by an air jacket 

 (which acts to retard the heat exchange between bath and sample) 

 immersed in a freezing mixture, the tube being fitted with a ther- 

 mometer graduated to hundredths of a degree (Beckmann type) 

 and a stirrer. As the sample cools it is stirred to minimize under- 

 cooling. When the sample is a pure solvent, the degree of under- 

 cooling is not important since the liquid remaining in equihbrium 

 •with, the separated solid is still pure liquid but, when the sample is a 

 solution, the final liquid phase will be a more concentrated solution 

 than before the solid (solvent) phase appears, and the observed freez- 

 ing point will be lower than the true value (for the original solution) 

 by a factor related to the degree of undercooling, the heat capacity of 

 the solution, and to the heat of fusion of the solvent. Undercooling 

 must be absolutely avoided or must be corrected for. It is difiicult 

 to avoid undercooling, especially when colloidal components such as 

 proteins, etc. are present in the solution. Where it can be safely as- 

 sumed (a) that the change in the freezing point depression with con- 

 centration is a linear function of concentration within the small con- 

 centration change resulting from separation of solvent (solid) due 

 to undercooling, (6) that no heat of dilution is involved, that is, if it 

 can be assumed that the solution is acting as an ideal solution, and (c) 

 that the solid separating consists only of the pure solvent component, 

 correction of the observed freezing point depression. A', to yield the 

 true value for the original solution. A, may be obtained bj' the rela- 

 tionship : 



