CHAPTER II 

 OSMOTIC PRESSURE (Cont'd) 



Measurement by Depression of Freezing Point 



The limitations in the use of the plasinolytic and hemolytic methods 

 in the precise measurement of the osmotic pressure of the body fluids 

 have rendered it necessary to find some physical method that will be 

 generally applicable. Because of technical difficulties, it is impracticable 

 to measure the pressure directly by employing an osmometer, so that 

 some indirect method, depending on a readily measurable physical prop- 

 erty which varies in proportion to the osmotic pressure of the dissolved 

 substances, must be used. Fortunately, one such exists in the property 

 which dissolved substances have in lowering the temperature at which 

 the pure solvent solidifies; the freezing point of pure water, for example, 

 is lowered when substances are dissolved in it, and the extent of this 

 lowering, with certain reservations which will be explained later (page 

 16), is proportional to the molecular concentration of the solution and 

 independent of the chemical nature of the substance dissolved. This 

 lowering of temperature is designated by the Greek letter A, and to 

 measure it a thermometer is used which is not only extremely sensitive 

 but in which the level of the mercury column can be adjusted so that it 

 stands at a convenient level on the scale corresponding to the freezing 

 point of whatever solvent was used in making the solution under investi- 

 gation (Beckmann's thermometer) (Fig. 4). The exact position on 

 the scale of this thermometer at which the pure solvent freezes having 

 been ascertained, the observation is repeated with the solution whose 

 osmotic pressure is to be determined. 



A gram-molecular solution in water (having therefore an osmotic pres- 

 sure of 170,240 mm. Hg) has a freezing point that is 1.86° C. lower than 

 that of pure water. This is known as the "freezing point constant," 

 and it varies for different solvents, being 3.9 for acetic acid and 4.9 

 for benzene. If an unknown watery solution is found to have a freez- 

 ing point that is A° C. lower than that of water, its osmotic pressure 



... .Axl7.024 _ 



will equal — ^-^ mm. Hg. 



10 



