II. OSMOTIC PRESStTRE MEASUREMENTS 63 



There exists a wide variety of choice among the static elevation types 

 of osmometers described in the Hteratiire (32-47). 



The dynamic eqiiihbrium osmometer, first used by Berkelej^ and 

 Hartley in their studies on sucrose sokitions in which they obtained 

 results in accurate agreement with those obtained by the static ele- 

 vation method of Morse and Frazer with the same solute, has the 

 advantage of rapid determination of the osmotic pressure, but this 

 type of instrument is somewhat more difficult to fill and operate than 

 are most of the static elevation apparatus. It has the limitation 

 also that, unless the activities of all diffusible components other than 

 the solvent are equ^l across the membrane when the osmotic pressure 

 is measured, the values of osmotic pressure so obtained are of no value 

 in the calculation of the molecular weight of the nondiffusible com- 

 ponent. In those cases, however, in which it is known that no dif- 

 fusible component other than the solvent is present in the system, or 

 where it is known that the activities (concentrations) of such dif- 

 fusible components are exactly equal in the two solutions on the op- 

 posite sides of the membrane, this method allows for a much quicker 

 estimation of the osmotic pressure due to the nondiffusible component 

 than does the static method. Many of the static elevation type os- 

 mometers utilize an applied constant external pressure approximately 

 equal to the osmotic pressure to be expected at equilibrium, thus 

 minimizing the degree to which solvent transfer across the membrane 

 must occur during the attainment of the final equilibrium. Most 

 dynamic equilibrium type osmometers can be readily adapted in 

 their use to the static elevation procedure. Descriptions of a number 

 of osmometers emploj^ed primarily in the dynamic equilibrium pro- 

 cedure can be found in references given under this heading {24-31). 



Since the type and specific construction of the osmometer that 

 will prove best adapted to a particular problem will necessarily vary 

 with the properties (molecular size, whether electrolyte or nonelec- 

 trolyte, solubility, amount available, etc.) of the solute, it is recom- 

 mended that the investigator, in light of his particular problem, con- 

 sider in detail the characteristics of a number of the instruments to 

 which reference has been made. When the objective of the experi- 

 ment is to obtain the molecular weight of a high molecular material 

 from osmotic pressure measurements, it is necessary to bear in mind 

 that some form of extrapolation to very low or zero concentration is 

 almost always necessary. The osmometer selected must be capable, 

 therefore, of yielding accurate values of osmotic pressure through a 



