II. OSMOTIC PRESSURE MEASUREMENTS 45 



solutes will constitute a number average molecular weight of the com- 

 ponents of the mixture but will give little information concerning 

 either the range of the molecular weights of the components or the 

 relative amounts of the various components present. The number 

 average molecular weight obtained from osmotic pressure measure- 

 ments is nevertheless of considerable value in following the degree of 

 polymerization during the preparation of high jiolymers. The 

 values of molecular weights obtained from osmotic pressure measure- 

 ments will be dependable in many cases only when calculated from 

 extrapolated values of the osmotic pressure/concentration relation- 

 ship obtained at infinite dilution and when such extraneous effects as 

 may arise from a Donnan membrane equilibrium are avoided or taken 

 into account. 



The use of membranes impermeable to high molecular substances 

 but freely permeable to other solute constituents of a solution makes 

 possible the determination of the fractional effect of the higher molec- 

 ular constituent upon the colligative properties of the solution. This 

 circumstance is taken advantage of in the estimation of the so-called 

 colloid osmotic pressure (oncotic pressure) of blood or of other bio- 

 logical fluids that commonly contain both high and low molecular 

 solutes, and also constitutes an important reason why direct osmotic 

 pressure measurements of the molecular weights of high molecular 

 substances is to be favored over the other methods mentioned above, 

 i.e., freezing point and vapor pressure methods. It is often difficult 

 to prepare proteins, gums, etc. entirely free of low molecular impuri- 

 ties. On a weight basis these low molecular solutes will have a rela- 

 tively great effect upon the colligative properties of the solution and, 

 even though present in very small amounts by weight, will lead to 

 large errors in the values of the molecular weight of the high molecular 

 component when calculated on the basis of the total effect upon the 

 properties of the solution obtained by freezing point depression or 

 vapor pressure decrease measurements. In the case of osmotic pres- 

 sure measurements, with membranes such as those referred to, the 

 low molecular substances distribute themselves at equal concentra- 

 tions on both sides of the membrane; this leads to no contribution 

 by them to the observed osmotic pressure. 



B. THEORETICAL CONSIDERATIONS 



Before considering methods and their possible applications in 

 biophysics, it seems desirable that a short summary of the theory in- 



