52 DAVID E. BRIGGS 



by one method of measurement will agree with that determined by 

 any other) it is not possible to predict from such a determination 

 what the value will be at any other concentration than that upon 

 which the measurement was actually made unless the system is either 

 (a) known to obey the laws of ideal dilute solutions or (6) some 

 knowledge is available as to the actual dependence of ai upon concen- 

 tration. For example, the observation that a red blood cell or a 

 muscle tissue slice swells as an "imperfect osmometer" may mean, 

 as is usually assumed to be the case, that the membrane of the cell 

 is not acting in a strictly semipermeable manner to the solutes within 

 and outside the cell; but it should not be forgotten that another 

 possible explanation may be that the cell solute contents do not affect 

 the activity of the solvent in a manner described by the ideal solution 

 laws. This would be the more probable the greater the percentage 

 of the total effect due to high molecular solutes within the cell. 

 Again, where osmotic pressure is used to measure the molecular 

 weight of a solute, particularly where the solute is a high molecular 

 substance, a determination of the osmotic pressure at a single concen- 

 tration of the solute may be insufficient. Determinations must be 

 made at several concentrations of increasing degrees of dilution such 

 that when a plot of 7r/c2 versus d is made, a dependable extrapolation 

 can be attained for the value of 7r/c2 at Ci = 0. Even then the calcu- 

 lated value for M2 obtained by inserting this value of t/c2 into van't 

 Hoff's equation will be in error if the relationship In ai = In iVi cannot 

 be safely assumed to hold at infinite dilution for the system under 

 study. 



3. Donnan Membrane Equilibria 



In the foregoing discussion of osmotic pressure theory, it was as- 

 sumed that the solutes involved were nonionizing substances or, if 

 capable of ionization, that the membrane used was impermeable to 

 all ions so derived, in which case the osmotic pressure would be the 

 summation of the osmotic increments due to each species of particle 

 present in the solution. Many naturally occurring high molecular 

 compounds such as proteins, gums, etc., for which osmotic pressure 

 methods are commonly emploj^ed in the determination of their 

 molecular weights, are electrolytes. These substances in solution are 

 capable of ionization, yielding polycharged ions of high molecular 

 weight, to which a membrane may be impermeable while it is com- 

 pletely permeable to the small ions (counter ions). 



