II. OSMOTIC PRESSURE MEASUREMENTS 55 



point of the protein (value of 22 near zero) and in solutions of salts 

 of fairly large C3 values {e.g., 0.1-0.4 M in sodium chloride). Under 

 these conditions the value of the second term in equation (14) should 

 become negligibly small. There are instances, however, in which a 

 protein may become agglomerated or even precipitated when brought 

 to its isoelectric point or when salt concentrations become high. Also 

 the isoelectric point of a protein may shift when salts are added to the 

 solution. In the former case, erroneous and erratic values for tt^ 

 would be encountered ; in the latter there is lack of certainty as to 

 the pH value at which z^ is zero. 



If 02 is kept constant (pH constant) and C3 is kept constant while 

 W2 is varied, the procedure employed by Scatchard, Batchelder, and 

 Brown (47) may be used to obtain the molecular weight of the high 

 molecular constituent. If it is assumed that equation (14) holds as 

 a limiting law {i.e., the protein in solution acts ideally at infinite dilu- 

 tion), when TTo/w^ versus w^ is plotted, both the second term (due to 

 TTj) and any nonideal properties of the protein itself in solution will 

 disappear in the limit of infinite dilution {w^ = 0), that is: 



limit (tt/wz) = RT/V0M2 



wi — >-0 



and the intercept value of ir^/wi will yield acceptable values of ilf 2- 



Deviations of actual solutions from ideality are marked in most 

 instances; a discussion of the relationships involved is too complex 

 to be included here. Reference should be made to papers by Adair 

 {10,11) and Scatchard {9). 



C. METHODS FOR MEASURING OSMOTIC PRESSURE 



1. Indirect Methods 



When it is desired to know the osmotic pressure of solutions con- 

 taining solutes of low molecular size for which it is not feasible to pre- 

 pare membranes to which these solutes are impermeable, or when the 

 solutes, though present mainly as micelles to which the easily obtain- 

 able membranes are impermeable, exhibit a tendency to dissociate 

 upon dilution into smaller solute molecules that are diffusible (such 

 as soaps in water solution), it is not possible to employ direct methods 

 for such determinations. Indirect methods, involving the measure- 

 ment of other colligative properties of the solutions, from which the 



