OSMOTIC PRESSURE 



179 



There is a second way of measuring z, namely, by determining 

 the concentration of Cl inside a 1 per cent gelatin chloride solu- 

 tion by titration. The Cl inside is partly in combination with H 

 (free HC1) and partly combined with gelatin. By titrating with 

 NaOH to pH 7.0 and making the correction for isoelectric 

 gelatin (as described in Chap. IV) we determine the value 

 z -f- y. y is known from the pH and by deducting y, we get z. 

 We made such determinations at the end of an osmotic experi- 



f y)(x-y) 



ment and calculated z also from 



y 



in the same 



experiment. Table XXXIV gives a comparison of the values 

 of z obtained in identical solutions by the two different methods. 



TABLE XXXIV. CONCENTRATIONS OF z X 10 5 N 



There is no wide divergence between the two sets of values, 

 yet enough to suggest that the calculated values of z may be 

 chiefly responsible for the discrepancy between calculated and 

 observed curves. The reader must remember that the value of 

 z is multiplied by 2.5 in the calculations of the osmotic pressure 

 (and, therefore, any error in the calculated osmotic pressure is 

 multiplied in the same way). 



3. THE INFLUENCE OF THE ADDITION OF SALTS 



It was first pointed out by R. S. Lillie that the addition of salt 

 to a gelatin solution depresses its osmotic pressure. It should, 

 however, be stated that this depressing effect does not occur at 

 the isoelectric point. When we add different salts to a gelatin 

 chloride solution of an initial pH 3.5 containing 1 gm. originally 

 isoelectric gelatin in 100 c.c. solution, the depressing effect of the 

 salt on osmotic pressure should according to the Donnan equa- 



