30 THEORY OF COLLOIDAL BEHAVIOR 



On the acid side of the isoelectric point, i.e., at pH<4.7, the 

 gelatin is in combination with the anion of the salt used. This 

 can be demonstrated in the same way by bringing different doses 

 of powdered gelatin to different pH and treating them for 1 hour 

 with a dilute solution of a salt whose anion easily betrays itself, 

 e.g., M/128 K 4 Fe(CN) 6 . If after this treatment the powdered 

 gelatin is washed six times or oftener with cold water to remove 

 the Fe(CN) 6 not in chemical combination with gelatin and if 1 

 per cent solutions of these different samples of gelatin are made, 

 it is found that when the pH is <4.7 the gelatin solution turns 

 blue after a few days (due to the formation of ferric salt), while 

 solutions of gelatin with a pH of 4.7 or above remain permanently 

 colorless (Fig. 2). Hence, gelatin enters into chemical combina- 

 tion with the anion Fe(CN) 6 only when pH is <4.7. The same 

 fact can be demonstrated through the addition of ferric salt when 

 gelatin has been treated with NaCNS, the anion CNS being in 

 combination with gelatin only where the pH is <4.7. Acid dyes, 

 like acid fuchsin, combine with gelatin only when the pH is 

 <4.7. x 



In this way it can be shown that when the pH is >4.7 gelatin 

 can combine only with cations; when the pH is <4.7 gelatin can 

 combine only with anions, while at pH 4.7 (the isoelectric point) 

 gelatin can combine with neither anion nor cation. The idea 

 that both ions are adsorbed or combine with a protein simultane- 

 ously is no longer tenable, since otherwise both ions of the salt 

 should have been discovered on both sides of the isoelectric point. 



It follows also that a protein solution is not adequately defined 

 by its concentration of protein but that the hydrogen ion con- 

 centration must also be known, since each protein occurs in three 

 different forms possibly isomers according to its hydrogen 

 ion concentration. 



Let us now return once more to the experiment in which doses 

 of powdered gelatin were brought to a different pH and subse- 



1 In these experiments it may happen that a few individual granules do 

 not give off their stain at the isoelectric point or on the alkaline side of the 

 isoelectric point, due probably to experimental shortcomings. When the 

 gelatin is melted the solution may show an indication of red. The difference 

 between the gelatin on the alkaline and on the acid side is, however, 

 sufficiently striking even if this slight error interferes. 



