90 THEORY OF COLLOIDAL BEHAVIOR 



augmenting effect of the H ion. The true reason for this will 

 appear in Chap. VIII. 



When an alkali, e.g., NaOH, is added to a protein, e.g., gelatin, 

 with a pH slightly above 4.7, at first more of the non-ionogenic 

 protein is transformed into metal proteinate, e.g., Na gelatinate; 

 and this raises the osmotic pressure, viscosity, and swelling 

 rapidly by causing an increase in the concentration of ionized 

 protein for a reason which will be given later. The cation of the 

 alkali, the Na ion, has a depressing effect on these properties, 

 and this depressing effect begins to be visible when the pH 

 exceeds a certain value. After this, with a further addition of 

 alkali, the depressing action of the cation (e.g., of Na) increases 

 more rapidly than the augmenting action of the OH ion. 



The addition of neutral salts of a concentration below N/16 

 to isoelectric gelatin has no effect on osmotic pressure, viscosity, 

 or swelling of the gelatin solution. When neutral salt is added 

 to a gelatin solution on either side of its isoelectric point only a 

 depressing action of that ion which has the opposite sign of 

 charge to the protein ion is observed. No augmenting action of 

 the ion with the same sign of charge as the protein is noticeable. 

 Thus, if CaC^or Na2SO4 is added to a solution of gelatin chloride 

 or gelatin nitrate only a depressing effect of the Cl or SO 4 ion 

 is observed but no augmenting effect of the Ca or Na ion; while 

 when these salts are added to a solution of a metal gelatinate only 

 a depressing effect of the Ca or Na ion is apparent but no aug- 

 menting effect of the anion. 1 The theoretical reason for these 

 effects will be given in Chap. VIII. 



An approximately 1.6 per cent solution of isoelectric gelatin 

 was prepared and brought to a pH of 4.0. The solution was made 

 0.8 per cent in regard to the originally isoelectric gelatin by adding 

 to 50 c.c. of the 1.6 per cent solution either 50 c.c. of H 2 O or of a 

 salt solution, e.g., NaCl, of different molecular concentration, 

 from M/8,192 to 1 M, taking care that the hydrogen ion concen- 

 tration remained the same. The time of outflow through a 

 viscometer was determined in the way described in Chap. V, 

 and the ratios of the time of outflow to that of water were plotted 

 as ordinates over the pH as abscissae (lower curve, Fig. 26). We 



1 The contents of this chapter are based on LOEB, J., J. Gen. Physiol., 

 vol. 3, p. 391, 1920-21. 



