28 THEORY OF COLLOIDAL BEHAVIOR 



whether both or only one of the two oppositely charged ions of a 

 salt enters into combination with gelatin. 



Such experiments show that at a given hydrogen ion concentra- 

 tion either the cation or only the anion or neither ion can combine 

 with a protein; and that it depends solely on the hydrogen ion 

 concentration of the solution which of the three possibilities exists. 1 



Proteins are amphoteric electrolytes which exist in three states, 

 according to their hydrogen ion concentration, namely, (a) as 

 non-ionogenic or isoelectric protein; (6) metal proteinate (e.g., 

 Na or Ca proteinate); and (c) protein-acid salts (e.g., protein 

 chloride, protein sulphate, etc.). We will use gelatin as an illus- 

 tration. At one definite hydrogen ion concentration, namely, 

 that of the isoelectric point, which in the case of gelatin lies at 

 10~ 4 - 7 N (or in S^rensen's logarithmic symbol at pH = 4.7), gelatin 

 can combine practically with neither anion nor cation of an elec- 

 trolyte. At a pH>4.7, gelatin can combine only with cations 

 (forming metal gelatinate, e.g., Na gelatinate); at a pH<4.7, 

 gelatin combines with anions (forming gelatin chloride, etc.). 

 This was proved in the following way: Doses of 1 gm. of finely 

 powdered commercial gelatin (going through sieve 60 but not 

 through 80), which happened to have a pH of 7.0, were brought to 

 different hydrogen ion concentrations by putting them for 1 hour 

 at about 15C. into 100 c.c. of HNO 3 solutions varying in concen- 

 tration from M/8,192 to M/8. Owing to the Donnan equilib- 

 rium the hydrogen ion concentration inside a gelatin granule is 

 lower than that outside. After this, each dose of 1 gm. of gelatin 

 was put on a filter, the acid being allowed to drain off, and each 

 dose was washed once or twice with 25 c.c. of cold water (at 5C. 

 or less) to remove the greater part of the acid between the 

 granules of the powdered gelatin. These different doses of 

 originally 1 gm. of gelatin, each of which now possessed a different 

 pH, were put for 1 hour each into a separate beaker containing 

 the same concentration, e.g., M/64, of silver nitrate at a temper- 

 ature of 15C. Each dose of powdered gelatin was then put on a 

 filter and washed with stirring six or eight times each with 25 c.c. 

 of ice-cold water. This washing serves the purpose of removing 

 the AgNO 3 held in solution between the granules, thus allowing 



1 LOEB, J., J. Gen. PhysioL, vol. 1, pp. 39, 237, 1918-19. Science, vol. 

 52, p. 449, 1920. /. chim. physique, vol. 18, p. 283, 1920. 



