VOVEMBEE 12, 1920] 



SCIENCE 



451 



that only the cation or only the anion or that 

 neither ion can combine at one time with a 

 protein; and that it depends solely on the 

 hydrogen ion concentration of the solution 

 which of the three possibilities exists. 



Proteins exist in three states, defined by 

 their hydrogen ion concentration, namely, (a) 

 as non-ionogenic or isoelectric protein, (h) 

 metal proteinate (e. g., !N"a or Ca proteinate), 

 and (c) protein-acid salts (e. g., protein 

 chloride, protein sulfate, etc.)- We will use 

 gelatin as an illustration. At one definite 

 hydrogen ion concentration, namely lO-*-^ N 

 (or in Sorensen's logarithmic symbol at 

 pH = 4.7), gelatin can combine practically 

 with neither anion nor cation of an electro- 

 lyte. At a pH > 4.7 it can combine only 

 with cations (forming metal gelatinate, e. g., 

 !N"a gelatinate), at a pH < 4.7 it 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 a different hydrogen ion concen- 

 tration by putting them for 1 hour at about 

 15° C. into 100 c.c. of HNOg solutions vary- 

 ing in concentration from M/8192 to M/8. 

 After this they were put on a filter, the acid 

 being allowed to drain off, and were washed 

 once or twice with 25 c.c. of cold water (of 

 5° C or less) to remove remnants of the acid 

 between the granules of the powdered gelatin. 

 These different doses of 1 gm. of gelatin now 

 possessing a different pH were all put for 1 

 hour into beakers containing the same con- 

 centration, e. g., M/64, of silver nitrate at a 

 temperature of 15° C. They were then put 

 on a filter and washed 6 or 8 times each with 

 26 c.c. of ice cold water; the wash water must 

 be cold since otherwise the particles will 

 coalesce and the washing will be incomplete. 

 This washing serves the purpose of removing 

 the AgNOg held in solution between the 

 granules, thus allowing us to ascertain where 

 the Ag is in combination with gelatin and 

 where it is not in combination, since the Ag 

 not in combination with gelatin can be re- 

 moved by the washing while the former can 



not, or at least only extremely slowly by alter- 

 ing the pH. After having removed the 

 AgNOg not in combination with gelatin by 

 washing with ice cold water we melt the gela- 

 tin by heating to 40° C, adding enough dis- 

 tilled water to bring the volume of each 

 gelatin solution to 100 c.c, determine the pH 

 of each solution potentiometrically or eolori- 

 metrically, and expose the solutions in test- 

 tubes to light, the previous manipulations 

 having been carried out in a dark room (with 

 the exception of the determination of pH, for 

 which only part of the gelatin solution was 

 used). In 20 minutes all the gelatin solu- 

 tions with a pH > 4.7, i. e., from pH 4.8 and 

 above, become opaque and then black, while 

 all the solutions of pH < 4.7, i. e., from 4.6 

 and below, remain transparent even when ex- 

 posed to light for months or years. The solu- 

 tions of pH 4.7 become opaque, but remain 

 white, no matter how long they may have 

 been exposed to light. At this pH — the iso- 

 electric point — gelatin is not in combination 

 with Ag, but it is insoluble. Hence the 

 cation Ag is only in chemical combination 

 with gelatin when the pH is > 4.7 At pH 

 4.7 or below gelatin is not able to combine 

 with Ag ionogenically. This statement was 

 confirmed by volumetric analysis. 



The same tests can be made for any other 

 cation the presence of which can be easily 

 demonstrated. Thus when powdered gelatin 

 of different pH is treated with NiClj and the 

 NiClg not in combination with gelatin be re- 

 ■ moved by washing with ice cold water, the 

 presence of Ni can be demonstrated in all 

 gelatin solutions with a pH > 4.7 by using 

 dimethyl glyoxime as an indicator. All gela- 

 tin solutions of pH of 4.8 or above assmne a 

 crimson color upon the addition of dimethyl- 

 glyoxime, while all the others remain color- 

 less. When we treat gelatin with copper 

 acetate, and wash afterwards, the gelatin is 

 blue and opaque when its pH is 4.8 or above, 

 but is colorless and clear for pH < 4.7. Most , 

 striking are the results with basic dyes, e. g., 

 basic fuchsin or neutral red, after sufficient 

 washing with cold water; only those gela- 



