CORRECTNESS OF THE CHEMICAL VIEWPOINT 29 



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 removed by the washing while the former can- 

 not, or at least only extremely slowly (by altering the pH). 

 After having removed the AgN0 3 not in combination with gelatin 

 by washing with cold water, the gelatin is melted by heating 

 to 40C., enough distilled water is added to bring the volume of 

 each gelatin solution to 100 c.c., the pH of a sample of each solu- 

 tion is determined potentiometrically, and the solutions are 

 exposed in test-tubes to light, the previous manipulations having 

 been carried out in a dark room (with the exception of the deter- 

 mination of pH, for which only part of the gelatin solution was 

 used). In 20 minutes all the gelatin solutions with a pH>4.7, 

 i.e., from pH 4.8 and above, upon exposure to strong light become 

 opaque and then brown or black, while all the solutions of pH < 

 4.7, i.e., from 4.6 and below, remain transparent even when 

 exposed to light for months or years (Fig. 1). The solutions of 

 pH = 4.7 become opaque, but remain white, no matter how long 

 they may have been exposed to light. At this pH the isoelectric 

 point gelatin is not in combination with Ag, but it is sparingly 

 soluble. 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 NiCl 2 , and the NiCl 2 not 

 in combination with gelatin be removed by washing with cold 

 water, the presence of Ni can be demonstrated in all gelatin 

 solutions with a pH >4.7 by using dimethylglyoxime as an indi- 

 cator. All gelatin solutions of pH of 4.8 or above assume a 

 crimson color upon the addition of dimethylglyoxime, while all 

 the others remain colorless. If we use copper instead of Ag or 

 Ni as a cation, treating gelatin with copper acetate, and washing 

 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 gelatin solutions 

 are red whose pH is above 4.7, while the others are colorless. 



