242 



AMPHOTERIC COLLOIDS. II 



with 3 m/1024 acid (pH = 4.7) was quite opaque but remained per- 

 fectly white; while the test-tube treated with m/256 was only slightly 

 opaque but remained also practically white. This shows that the gela- 

 tin treated with 3 m/'1024 and whose pH was 4.7 was also entirely free 

 from ionized silver. Fig. 2 is a photograph of these test-tubes. This 

 experiment was repeated several times with the same result. It is a 

 very striking demonstration experiment. 



On the more acid side, where the gelatin was free from silver, the 

 gelatin existed as gelatin nitrate as we shall prove presently. This 

 then shows that at the isoelectric point and on the acid side from the 

 isoelectric point a cation cannot combine with gelathi, while on the 

 more alkaline side from the isoelectric point such a combination 

 occurs. 



Table I gives the cc. of 0.01 n Ag found in combination with 0.25 

 gm. of gelatin. 



TABLE I. 



Cc. 0.01 N Ag in combination with 0.25 gm. of gelatin at different pH. 



pH 



Cc. 0.01 N Ag combined 

 with 0.25 gm. of gelatin. , 



3.6 

 0.5 



3.8 

 0.3 



3.8 

 0.4 



3.9 

 0.3 



4.1 

 0.2 



4.3 

 0.2 



4.6 

 0.2 



4.7 



. 55 



5.0 

 1.25 



5.3 

 3.2 



5.7 

 4.0 



6.1 



4.85 



6.1 

 4.6 



6.4 

 4.9 



The small values of 0.2 cc. or even 0.5 cc. found in the gelatin on 

 the acid side from the isoelectric point are due to incomplete removal 

 by washing; an increase in the number of w^ashings would probably 

 have removed these traces also. Where 0.5 or less cc. of 0.01 n Ag 

 was found in 0.25 gm. of gelatin, light no longer blackened the gela- 

 tin. As soon as pH became greater than 4.7 the value of Ag found 

 rose suddenly. 



Fig. 3 represents the physical properties of gelatin treated first 

 with m/16 AgNOs and then brought to different hydrogen ion concen- 

 trations by treatment with different concentrations of HNO3. 1 gm. 

 of powdered gelatin was first put for 30 minutes into 100 cc. of M/''16 

 AgNOs (pH about 6.9) and the excess of salt was then washed away 

 (in the manner described). Then each gm. of gelatin was perfused 

 three times with a definite concentration of HNO3 and the excess of 

 acid was washed away by four perfusions with distilled water. The 



