574 



AMPHOTERIC COLLOIDS. 



The values found for HNO3 are slightly less than those found for 

 HBr and HCl. This harmonizes with the fact that the effect of 

 HNO.i on the physical properties of gelatin is also slightly less than 

 the effect of HCl or HBr for the same pH. A comparison of the 

 figures for NaOH values for HNO3 and for the PO4 values, found by 

 titration for PO4 (Table IV, Rows 1 and 3) shows for the two values 

 practically the ratio of 1 : 3 at the same pH; i.e., three times as much 

 H3PO4 as HNO3 is in combination with the same mass of gelatin. 

 The figures for HNO3 and oxalic acid (Rows 1 and 2, Table IV) give 

 the ratio of approximately 1:2. 



We therefore see in these figures the proof that actually one mole- 

 cule each of phosphoric and oxalic acids is in combination with only one 

 molecule of gelatin; and we see in this the explanation for the 



TABLE IV. 



Cc. of 0.01 N acid in combination with 10 cc. of a 1 per cent gelatin solution at 



different pH. 



fact that they do not depress the osmotic pressure as does H2SO4, 

 which behaves like a dibasic acid towards gelatin. 



What has been proved for oxalic and phosphoric acids holds also 

 for tartaric, succinic, and citric acids. These latter two acids are 

 very weak and thus relatively large quantities of acid have to be added 

 to bring the solution to the desired pH. The same is true for acetic 

 acid. The equihbrium conditions seem to be such that a very large 

 amount of undissociated acid is found inside the collodion bag and it 

 seems for the present impossible to determine by titration with NaOH 

 the amount of these acids in actual combination with gelatin. 



The fact that all the dibasic and tribasic acids mentioned (with the 

 exception of H2SO4) behave towards gelatin practically like mono- 

 basic acids is a further proof for the theory that the influence of acids 

 on gelatin is determined by the relative amount of gelatin salt formed 



