564 AMPHOTERIC COLLOIDS. V 



— ) corrected for the pH {i.e., after deduction of the conduc- 



ohms / 

 tivity of a free acid solution with the pH found in the gelatin soluiion). 



Rows 9 to 12 give the measurements of the pure acid solution (free 

 from gelatin) surrounding the collodion bag containing the gelatin 

 solution. The pH in the outside solution is always lower than inside 

 the bag and this difference may find its explanation on the basis of 

 the theory of equihbrium developed by Donnan for such cases. The 

 values for titration for Br by the Volhard method and for the titra- 

 tion for HBr by titration with NaOH are practically identical for 

 this pure acid solution, thus showing that the degree of accuracy of 

 the method of titration was adequate. 



In Table II the measurements for 1 per cent isoelectric gelatin 

 treated with H2SO4 are given. Here we can ascertain the amount of 

 SO4 in combination with 0.1 gm. of gelatin only indirectly by titrat- 

 ing with NaOH, applying the two corrections mentioned. All the 

 other measurements are the same as in the experiment with HBr. 



A comparison of the corrected NaOH values in Row 4, Table II, 

 with those in Row 6, Table I, shows that they are practically identical 

 for the same pH. Thus for pH = 3.35 the corrected NaOH value 

 is 4.4 in the case of gelatin sulfate, and 4.35 for gelatin bromide. 

 Since the experiments with HBr have shown that these corrected 

 NaOH values are identical with the Br values we can state, that for a 

 given pH 1 gm. of gelatin is in combination with twice as many 

 atoms of Br as with radicles of SO4; or in other words, that H2SO4 

 behaves toward gelatin as a dibasic acid. Fig. 1 illustrates this iden- 

 tity of the curves for NaOH values of the two acids. In spite of the 

 identity of equivalents of these two ions in combination with 1 gm. of 

 gelatin the osmotic pressures are very different for the two types of 

 gelatin salts. This is illustrated by Fig. 2, where the ordinates rep- 

 resent the osmotic pressures and the pH are the abscissae. It is 

 obvious that the values of the osmotic pressure of gelatin bromide are 

 considerably higher than the values for the osmotic pressure of gela- 

 tin sulfate. The reader will notice that the maximum osmotic pres- 

 sure for gelatin bromide is about 325 mm., which is identical with 

 the maximal osmotic pressure found for gelatin salts with univalent 

 cation, €.g. sodium gelatinate; while the maximum value for gelatin 

 sulfate is about 130, almost identical with the maximum for calcium 



