JACQUES LOEB 379 



pH > 4.7 is a further proof of the correctness of the purely chemical 

 theory according to which for pH > 4.7 ionized gelatin can only exist 

 as an anion, not capable of holding HBr in combination. 



When we titrate 25 cc. of 1 per cent gelatin solution at the iso- 

 electric point with 0.01 n NaOH, we find that it acts as an acid, re- 

 quiring between 5.25 and 5.5 cc. 0.01 n NaOH for neutralization 

 against phenolphthalein. (The number of cc. 0.01 n NaOH required 

 to neutralize 25 cc. of 1 per cent gelatin we will call the NaOH num- 

 ber of gelatin.) When the pH of common gelatin, not treated with 

 acid, is greater than 4.7 the NaOH number becomes less than 5.25 

 cc, probably on account of the fact that part of the gelatin exists 

 as a metal gelatinate (probably chiefly Ca gelatinate) owing to iono- 

 genic impurities remaining from the process of manufacture. Our 

 previous papers have shown that at the isoelectric point gelatin is 

 compelled to give off these ionogenic impurities. 



On the more acid side from the isoelectric point the gelatin con- 

 tains Br and the Br number increases with decreasing pH. With the 

 exception of a small fraction this Br is held in combination with the 

 gelatin as can be shown on the basis of titration with NaOH of the 

 gelatin treated previously with HBr and possessing a pH < 4.7. In 

 such a titration the gelatin solution whose pH < 4.7 is gradually 

 rendered more alkaline through the addition of NaOH until finally 

 its pH becomes equal to 4.7, and when that happens all the HBr held 

 in combination with gelatin must be set free. As a consequence in a 

 titration of gelatin bromide with NaOH two acids must be saturated 

 with NaOH, the pure gelatin, and the HBr set free when during the 

 process of titration the gelatin reaches its isoelectric point. It fol- 

 lows from this that the NaOH number found in this case must equal 

 the sum of the Br number of the gelatin plus the NaOH number for 

 gelatin at the isoelectric point ; regardless of how the gelatin had been 

 treated before and regardless of the pH for which this rule is tried 

 out. 



If we denote a given pH with n, the NaOH number at this pH = n as 

 " (NaOH)„," the NaOH number at the isoelectric point with "NaOH 

 (isoelectric)," and the Br number at pH = n with "Brn", then the 

 following equation will hold: (NaOH)n = NaOH (isoelectric) -f-Brn. 

 In Table I, I have selected at random four experiments in which the 



