JACQUES LOEB 561 



In addition a second correction is needed as explained in a previous 

 paper. The gelatin treated with acid contains some free acid, the 

 titration value of which has also to be deducted from the titration 

 number. This correction can be found by titration of the same acid 

 at the same pH but free from gelatin. 



With the aid of these two corrections we are also able to investi- 

 gate the effect of acid on gelatin in the presence of the acid. It was 

 observed that the maximal osmotic pressure of gelatin chloride or 

 sulfate is reached at a pH of between 3.3 to 3.5. This maximum is 

 never reached when we wash the gelatin four or six times with H2O 

 after a treatment with an acid of moderate concentration, as was 

 done in our previous experiments, since in this washing the acid 

 formed by hydrolytic dissociation is removed, which causes new 

 hydrolysis and in this way in successive washings a rapid decline 

 of the pH results, as will be shown more definitely in a subsequent 

 paper. 



Since it was necessary for our purpose to be always sure of ascer- 

 taining the maximal osmotic pressure a 1 per cent solution of iso- 

 electric gelatin can reach when treated with different acids, we were 

 forced to adopt a different method from that described in the pre- 

 ceding papers. We melted 1 gm. of gelatin rendered isoelectric in 

 the manner described in a previous paper and then added different 

 quantities of 0.01 n or 0.1 n acid and made up the volume to 100 cc* 

 This solution was put into a collodion bag of a capacity of about 

 50 cc. and the bag was put into a beaker containing 400 cc. of the 

 same percentage of acid in distilled water as that added to the gela- 

 tin solution. The collodion bag was closed tightly with a perforated 

 rubber stopper containing a glass tube serving as a manometer to 

 measure the osmotic pressure of the gelatin solution. After about 18 

 hours, when osmotic equilibrium was reached, the pH, the conductivi- 

 ties, and the titration numbers of the acid inside and outside the 

 bag were determined, as were also the osmotic pressures. 



■* For the sake of briefness we shall designate such solutions as 1 per cent gela- 

 tin solutions, although this does not take into account the increase in weight of 

 the gelatin due to the combination with another ion. 



