JACQUES LOEB 89 



probably in the fact that he compared the effects of the addition of 

 equal quantities of acid to a protein while we compared the effects of 

 different acids at equal hydrogen ion concentrations of the solutions. 



II. Action of Weak and Strong Monobasic Acids. 



We will first indicate why it is necessary to choose equal hydrogen 

 ion concentrations as a standard of comparison instead of using equal 

 quantities of acid. We always use as standard material isoelectric 

 protein, to which we add enough acid or alkali to bring it to the de- 

 sired hydrogen ion concentration. In the case of gelatin we proceed 

 usually in this way; we add to 1 gm. of powdered gelatin brought to 

 the isoelectric point (pH = 4.7) small quantities of acid to bring the 

 samples to the desired hydrogen ion concentration, then melt the 

 gelatin, and bring the volume of the solution to 100 cc. by adding 

 H2O. The pH is then determined. It is hardly necessary to state 

 that it requires greater quantities of weak than of strong acid to 

 bring the gelatin to the same pH on the acid side of the isoelectric 

 point. Fig. 1 gives the quantities of 0.1 N acetic, monochloracetic, 

 dichloracetic, and trichloracetic acids required to bring 1 gm. of 

 isoelectric gelatin in a 1 per cent solution to the same pH. The 

 abscissae in Fig. 1 are the pH of the gelatin solution resulting from 

 the addition of acid and the ordinates are the numbers of cc. of 

 0.1 N acid which must be contained in 100 cc. of a 1 per cent solu- 

 tion of originally isoelectric gelatin to produce the pH. The curve 

 indicates that the quantity of acid required is the less the stronger 

 the acid. If we now measure the osmotic pressure of the solu- 

 tions of the four gelatin acetates (all 1 per cent in regard to the 

 originally isoelectric gelatin) and if we plot the curves with the values 

 for the osmotic pressure as ordinates and the pH as abscissae, the curves 

 for all four gelatin-acetate salts are practically identical (Fig. 2) . All 

 four curves have a minimum at the isoelectric point pH = 4.7 ; they all 

 rise identically with a diminution in pH {i.e. a rise in hydrogen ion 

 concentration) ; all reach their maximum at a pH of about 3.5, and all 

 drop almost as steeply with a further diminution of pH as they rise 

 on the ascending side of the curve. The drop will not be discussed 

 in this paper. 



