■ JACQUES LOEB 45 



ascertained and is put under the figures for the logarithms of the 

 concentration. 



We now will in turn discuss the four main groups of curves. 



Fig. 1 gives the curves for gelatin treated for 30 minutes with HCl 

 alone and then freed from the supernatant and intercapillary acid 

 (with the exception of that formed by hydrolysis of gelatin chloride). 

 The striking fact is that the conductivity, osmotic pressure, swelling, 

 and alcohol number (and viscosity, though this curve is omitted) 

 have all a minimum near a point where the hydrogen ion concentra- 

 tion pH = 4.7; i.e., at about the isoelectric point as determined for 

 gelatin solution by Michaelis on the basis of migration experiments. 

 We may add that the transparency of the solution was also a minimum 

 at the isoelectric point, where the solution was generally quite opaque 

 after standing over night. 



Though it was already known^ that the viscosity and the swelling 

 of gelatin are a minimum at pH = 4.7, our experiments for the first 

 time furnish the proof that this is also the exact location of the 

 minimum for conductivity, osmotic pressure, alcohol number, and 

 transparency. 



The second fact of significance is the sudden rise of the system of 

 curves on either side of the isoelectric point. Since the conductivity 

 curve is the direct expression of the degree of electrolytic dissociation 

 existing in the gelatin solution (which had been freed from all 

 ions except those formed by electrolytic and hydrolytic dissociation 

 of the gelatin salt), these curves may be considered as a function 

 of the degree of electrolytic dissociation of the gelatin. 



Gelatin may exist in the form of non-ionized gelatin on both sides of 

 the isoelectric point, but we shall endeavor to show that ionized 

 gelatin exists in a different ionic state on the two sides of the iso- 

 electric point. On the right, less acid side of the isoelectric point it 



- + - + 



can only exist as a negative ion {e.g. gelatin-H or gelatin-Na, etc.), 



while on the left, more acid side of the isoelectric point it can only 



+ - + - 



exist as a cation {e.g. gelatin-Cl or gelatin-OH). When we study the 



curves for gelatin treated previously with HCl and freed from the 



excess of HCl by washing as described, the two branches of the curves 



on the two sides of the isoelectric point are asymmetrical (Fig. 1) . 



