JACQUES LOEB 693 



about 3.5, and then drop again; and second, that only the valency 

 of the ion in combination with a protein influences its osmotic pressure 

 (or degree of swelling, etc.), while the specific nature of the ion 

 aside from its valency has no influence. The latter fact is really the 

 crucial point which decides between colloid chemistry and classical 

 physical chemistry. If only the valency of the ion in combination 

 with a protein is of importance and if gelatin-acid salts of the same 

 pH and the same concentration of originally isoelectric gelatin have 

 the same osmotic pressure, provided the anion of the gelatin-acid 

 salt has the same valency, a suspicion must arise that we are dealing 

 with some equilibrium condition for which classical physical chem- 

 istry is able to account. The writer has shown in preceding papers 

 that gelatin chloride, nitrate, acetate, succinate, tartrate, citrate, 

 and phosphate have at the same pH and the same concentration of 

 originally isoelectric gelatin the same osmotic pressure ; and it was 

 shown by titration curves that the anion in all these salts is mono- 

 valent. The titration curves show also that the anion in gelatin 

 sulfate is bivalent and we have found that the osmotic pressure of 

 gelatin sulfate is less than half that of gelatin chloride or phosphate 

 at the same pH and for the same concentration of originally isoelec- 

 tric gelatin.^ 



Fig. 1 illustrates this valency effect in the observed osmotic pres- 

 sure. The curves for the observed osmotic pressure of gelatin chlo- 

 ride and gelatin phosphate are identical while the curve for gelatin 

 sulfate is considerably lower. 



It is the purpose of this paper to show that we can calculate with a 

 fair degree of accuracy the osmotic pressure of gelatin solutions on 

 the assumption of the validity of Donnan's equilibrium equation 

 and the validity of van't Hoff's theory of osmotic pressure. 



//. Theoretical Data. 



A gelatin chloride solution contains free hydrochloric acid, gelatin 

 chloride (which dissociates electrolytically like any other salt in 

 watery solution), and non-ionogenic protein molecules. A 1 per 

 cent gelatin chloride solution of about pH 3.5 is in equilibrium with a 



^Loeb, J., /. Gen. Physiol., 1920-21, iii, 85, 247, 2>9V, Science, 1920, lii, 449. 



