212 
CHEMISTRY: J. LOEB 
Proc. N. a. S. 
hydrogen ion concentration is the isoelectric point. When acid is added 
to a solution of isoelectric gelatin, part of the acid combines with the 
gelatin to form gelatin-acid salts, and the proportion of isoelectric gelatin 
transformed into gelatin-acid salt increases with the amount of acid added. 
There exists always a definite equilibrium betvveen free acid, gelatin- 
acid salt, and isoelectric gelatin. Since the physical properties of iso- 
electric gelatin are different from those of the gelatin-acid salt it is neces- 
sary to define a gelatin solution not only by the concentration of gelatin 
present but also by its hydrogen ion concentration. The writer was 
able to show that gelatin forms monogelatin salts with many dibasic or 
tribasic acids, e.g., oxalic, tartaric, and phosphoric acids, while it forms 
digelatin salts with sulfuric acid.^ 
When we add a base, e.g., NaOH or Ca(0H)2 to isoelectric gelatin, 
metal gelatinates are formed, and the relative amount of non-ionogenic 
gelatin transformed into metal gelatinate also depends upon the hydrogen 
ion concentration.^ 
II. We can measure the osmotic pressure of gelatin solutions by putting 
them into collodion bags (cast in the form of Erlenmeyer flasks) closed 
with a rubber stopper perforated by a glass tube which serves as a man- 
ometer. A 1% solution of gelatin to which a certain amount of acid or 
alkali had been added was put into the collodion bag which was dipped 
into a beaker containing water of the same hydrogen ion concentration 
as that inside the collodion bag. One per cent solutions of gelatin-acid 
salts had the highest osmotic pressure when the was about 3.4 and 
solutions of metal gelatinates had their highest osmotic pressure at a 
pH of about 8.4. At about 24° the permanent osmotic pressure was 
reached in less than 20 hours. 
The influence of ions on the osmotic pressure of gelatin solutions was 
as follows : 
(1) The osmotic pressure of 1% solutions of metal gelatinates of a pg 
of about 8.4 is a little over twice (but less than three times) as great when 
the metal is monovalent than when it is bivalent. 
(2) The osmotic pressure of 1% gelatin-acid solutions of 3.4 is a 
little over twice (but less than three times) as great when the acid anion 
is monovalent than in the case of SO4. 
(3) The addition of neutral salts or alkalies to 1% metal gelatinate 
solutions of 8.4 lowers the osmotic pressure of the solution and the de- 
pressing effect increases with the valency of the cation of the electrolyte 
added. 
(4) The addition of neutral salts or acids to 1% gelatin-acid solutions 
of pH 3.4 lowers the osmotic pressure of the solution and the depressing 
effect increases with the valency of the anion.^ 
This influence of electrolytes on the osmotic pressure of gelatin solu- 
tions has no connection with the fact that gelatin is a colloid.^ This 
