440 



SCIENCE 



[N. S. Vol. L. No. 1297 



osmotic pressure, depend therefore not only 

 upon the concentration of the gelatin in 

 solution but also upon the hydrogen ion con- 

 centration. 



Colloid chemists usually state only the 

 amount of acid added to a protein without 

 measuring the hydrogen ion concentration of 

 their protein solution. The effect of the 

 addition of the same amount of acid upon 

 the chemical and physical properties of 

 gelatin is entirely different according to the 

 hydrogen ion concentration of the gelatin 

 used. When a slight amount of acid is 

 added to isoelectric gelatin it will increase 

 its osmotic pressure while the same amoimt 

 of acid if added to gelatin with a pH = 3.3 

 or to neutral gelatin (pH = 7.0) will dimin- 

 ish its osmotic pressure. Since the hydrogen 

 ion concentration of commercial gelatin 

 varies and since, moreover, the combining 

 power of different acids with gelatin varies 

 also,* the results obtained by the addition of 

 electrolytes without measurement of the hy- 

 drogen ion concentration are irregular and 

 confusing. 



In addition, the properties of gelatin salts 

 depend upon at least two more variables, 

 namely, the nature of the ion in combination 

 with the gelatin and the concentration of 

 electrolyte present. When we transform 1 

 per cent, solutions of isoelectric gelatin into 

 sodium gelatinate and calcium gelatinate both 

 possessing the same hydrogen ion concentra- 

 tion (e. g., 10"") the sodiiun gelatinate has 

 an osmotic pressure more than twice as 

 great as the calcium gelatinate. This differ- 

 ence is not due to a difference in the degree 

 of electrolytic dissociation since both solu- 

 tions have the same conductivity.^ When we 

 add increasing quantities of neutral salts or 

 alkalies to the two solutions the osmotic 

 pressure is depressed in both solutions and 

 if enough is added the osmotic pressure falls 

 to almost zero in both solutions. (If we add 

 acid, the same will occur but for another 

 reason, the metal gelatinate being brought to 

 the isoelectric point, and, by addition of more 



*Loeb, J., J. Gen. Physiol, 1918-19, I., 559. 

 6 Loeb, J., J. Gen. Physiol, 1918-19, I., 483. 



acid, being transformed into gelatin-acid 

 salts.) 



The same difference as between sodium and 

 calcium gelatinate exists between gelatin 

 chloride and gelatin sulfate* and this differ- 

 ence is also obliterated when neutral salt or 

 acid is added to the solution. (The addition 

 of an excess of alkali would transform the 

 gelatin acid into isoelectric gelatin and 

 finally into metal gelatinate.) 



If we wish to investigate the specific effect 

 of different ions on the physical properties 

 of gelatin (or of proteins in general) it is 

 therefore necessary to avoid an excess of 

 electrolytes. The writer proceeds in the fol- 

 lowing way. Finely granulated (commercial) 

 gelatin is brought to the isoelectric point by 

 the method described in the writer's previous 

 publications. Isoelectric gelatin if properly 

 washed will lose its ionogenic impurities. 

 Just enough acid or alkali is then added to 

 1 gm. of isoelectric gelatin to produce a 

 gelatin salt (either gelatin acid or metal 

 gelatinate) of the desired hydrogen ion con- 

 centration. Since there exists an equilib- 

 rimn between free acid (or free alkali) 

 gelatin salt and isoelectric (non-ionogenic) 

 gelatin two solutions of metal gelatinate (e. g., 

 Na gelatinate and Ca gelatinate) each con- 

 taining 1 gm. of isoelectric gelatin and each 

 possessing the same hydrogen ion concentra- 

 tion contain the same proportion of metal 

 gelatinate and non-ionogenic gelatin. Differ- 

 ences in the physical properties of these two 

 solutions may be ascribed to differences in 

 the effect of the metal ion in combination 

 with the gelatin. The same is true for solu- 

 tions of gelatin chloride and gelatin sulfate 

 of the same hydrogen ion concentration if 

 prepared from isoelectric gelatin of the same 

 concentration. If this procedui'e is not fol- 

 lowed, erroneous results will be obtained such 

 as are found in the textbooks of colloid 

 chemistry. Thus it is generally stated that 

 acids and alkalies increase the osmotic 

 pressure of gelatin while neutral salts depress 

 it. This statement is entirely wrong and due 

 to the fact that the experimenter responsible 

 for this statement did not work with gelatin 



