34 THEORY OF COLLOIDAL BEHAVIOR 



replaced by the Mg of the MgSO 4 resulting in the formation of 

 magnesium gelatinate. The SO 4 , however, cannot affect the 

 properties of Na gelatinate since it cannot (or can practically not) 

 combine with the gelatin. When, however, we mix gelatin 

 chloride with MgSO4, only the SO4 can affect the properties of 

 the gelatin salt, since the SO 4 can replace the Cl in the gelatin 

 chloride resulting in the formation of gelatin sulphate. The Mg, 

 however, cannot (or can practically not) enter into combination 

 with gelatin chloride and hence cannot affect its properties. 



When we alter the pH of a gelatin-acid salt, e.g., gelatin 

 chloride, by adding alkali, e.g., NaOH, it will cease to be gelatin 

 chloride as soon as the pH is 4.7 because at this pH the Cl will be 

 given off by the gelatin and the latter will be transformed into the 

 chemically inert isoelectric or non-ionogenic gelatin and into 

 NaCl. The isoelectric gelatin can combine practically neither 

 with anions nor with cations. When we add more NaOH so that 

 the pH is >4.7, Na gelatinate will be formed. At no time can 

 metal gelatinate (e.g., Na gelatinate) and gelatin-acid salt (e.g., 

 gelatin chloride) exist simultaneously (except in traces beyond 

 the limits of analytical demonstration). When we have Na 

 gelatinate and add acid, e.g., HC1, the gelatin salt will give off its 

 Na and become isoelectric gelatin as soon as pH = 4.7. This 

 isoelectric gelatin is chemically inert being practically unable to 

 combine with either anion or cation. When we add more HC1, 

 gelatin chloride will be formed. 



These experiments show that proteins behave like amphoteric 

 electrolytes, forming definite salts with acids or bases, but that 

 they cannot combine simultaneously with the cation and the 

 anion of a neutral salt. The idea of the existence of adsorption 

 compounds between non-ionized molecules of proteins and mole- 

 cules of neutral salts is not in harmony with these experiments. 



In 1918 the writer 1 published a simple method of preparing 

 ash-free proteins based on the fact that at the isoelectric point 

 proteins can combine neither with anions nor with cations. 

 Hence, if we wish to prepare gelatin or casein free from ionogenic 

 impurities, we must bring these proteins in powdered form to the 

 isoelectric point and then wash them. This is of importance for 

 all industries using proteins as well as for scientific work. In the 

 , J., J. Gen. PhysioL, vol. 1, p. 237, 1918-19. 



