AMPHOTERIC COLLOIDS. 



II. Volumetric Analysis of Ion-Protein Compounds; the Sig- 

 nificance OF THE Isoelectric Point for the Purifi- 

 cation OF Amphoteric Colloids. 



By JACQUES LOEB. 

 {From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, August 28, 1918.) 



introduction. 



In order to understand the influence of the hydrogen ion concen- 

 tration upon the chemical and physical behavior of proteins we may 

 well keep in mind the role of the hydrogen ion concentration upon the 

 reactions of an amphoteric metal hydroxide, like aluminium or chromic 

 hydroxide, whose chemical constitution is better known than that of 

 the proteins. 



"Aluminium hydroxide dissolves in acids. From its solution in hydrochloric 

 acid, an aluminium salt, aluminium chloride AICI3, 6H2O, is obtained. It also 

 combines with strong bases, dissolving for instance in a solution of sodium hy- 

 droxide and forming an aluminate, NaA102 .... According to the best knowledge 

 we have on the subject, the molecule of aluminium hydroxide has the following 

 structure or arrangement of its atoms Al(-0-H)3. It is readily seen that the 

 cleavage of the molecules may produce, either aluminium and hydroxide ions, 

 characteristic ions of a base, or aluminate and hydrogen ions, characteristic ions of 

 an acid: 



A1+++ + 3 OH ^ Al(-0-H)3 ?=i AIO2 -F H+ + H20."i 



It is obvious that between the action of acids and of bases producing 

 these two types of electrolytic dissociation there must be one hydrogen 

 ion concentration in which aluminium hydroxide is practically neither 

 able to form A1+++ nor AIO2 ions, and this would be the isoelectric 

 point. 



^ Stieglitz, J., The elements of qualitative chemical analysis, New York, 1911, 

 171. 



237 



