

 34 ELECTRIC CHARGE ON NATURAL SOLUBLE 



of natural proteins is, however, the evaluation of the values of 

 both K and K 6 . The methods employed for measuring these 

 two constants in simpler amphoteric electrolytes, especially 

 those directed to the determination of the dissociation equili- 

 brium of their salts, are scarcely applicable to natural proteins, 

 for reasons which will appear below. More recent attempts to 

 determine K and K 6 directly by a special method are not yet 

 concluded. 



j 



The methods hitherto employed for determining ^~ depend 



j^b 

 on two principles : 



I. The direct discovery of the iso-electric condition of a 

 protein by means of the application of an electric field 

 in an electrophoresis apparatus, the hydrion content 

 of the solution being varied. 



II. The determination of the hydrion concentration which 

 shows, by various tests, a maximum of neutral 

 particles. 



We must next make some observations on the first method of 

 determining the iso-electric point by electrophoresis, the 

 discussion of the second method being deferred to the following 

 chapter.* 



The technical difficulty of this determination lies mostly in 

 the fact that even with the most precise procedure the change 

 from movement to the anode to movement to the cathode does 

 not occur at a sharply defined point. It more frequently 

 happens that the movement is in both directions (or, less 

 frequently, no movement occurs) over a wide range of hydrion 

 concentration. Sometimes, indeed, in different observations 

 the direction is uncertain. One is therefore forced to take the 

 mean of the limiting values which show definite migration in 

 either sense. 



The following facts are important when considering migration 

 in both directions. If the electrophoresis apparatus is filled 

 and left to itself with the taps open, traces of albumin can be 



* A third and recent method of determining the hydrion concentra- 

 tion which remains unaltered on varied additions of ampholyte is due to 

 Soiensen, and will be dealt with at the end of the following chapter. 



