304 PROPERTIES OF AMINO-ACIDS AND PEPTIDES 



other cause. An equilibrium relation between the two (or more) 

 unionized forms would exist as a rule. This conception complicates 

 somewhat the theoretical treatment of amphoteric electrolytes pre- 

 sented by Bredig,"" Walker,^^ and others by the addition of an equa- 

 tion representing such an equilibrium and by introducing the concen- 

 tration of the suitable molecular species in the equations representing 

 the ionization constants. In strongly acid or alkaline solution, how- 

 ever, practically only one form would be present. 



The difference in the ionization of the ampholyte is not always 

 brought out clearly. For example, with a protein, the two kinds of 

 ions are sometimes indicated as Na+ (Protein*) and (Protein+) Cl~. 

 As a matter of fact, rarely, if ever, do the ions (Protein") and (Pro- 

 tein +) have even the same chemical composition. This may be indi- 

 cated by taking the simple case of glycine. Here, the ions are 

 C1-(NH3CH2C02H)+ or C1-(C2H602N)+, and (NH2CH2C02)-Na+ or 

 (C2H402N)-Na+, or (HONH3CH2C02)-Na+ or (C2H603N)-Na+, 

 omitting possible hydration of the ions. 



The consideration of the transition point spoken of in connection 

 with the different kinds of ionization is a more difficult one. It can 

 best be treated from the side of the isoelectric points of the amphoteric 

 substances. 



The definitions of isoelectric point given by various workers have 

 been contradictory at times. The most satisfactory definition appears 

 to be that hydrogen ion concentration at which the properties such 

 as electrical conductivity, viscosity, solubility, etc., when studied over 

 extended ranges of acidity in moderately dilute solutions, show a point 

 of inflexion.!^ Chemically this would be interpreted as stating that in 

 more acid solutions the substance ionizes with the complex as part of 

 the cation, in more alkaline solutions, as part of the anion. At the 

 isoelectric point combination with added acid or base and accompa- 

 nying ionization is a minimum, or the substance is in a maximum 

 uncombined state. The method of cataphoresis in which the hydro- 

 gen ion concentration is determined at which the ampholyte does not 

 migrate in solution or migrates in both directions under the influence 



20 Bredig, G., Z. Elektrochem., 1899-1900, vi, 33. 



21 Walker, J., Proc. Roy. Soc. London, Series B, 1904, Ixxiii, 155; 1905, Ixxiv, 

 271; Z. physik. Chem., 1904, xlix, 82; 1905, li, 706. 



