68 HYDROGEN ION CONCENTRATION 



this molecular species. Hence in such a case we may consider an 

 ion as the undissociated molecule, whereas in the past we have been 

 accustomed to consider the electroneutral molecule as the undis- 

 sociated form. This ion then dissociates, similarly to a dibasic acid, 

 in the following way: 

 1st Stage: 



+NH3-R-COOH -^ +NH3-R-COO-+H+ 



2nd Stage: 



+NH3-R-COO- -^ NH2-R-COO-+H+ 



The equilibrium constants are, for the first stage: 



[+NH3-R-COO-] [H+] 

 [+NH3-R-COOH] ~ ' 



and for the second stage : 



[NH2-R-COO-] [H+] 

 [+NH3-R-COO-] ~ ' 



The relation of ki and k2 to the hitherto employed (and still widely 

 used and for most practical purposes usable) values ka and kb is: 



ka = k2 



kw 



For the aromatic amino acids (such as aminobenzoic acid) this 

 conception does not apply so well. In these the molecular species 

 NH2 — R — COOH may actually exist in measurable concentrations 

 along with the ampholyte ion +NH3— R — C00~. As for the am- 

 photeric electrolytes with weaker acidogenic radicals, such as amino- 

 phenol, on the other hand, the older conception is the more correct 

 one. 



Glycocoll is a stronger acid than acetic acid and a stronger base 

 than ammonia. Its inner salt (the ampholyte-ion) is therefore 

 extremely little dissociated hydrolytically which corresponds to 

 stating that there are very few molecules in the form NH2 — CH2 — 

 COOH. 



It is very probable that also the proteins in the isoelectric state 

 exist in the ampholyte-ion form. The amphoteric nature of such ions 



