74 HYDROGEN ION CONCENTRATION 



or, as it is customarily written 



CH3COO NH4 + HoO ;^ CH3COOH + NH4OH 



the amphoteric ion Hkewise yields by hydrolysis the ordinary amino- 

 acid molecule: 



+NH3-R-C00-^ NH2-R-COOH 



Since one mol of amino-acid yields one mol of amphoteric ion, 

 it follows that for a given amino-acid the concentration of the 

 amphoteric ion must be an equivalent fraction of the concentration 

 of the undissociated amino-acid. There is no direct method up to 

 the present time of demonstrating concretely the existence of the 

 amphoteric ions. Because of their very small concentration they 

 are not capable of disturbing to a measurable extent the equilibrium 

 among the other known dissociation products of amino-acids. 



20. The hydrogen ion concentration of a pure solution of ampholyte 



It is apparent that a pure aqueous solution of an ampholyte must 

 have a neutral reaction whenever ka = kb, that it must be acid in 

 reaction when ka > kb and alkaline when ka < kb. The exact 

 calculation of the hydrion concentration, however, is not a simple 

 process. S0rensen^^ adopted the following method of calculation. 

 Let A represent the total concentration of the amino-acid and A+, 

 A~ and U the concentrations of its cations, anions and undissociated 

 portion respectively. It then follows 



A+ + A- + U = A (I) 



A+ X H+ = k X U (II) 



A+ X OH- = kli X U (III) 



H+ X OH- = kw (IV) 



and furthermore, in accordance with the law of electroneutrality, 



A+ + H+ = A- + OH- (V) 



If the values of A, ka and kb be given, then the five unknown 

 values of U, A+, A~, [H+] and [0H~] may be calculated by solving 



" S. P. L. Sorensen, Ergebn. d. Physiol. 12, 495 ff. (1912). 



