1904.] 



Theory of Amphoteric Electrolytes. 



155 



" Theory of Amphoteric Electrolytes." By Professor James 

 Walker, F.K.S,, University College, Dundee. Eeceived 

 February 3, — Read February 18, 1904. 



During the, past few years considerable attention has been devoted 

 to the behaviour of amphoteric electrolytes, i.e., of substances capable 

 of behaving as acids towards bases and as bases towards acids. The 

 most exhaustive investigation of such substances is that by 

 Winkelblech,* who determined by the customary hydrolytic methods 

 the .dissociation constants of a number of amino-acids, both with 

 respect to their ionisation as acids and as bases. 



The ionisation theory of these amphoteric electrolytes has not yet, 

 however, been fully worked out, although the fundamental equilibrium 

 equations have already been stated by Bredig,f and it is the object 

 of this paper to present the theory from the standpoint of the law of 

 mass action and Arrhenius's theory of electrolytic dissociation. 



It is necessary first to deduce Ostwalcl's dilution law for simple 

 electrolytes in the form in which we shall afterwards meet with it. 

 When an acid, say acetic acid, is dissolved in water, the equilibrium 

 between the ions present in the solution is expressed by means of two 

 equations, one regulating the equilibrium between hydrion and the 

 hydroxiclion derived from the water, the other regulating the 

 equilibrium of hydrion and the anion of the acid. Let the active 

 masses (molecular concentrations) of the various substances involved 

 be expressed as follows : 



Hydrion H+ Hydroxidion OH" Anion X" Unionised acid HX 

 a b c u 



then, from the law of mass action, 



ab = K (1), ac = k a u (2), 



where K is the constant ionic product for water, which includes within 

 it the constant active mass of water, and k a is the dissociation constant 

 of the acid. Now in order that the solution may be electrically 

 neutral, the concentration of the positive ion must be equal to the sum 

 of the concentrations of the negative ions, i.e., 



a = b + c (3) 



Summing (1) and (2) and substituting a for b + r, we obtain 



a*~K+h a u (4). 



* * Zeit. f. physikal. Chem.,' vol. 36, p. 546, 1901. 

 t 'Zeit. f. Elektrochemie,' vol. 6, p. 34, 1899. 



