54 HYDROGEN ION CONCENTRATION 



All of the above applies entirely and directly to the dissociation 

 of the bases, and all that is needed is to substitute [0H~] for [H+] 

 and pOH for pH. 



Therefore for a base, 



[0H-] " k 



1 + -—■ 1 + 



k [0H-] 



But 



[0H-] = — 



[H+] 



therefore, substituting for [0H~] above, 



The a-curve of a base with a dissociation constant k is, therefore, 



identical with the p-curve of an acid, whose dissociation constant 



. kw 



is -j^, and the p-curve of a base of constant k is identical with the 



kw 

 a-curve of an acid whose constant is -^. 



k 



A simple verification of the course of a dissociation curve will be 

 found in the following experiment. p-Nitrophenol is a weak acid 

 whose ions are yellow and whose undissociated molecule is color- 

 less. If the same small amount of this indicator be added to a 

 series of solutions adjusted to varying pH values by the acetate 

 buffers, then the degree of dissociation a of the p-nitrophenol and 

 simultaneously the intensity of its color will vary with the pH of 

 the solution. In figure 11, p. 98, the pH is given on the abscissa 

 and the intensity of color, or the degree of dissociation a, of the dye 

 on the ordinate. The course of the curve corresponds exactly to 

 the theoretical a-curve, and, if pK be determined graphically by 

 the method given above it will be found to have the value of 7.18, 

 while ahnost the same value, 7.28, was observed by means of the 

 conductivity method (calculated for the same temperature, 18° 

 from the data of Euler and Bolin and also of Lunden). 



