140 HYDROGEN ION CONCENTRATION 



and the mass law demands that 



(5) 

 (6) 



(I) 



(II) 



(III) 



1 + ^h^+l 



Of these functions that for p is in the simplest form. In the 

 absence of salt-formation its usual form is 



1 



-t 



(la) 



i+ 



The value ka in (la) is substituted in (I) by ka( r" + 1), and this 



■kg 



latter value is constant for a given concentration of added cations. 



This p-curve is therefore identical with that of any other acid whose 



i+ . 



dissociation constant amounts to ka (r~ + 1) without true salt forma- 



ks 



tion. The meaning of this in other words is as follows: If we vary 



the pH of a very dilute acetic acid solution by the addition of a very 



small amount of NaOH, then the p-curve has its originally derived 



form (page 000). But if this solution also contains an excess of a 



salt in constant concentration, then the resulting p-curve is such 



as it would be of an acid with a somewhat greater dissociation 



i+ 



constant k'a = ka (r" + 1). Graphically this is shown by a small 



ks 



horizontal displacement of the p-curve, this horizontal displacement 



i+ 

 is to the extent of —log (r" + 1). This is shown in figure 17 for 



ks 



several assumed values of ks, and in figure 20 for various values of 1.+ 



