SOIL REACTION 



421 



action is clear. Thus, suppose there is present a mixture of a 

 weak acid, e.g. acetic acid, and one of its salts, e.g. sodium 

 acetate. The acetic acid is only slightly ionised, so that 



[H-]^JAc1 

 [HAc] 



= Ka = i8-o X 10 



- 6 



The sodium acetate, on the other hand, is a strong electrolyte, 

 and a very large proportion of it is ionised. This involves 

 an enormous increase in [Ac'], and in order to keep Ka 

 of the acetic acid constant [H'] must be decreased, the result 

 being that such a solution is by no means so sensitive to additions 

 of acid or alkali as is pure water or pure aqueous acetic acid in 

 the absence of the salt. This is seen very clearly in Fig. i, * 



10 



20 30 40 50 60 70 



>■ Per cent of NaOH 



N 



90 100 



Fig. I. — Neutralisation curve of — acetic acid. 



10 



which represents the change in [H"] of — acetic acid when it 



is titrated with — alkali. At either end of the curve when only 

 10 



acetic acid or NaAc is present the addition of 10 per cent, of 



NaOH will cause a large alteration in Ph. At the point X, 



however, where the acetate content of the solution consists of 



50 per cent, acetic acid and 50 per cent. Na-acetate, the addition 



of 10 per cent, of NaOH or of acetic acid will produce only an 



insignificant effect on Ph- 



Similar considerations apply to a weak base in the presence 



of one of its salts. 



1 Reproduced from Prideaux's Theory and Use oj Indicators (Constable), 

 I9i7» ?• 228. 



