CH. I.] " BUFFERS." 17 



have apparently the same acidity. But actually the 

 hydrochloric acid has an (H) over 60 times greater than 

 the acetic acid. The reason for this is that the acetic 

 acid is only very slightly ionised, the amount ionised 

 being a certain proportion of the total acid present. 

 As soon as the ionised part has been removed by the 

 addition of a base, a further fraction of the previously un- 

 dissociated acid is ionised. This process is repeated with 

 further additions of alkali until the whole of the acid 

 originally present has become dissociated, and its hydrogen 

 ions have united with the hydroxyl ions of the base. The 

 next trace of added alkali reacts with the indicator to give 

 a pink colour. Titration therefore only gives us an index 

 of the capacity of the solution to neutralise acids or alkalies ; 

 it does not give us information concerning the potential of 

 the hydrogen ions, i.e. the P H . 



"Buffers." A single drop of 0-02 N.HC1 added 'to a 

 litre of pure water at 18 C. would cause a change in P H 

 from 7-07 to about 6. A trace of diffusible alkali from a 

 glass bottle might change the P H to 8, or even higher, 

 whereas exposure to the CO 2 of the air might cause a drop 

 to about 6. Thus it is extremely difficult to maintain any 

 constancy of P H in such a solution. But with certain 

 substances present the addition of a small amount of acid 

 or alkali causes only a minimal change in P H . Such 

 substances are called " Buffers." Various solutions are 

 used for this purpose, such as phosphates, citrates, borates, 

 and acetates. Let us consider the case of a solution of 

 sodium acetate, to which is added a small amount of 

 hydrochloric acid. Both substances are freely dissociated 

 so that the following ions are originally present, Na, 

 ( CH 3 .COO, H, Cl. Now acetic acid is a weak acid, which 

 means that CH 3 .COO and H ions can exist together only in 

 very low concentrations. We therefore get 



Na + CH 3 .COO + H + Cl = CH 3 .COOH + Na + Cl. 



Thus the H ions of the added hydrochloric acid nearly 

 disappear, owing to the presence of the buffer sodium 



