BFFFEPx SOLUTIONS 17 



same. The only el'Icft ui (.liluliun is to inciease the value 

 of a slightly as there is slightly more dissociation in dilute 

 solutions than in concentrated ones ; hence dilution of a 

 buffer solution will decrease the hydrogen ion concentra- 

 tion very slightly, that is the pH value will become very 

 slightly higher. 



If a strong acid such as hydrochloric acid is added to 

 a buffer solution the hydrogen ions to which it gives rise 

 will immediately combine with acetate ions to give 

 imdissociated acetic acid, and consequently there will be 

 but a slight change of hydrogen ion concentration. 

 Similarly, if an alkali such as sodium hydroxide is added, 

 the hydroxyl ions will combine with the hydrogen ions 

 to form water, and once more there will be very little 

 change in hydrogen ion concentration. This perhaps 

 becomes more clear in terms of the acid and conjugate 

 base view. Hydrogen ions resulting from addition of acid 

 to the systems are " neutralised " by the acetate ions 

 acting as a weak base : — 



H.OH2+ + A- ^=^ HA + H.O 



(acetate (acetic 

 ion) acid) 



Whilst hydroxyl ions are " neutralised " by the reaction 



OH- + HA ^=^ HoO + A- 



(acetic (acetate 



acid) ion) 



When the buffer is a weak base and one of its salts such as 

 ammonia and ammonium acetate the corresponding 

 reactions are 



H.OH,+ + NH3 ^=^ XH4+ + HoO 



(ammonia) (ammonium 



ion) 



and 



OH- + NH4+ ^=^ H.O + NH3 



(ammonium (ammonia) 



ion) 



The better the buffer the more acid or alkali is required 

 to alter the pK of the solution by a given amount. 



