Please explain the mechanism of acidic buffer with an example.

An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts. The common example would be a mixture of ethanoic acid and sodium ethanoate in solution. In this case, if the solution contained equal molar concentrations of both the acid and the salt, it would have a pH of 4.76.The pH of the buffer can change by changing the ratio of acid to salt, or by choosing a different acid and one of its salts.

Ethanoic acid is a weak acid, and the position of this equilibrium will be well to the left:

Adding sodium ethanoate to this adds lots of extra ethanoate ions. According to Le Chatelier's Principle, that will shift the position of the equilibrium even further to the left.

The solution will therefore contain these important things:

• lots of un-ionised ethanoic acid;
• lots of ethanoate ions from the sodium ethanoate;
• enough hydrogen ions to make the solution acidic.

Mechanism of Buffer solution when an acid is added:

If an acid is added to above buffer solution it will increase the concentration of hydrogen ions in solution. Hydrogen ions will combine with the ethanoate ions to make ethanoic acid. Most of the new hydrogen ions are used up in this way.Since most of the new hydrogen ions are removed, the pH won't change very much.

Mechanism of Buffer when a base is added:

Alkaline solutions contain hydroxide ions and the buffer solution removes most of these hydroxide ion by reacting with hydrogen ions . As Hydrogen ions get consumed by the hydroxide ions it leads to dissociation of acetic acid molecule to furnish the hydrogen ion further and maintain the equilibrium. Hence the pH of the system will not change.