Question

If after titration, you find that the OTC buffer handles the addition of acid much better than base, explain how this is made possible chemically. In other words, how is the buffer designed specifically to absorb more acid than base?   Be specific and detailed in your answers.

Answer 1

When a strong acid (H₃O⁺) is added to a buffer solution the conjugate base present in the buffer consumes the hydronium ion converting it into water and the weak acid of the conjugate base.

A^-(aq) + H₃O⁺(aq) --> H₂O(l) + HA(aq)

This results in a decrease in the amount of conjugate base present and an increase in the amount of the weak acid. The pH of the buffer solution decreases by a very small amount because of this ( a lot less than if the buffer system was not present).

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 tip the position of the equilibrium even further to the left.
	Note: If you don't understand Le Chatelier's Principle, follow this link before you go any further, and make sure that you understand about the effect of changes of concentration on the position of equilibrium. Use the BACK button on your browser to return to this page.
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. Other things (like water and sodium ions) which are present aren't important to the argument. Adding an acid to this buffer solution The buffer solution must remove most of the new hydrogen ions otherwise the pH would drop markedly. Hydrogen ions combine with the ethanoate ions to make ethanoic acid. Although the reaction is reversible, since the ethanoic acid is a weak acid, most of the new hydrogen ions are removed in this way. Since most of the new hydrogen ions are removed, the pH won't change very much.