How do acid buffer solutions work?

Firstly, we have to understand what a buffer is. A buffer is a solution which resists changes in pH when small amounts of acid or base are added. The key word here is small amounts, as if too much acid or base is added then the position of the equilibrium in the dissociation equation will change, altering the pH of the solution. An acidic buffer is usually made up of a large quantity of a weak acid and its corresponding salt, ie: ethanoic acid and sodium ethanoate. This produces and equilibrium equation of: CH₃COOH < > CH₃COO^- + H⁺ Here we have to understand that the hydrogen ion concentration is low as ethanoic acid is a weak acid and so only dissociates by a small amount. Also the concentration of the ethanoic acid and ethanoate ions are high as due to the added sodium ethanoate.
So what happens on addition of acid? The addition of acid causes the hydrogen ion concentration to increase, and due to Le Chatelier's principle, the extra hydrogen ions react with the excess ethanoate ions shifting the position of equilibrium to the left, returning the hydrogen ion concentration back to normal. So there is no altering in pH.On the addition of alkaline, the added hydroxide ions react with the already present hydrogen ions, producing water. This leads to a reduction in the overall hydrogen ion concentration, and so the position of equilibrium shifts to the right now, causing the excess ethanoic acid to dissociate more to produce extra hydrogen ions to prevent the pH from rising.