A buffer solution is a system that minimises pH changes when small amounts of an acid or a base are added.

The two methods to prepare an acidic buffer solution are:

1. Mix a solution of a weak acid with a solution of one of its salts, for example, mix together ethanoic acid and sodium ethanoate.
2. Partially neutralise a weak acid with an alkali, for example, partially neutralise ethanoic acid with sodium hydroxide.

The resulting solutions in both methods contains the weak acid and its conjugate base, for example, ethanoic acid and ethanoate ions.

Mix a solution of a weak base with the salt of its conjugate acid, for example, mix together ammonia and ammonium chloride.

The resulting solution contains the weak base and its conjugate acid, for example, ammonia and ammonium ions.

When acid is added to an acidic buffer solution:

1. The concentration of H⁺ ions increases.
2. The excess H⁺ ions react with the conjugate base (A^-) to form the weak acid (HA).
3. The equilibrium position shifts to the left, consuming most of the added H⁺ ions and minimising the change in pH.

When alkali is added to an acidic buffer solution:

1. The concentration of OH^- ions increases.
2. The added OH^- ions react with H⁺ ions from the buffer, forming water: H⁺_(aq) + OH^-_(aq) ➔ H₂O_(l)
3. To replenish the H⁺ ions, the weak acid (HA) dissociates further, shifting the equilibrium position to the right and minimising the change in pH. 

The body converts carbonic acid to carbon dioxide gas, which is exhaled by the lungs. This prevents the blood from becoming too acidic.

H₂CO_3(aq) ⇌ CO_2(aq) + H₂O_(l)

When acid is added to the carbonic acid-hydrogencarbonate buffer system in blood:

1. The concentration of H⁺ ions increases.
2. The excess H⁺ ions react with the conjugate base (HCO₃^-) to form carbonic acid (H₂CO₃).
3. The equilibrium shifts to the left, consuming most of the added H⁺ ions and minimising the change in blood pH.

The buffer solution loses its ability to resist pH changes, and further additions of acid or alkali will lead to significant pH changes.

When alkali is added to the carbonic acid-hydrogencarbonate buffer system in blood:

1. The concentration of OH^- ions increases.
2. The added OH^- ions react with H⁺ ions from the buffer, forming water: H⁺_(aq) + OH^-_(aq) ➔ H₂O_(l)
3. To replenish the H⁺ ions, carbonic acid (H₂CO₃) dissociates further, shifting the equilibrium to the right and minimising the change in blood pH