Buffer Solutions

The extent of dissociation of a weak acid can be decreased by adding a salt containing a common ion. The change in the extent of dissociation can also be varied by changing the concentration of the common ion. The mixture of a weak base and a salt of common ion also behave in the same way. These aqueous solutions containing weak acids or bases and a salt of common ion are called buffer solutions.

Buffer solutions are the ones that resist a change in its pH on adding a small amount of an acid or a base.

In laboratory reactions, in industrial processes and in the bodies of plants and animals, it is often necessary to keep the pH nearly constant despite the addition of acids and bases. The oxygen carrying capacity of haemoglobin in blood and activity of the enzymes in cells depends very strongly on the pH of body fluids. pH of the blood is very close to 7.4 and pH of saliva is close to 6.8. Animals and plants are protected against sharp changes in pH by the presence of buffers.

There are two kinds of commonly used buffer-solutions:

  1. A weak acid and a soluble ionic salt of the weak acid. For example, acetic acid and sodium acetate; CH3COOH + CH3COONa
  2. A weak base and a soluble ionic salt of the weak base. For example, ammonium hydroxide and ammonium chloride; NH4OH + NH4Cl

The buffers with pH less than 7 are called acidic buffers and those with pH above 7 are called basic buffers. Acetic acid - sodium acetate buffer is an example of acidic buffer while ammonium hydroxide - ammonium chloride is a basic buffer.

Buffer Action

A buffer system contains a conjugate acid-base pair and the concentrations of these two are quite high as compared to that of the hydronium ions. These are called as the acid reserve and the base reserve respectively.

The added acid or base reacts with these reserves and gets consumed without significantly altering the hydronium ion concentration and therefore the pH does not change significantly.

Henderson-Hasselbalch Equation

This equation relates the pH of a given buffer solution to the concentrations of its components - weak acid / salt or weak base / salt.