Buffer solution

The solution which can resist the change in pH on addition of small amount of strong acid or strong base is called buffer solution. There are two types of buffer in general.

1. Acidic buffer

2. Basic buffer

1. Acidic buffer: The buffer solution having pH value in acidic range is called acidic buffer. This buffer solution is prepared by mixing equimolar solution of weak acid and weak base.

Example: Boric acid and borax, acetic acid and sodium acetate etc.

2. Basic Buffer: The buffer solution having pH value in basic range is called basic buffer. This buffer solution is called basic buffer. This buffer solution can be prepared by mixing equimolar solution of weak base and its salt with strong acid. Example: glycerine and glycerine hydrogen chloride, ammonium hydroxide and ammonium chloride  etc.

Buffer capacity

The number of moles of compound required to change to change the pH of the one litre buffer solution is by unity is called buffer capacity. Generally,

$$\text{Buffer Capacity} = \frac{\text{Number of moles of acid or base }}{\text{Change in pH}}$$

Mechanism of Buffer Action:

Let us consider the working on an acidic buffer CH3COOH CH3COONa.  In this buffer solution, following equilibria may exists:

$$\ce{CH3COOH  <=> CH3COO^- + H^+}$$

$$\ce{CH3COONa  -> CH3COO^- + Na+ }$$

$$\ce{H2O <=> H^+ + OH^-}$$

If HCL is added to the solution, it gives H+ ion. Thus formed H+ ion combine with CH3COO  to form weakly ionizable CH3COOH there by maintaining constant pH value.

$$\ce{$\underset{\text{From Buffer}}{\ce{CH3COO^-}}$ + $\underset{\text{From Acid (HCl)}}{\ce{H^+}}$  <=> CH3COOH}$$

When NaOH is added to the solution, it provide OH– ions. These OH ions will react with the H+ ions from the buffer solution to form unionized water molecules. Hence, the pH value of the solution remains constant. The reaction process is given below:

$$\ce{$\underset{\text{From Buffer}}{\ce{H^+}}$ + $\underset{\text{From Base (NaOH)}}{\ce{OH^-}}$ -> H2O}$$

In this way the addition of small amount of acid or base is neutralized.

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