Buffer capacity:

The property of a buffer solution to resist alteration in its pH value is known as buffer capacity. It has been found that if the ratio [Salt]/[Acid] or [Salt]/[Base]  is unity, the pH of a particular buffer does not change at all. Buffer capacity is defined quantitatively as number of moles of acid or base added in one litre of solution as to change the pH by unity,

i.e..

Buffer capacity

(φ) = (No.of moles of acid or base added to 1 litre)/(Change in pH)

or   φ = δb/δ(ph) 

where δb → number of moles of acid or base added to 1 litre solution and δ(pH) → change in pH.

Buffer capacity is maximum:

(i)  When [Salt] = [Acid], i.e., pH = pK_a for acid buffer

(ii) When [Salt] = [Base], i.e., pOH = pK_b for base buffer under above conditions, the buffer is called efficient.

Utility of buffer solutions in analytical chemistry

Buffers are used:

(i)  To determine the pH with the help of indicators.

(ii)  For the removal of phosphate ion in the qualitative inorganic analysis after second group using CH₃COOH + CH₃COONa buffer.

(iii) For the precipitation of lead chromate quantitatively in gravimetric analysis, the buffer, CH₃COOH + CH₃COONa, is used.

(iv) For precipitation of hydroxides of third group of qualitative analysis, a buffer, NH₄Cl + NH₄OH, is used.

(v)  A buffer solution of NH₄Cl, NH₄OH, and (NH₄)₂CO₃ is used for precipitation of carbonates of fifth group in qualitative inorganic analysis.

(vi) The pH of intracellular fluid, blood is naturally maintained. This maintenance of pH is essential to sustain life because, enzyme catalysis is pH sensitive process. The normal pH of blood plasma is 7.4. Following two buffers in the blood help to maintain pH (7.4).

       (a) Buffer of carbonic acid (H₂CO₃ and NaHCO₃)

       (b) Buffer of phosphoric acid (H₂P0₄, HPO^2-)

Buffers are used in industrial processes such as manufacture of paper, dyes, inks, paints, drugs, etc. Buffers are also em­ployed in agriculture, dairy products and preservation of vari­ous types of foods and fruits.