292.

Explain the terms: buffer solution and buffer capacity.

Buffer solution: A buffer solution may be defined as a solution which resists change in its pH on the addition of water or small amount of acid or base. The ability of the buffer solution to resist change in its pH on the addition of acid or base is called buffer action.

Types of buffer solutions : Buffer solutions are of two types:
(i) Acidic buffer. An acidic buffer is a mixture (equimolar) of a weak acid and its salt with strong base. For example CH₃COOH and CH₃COONa ; H₂CO₃ ; H₃PO₄ and K₃PO₄.

(ii) Basic buffer. A basic buffer is a mixture (equimolar) of a weak base and its salt with strong acid. For example NH₄OH and NH₄Cl ; Ca(OH)₂ and CaSO₄.
Buffer capacity or Buffer index: The buffer capacity is defined as the amount (number of moles) of the acid or base which when added to 1 litre of the given buffer solution change its pH by unity i.e. 
           

where d[B] = change in the concentration of base
          d[A] = change in the concentration of acid
  or     dpH = change in pH of the buffer solution
     Buffer capacity is always positive, because
  (i) On adding acid to buffer, its pH is decreased i.e. d[A] is positive but dpH is negative so that
(ii) On adding base to a buffer solution, its pH increases, i.e. d[B] and dpH are both positive and hence