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The pH of acidic and basic buffer can be calculated by Henderson – Hasselbalch equations. Consider an acidic buffer HA + A - HA H + + A - K a = [H + ] [A - ] / [HA] [H + ] = K a [HA]/[A - ] [H + ] = K a [acid]/[salt] There fore pH = -log[H + ] pH = pK a + log [salt]/[acid] when, [salt]/[acid] = 1 , pH = pK a Since pK a of an acid is a constant at constant temperature, the pH of the buffer is constant. Thus buffer capacity is maximum in a solution containing equivalent amount of acid and its salt. The pH of basic buffer is also given by Henderson – Hasselbalch equation BOH B + + OH - K b = [B + ][OH - ]/[BOH] [OH - ] = K b [BOH]/[B + ] pOH = pK b + log [salt]/[base] pH = 14 – pOH = 14 – [pK b + log [salt]/[base]]

The property of a buffer solution to resist change in its pH value even when small amounts of the acid or the base are added to it is called buffer action . Consider the acidic buffer solution containing acetic acid and sodium acetate. They dissociate as CH3COONa <=======> CH3COO - + H + CH3COONa <=======> CH3COO - + Na + When a few drops of an acid, HCl is added to this buffer solution, the H + ions combine with CH3COO - ions to form weakly ionized molecules of CH3COOH. CH3COO - + H + <=======> CH3COOH Thus H + ion concentration does not change and hence the pH of the solution remains constant. When a few drops of base, NaOH is added to the buffer solution, hydroxyl ions of the base neutralize the acid, forming salt and water. Similarly, in a basic buffer solution of NH4OH and NH4Cl, they dissociates as NH4OH <======> NH4 + + OH - NH4Cl ----------> NH4 + + Cl - When a few drops of a base added, the OH - ions given by it combine with NH4 + ions to ...