small amounts of acid or alkali are called Buffer Solutions. Buffer solutions of known pH can be prepared from the knowledge of p K a of the acid or p K b of base and by controlling the ratio of the salt and acid or salt and base. A mixture of acetic acid and sodium acetate acts as buffer solution around pH . and a mixture of ammonium chloride and ammonium hydroxide acts as a buffer around pH .
. You will learn more about buffer solutions in higher classes. . .
Designing Buffer Solution Knowledge of p K a , p K b and equilibrium constant help us to prepare the buffer solution of known pH. Let us see how we can do this. Preparation of Acidic Buffer To prepare a buffer of acidic pH we use weak acid and its salt formed with strong base. We develop the equation relating the pH, the equilibrium constant, K a of weak acid and ratio of concentration of weak acid and its conjugate base.
For the general case where the weak acid HA ionises in water, HA + H O H O + + A – For which we can write the expression Rearranging the expression we have, Taking logarithm on both the sides and rearranging the terms we get — Or ( . ) ( . ) The expression ( . ) is known as Henderson–Hasselbalch equation.
The quantity is the ratio of concentration of conjugate base (anion) of the acid and the acid present in the mixture. Since acid is a weak acid, it ionises to a very little extent and concentration of [HA] is negligibly different from concentration of acid taken to form buffer. Also, most of the conjugate base, [A — ], comes from the ionisation of salt of the acid. Therefore, the concentration of conjugate base will be negligibly different from the concentration of salt.
Thus, equation ( . ) takes the form: pK a H= [Salt] [Acid] + log In the equation ( . ), if the concentration of [A — ] is equal to the concentration of [HA], then pH = p