Question

An acidic buffer solution can be prepared by mixing the solutions of

• A. Ammonium acetate and acetic acid
• B. Ammonium chloride and ammonium hydroxide
• C. Sulphuric acid and sodium hydroxide
• D. Sodium chloride and sodium hydroxide

Answer 1

The correct option is A

Ammonium acetate and acetic acid

A buffer solution is a solution which resists any change in its pH value on dilution or an addition of solution of an acid or alkali. The process by which added $H^{+}$ or $O{H}^{-}$ ions are removed so that pH remains constant is known as buffer action.

Acid buffer solutions: An acid buffer solution consists of solution of a weak acid and its salt with strong base. The best known example is a mixture of solution of acetic acid and sodium acetate.

$C{H}_{3}COOH\rightleftharpoons C{H}_{3}CO{O}^{-}+H^{+}$(Weakly ionized)

$C{H}_{3}COONa\rightleftharpoons C{H}_{3}CO{O}^{-}+N{a}^{+}$(Strongly ionized)

Sodium acetate, being salt, ionises completely to form $C{H}_{3}CO{O}^{-}$ and $N{a}^{+}$ ions. On the other hand, acetic acid being a weak acid ionises very less. Moreover, its ionisation is further suppressed by the acetate ions from sodium acetate (common ion effect). When a few drops of an acid (say HCl) are added to it, the $H^{+}$ ions from the added acid combine with excess of acetate ions to form feebly or nearly unionised $C{H}_{3}COOH$. Thus there is no rise in $H^{+}$ ion concentration and the pH remains unchanged. When a few drops of an acid (say HCl) are added to it, the $H^{+}$ ions from the added acid combine with excess of acetate ions to form feebly or nearly unionised $C{H}_{3}COOH$. Thus there is no rise in $H^{+}$ ion concentration and the pH remains unchanged. On the other hand, when a few drops of base (say $NaOH$) are added, OH- of the added base reacts with acetic acid to form water (unionisable) and acetate ions.

Thus there is no increase in the $O{H}^{-}$ ion concentration, and hence the pH of the solution remains constant. pH values of acid buffers are less than 7 and can be calculated from the following Henderson's equation :

$pH$ = $p{K}_a+log\frac{\left[salt\right]}{\left[Acid\right]}$

Basic buffer solution : A basic buffer solution consists of a mixture of a weak base and its salt with strong acid. The best known example is a mixture of ammonium hydroxide and ammonium chloride.

$N{H}_{4}OH\rightleftharpoons N{H}_4^{+}+O{H}^{-}$

$N{H}_{4}Cl\rightleftharpoons N{H}_4^{+}+C{l}^{-}$

The $N{H}_4^{+}$ ions from completely ionised $N{H}_{4}Cl$ suppress the ionisation of $N{H}_{4}OH$ which is already a weak ionisable substance. Thus the concentration of $O{H}^{-}$ ions remains constant. When a few drops of a base (say $NaOH$) are added, the $O{H}^{-}$ ions (from $NaOH$) combine with $N{H}_4^{+}$ ions to form nearly unionised $N{H}_{4}OH$ and thus the concentration of $O{H}^{-}$ ions and hence pH value remains constant.