Buffer Solution/unit-7

Buffer Solution:

Generally, P^H of a solution changes on addition of small amount of acids or bases to it. But if the solution contains a weak acid and its conjugate base, or a weak base and its conjugate acids, such a solution can resist change in P^H and is called a buffer solution.

“A buffer solution is the solution which can resist the change in P^H on addition of small amount of acid or base. The ability of buffer solution to resist change in P^H on addition of acid or base is called buffer action.”

Depending upon P^H values, buffer solutions are divided into two classes. If the P^H of the buffer solution is less than 7, it is called acidic buffer and if it is more than 7, it is called basic buffer.

Strong Acid Buffers:

A strong acid such as nitric acid or hydrochloric acid can act as a buffer with a low P^H. Strong acids arc completely ionized in aqueous solution and there the concentration of hydrogen ions is high. The addition of a small amount of acid or base to the acid will have negligible effect on the P^H of the solution.

For example, if 1 cc of 0.1 M hydrochloric acid is added to 100 cc of 0.01 M nitric acid, the P^H of the solution changes from 2 to 1.96, which is a negligible change. On the other hand if 1 cc of 0.1 M hydrochloric acid is added to 100 cc of pure water P^H changes from 7.00 to 4.00. Thus, pure water does not act as a buffer.

Strong Base Buffers:

A strong base such as NaOH, KOH etc. can act as a buffer with a high P^H. The addition of small amount of acid or base has negligible effect on P^H of solution of such bases.

For example, when 1 cc of 0.1 M HCI is added to 100 cc of 0.01 M NaOH solution, the P^H changes from 12 to 11.96, which is negligible change.

Weak Acid Buffers:

Buffer solutions with P^H range 4 to 7 can be prepared from weak acids and their salts with strong bases. For example, acetic acid and sodium acetate solution.

Let us understand how an acidic buffer such as solution of acetic acid (weakly dissociated) and sodium acetate (largely dissociated) resists the change in P^H. This solution contains a large amount of sodium and acetate ions and a large amount of undissociated acetic acid molecules along with a small amount of hydronium ions.

CH₃COOH (aq) + H₂O (l)  —–> CH₃COO^־ (aq) + H₃O⁺ (aq)

CH₃COONa (aq) ——->  CH₃COO^־ (aq) + Na⁺ (aq)

Suppose, a few drops of HCl (aq) are added to this buffer solution. This would provide H₃O⁺ ions. These H₃O⁺ ions would combine with CH₃COO^־ ions to form weakly ionized acetic acid molecules as shown below:

CH₃COO^־ (aq) + H₃O⁺ (aq)  ——>  CH3COOH (aq) + H₂O (l)

From buffer       From HCl

Since the additional H₃O⁺ ions are neutralised by CH₃COO^־ ions in the solution, there will be no change in its P^H value. On the other hand, if a few drops of NaOH are added to the buffer solution, it would provide ־OH ions. These ־OH ions will combine with H₃O⁺ ions present in the buffer solution to form unionized water molecules. This would result in the greater ionization of acetic acid in order to restore the concentration of H₃O⁺ ions to its original value.

H₃O⁺ (aq)      + ־OH (aq)  ——-> 2H₂O (l)
from buffer      from NaOH

Therefore, pH of the solution remains unchanged.

Weak Base Buffers:

Buffer solutions with P^H values between 7 and 10 can be prepared from weak bases and their salts with strong acids. For example, a solution containing NH₄OH and NH₄C1 acts as a buffer.

Let us understand how a solution containing ammonium hydroxide and ammonium chloride acts as a buffer and resists change in P^H. In case of a basic buffer solution containing equimolar quantities of ammonium hydroxide and ammonium chloride, there is a large concentration of ammonium ions, chloride ions, undissociated ammonium hydroxide and small amount of ־OH ions.

NH₄OH (aq) ——->   NH₄⁺ (aq) + ־OH (aq)

NH₄C1 (aq)  ——–>  NH₄⁺ (aq) + C1־ (aq)

When a few drops of HC1 (aq) are added, the additional H₃O⁺ ions are neutralised by OH- ions present in the buffer.

־OH (aq)    +    H₃O⁺(aq)   ——->  2H₂O(l)

From buffer     From acid

As some of OH ⁻ ions from NH₄OH combine with H₃O⁺ ions from the acid, it would result in the greater ionization of NH₄OH to restore the OH ⁻ion concentration.

On the other hand, when a few drops of NaOH solution are added to the buffer solution, it would give OH −ions. The additional OH ⁻ions will combine with NH₄⁺ ions.

NH₄⁺ (aq) +   OH ⁻ (aq) ——->  NH₄OH (aq)
From buffer   From NaOH

If we observe the titration curves (Fig. 7.3) we find that in certain regions (flat portions of the titration curve) the p^H of the solution is insensitive to small changes in the concentration of acid or base. These portions of the curve are known as buffer regions.