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Buffer Solutions

Generally, pH of the solution changes on addition of small amounts 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 pH and is called a buffer solution.

A buffer solution is a solution which can resist the change in pH on addition of small amount of acid or base. The ability of buffer solution to resist change in pH on addition of acid or base is called buffer action.

Depending upon pH values, buffer solutions are divided into two classes. If the pH 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.

 

WEAK ACID BUFFERS

Buffer solutions with pH range 4 to 7 can be prepared from weak acids and their salts with strong bases. For example, acetic acid and sodium acetate are commonly used for this purpose.

Now let us understand how an acidic buffer such as solution of acetic acid (weakly dissociated) and sodium acetate (largely dissociated) resists the change in pH. 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.

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

Since the additional H3O+ ions are neutralised by CH3COO- ions in the solution, there will be no change in its pH 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 Hp+ 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 H2O+ ions to its original value.

WEAK BASE BUFFERS

Buffer solutions with pH values between 7 and 10 can be prepared from weak bases and their salts with strong acids. For example, a solution containing NH3 and NH4Cl acts as a buffer.

Now let us understand how a solution containing ammonia and ammonium chloride acts as a buffer and resists change in pH.

In case of a basic buffer solution containing equimolar quantities of ammonia and ammonium chloride, there is a large concentration of ammonium ions, chloride ions, ammonia and small amount of OH- ions.

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

As some of OH- ions from NH4OH combine with H3O+ ions from the acid, it would result in the greater ionization of aqueous ammonia 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 NH/ ions to form aqueous ammonia.

As some of OH- ions from NH4OH combine with H3O+ ions from the acid, it would result in the greater ionization of aqueous ammonia 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 NH/ ions to form aqueous ammonia.

If we observe the titration curves (Fig. 27 .2) we find that in certain regions (flat portions of the titration curve) the pH 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.

Some common buffer systems are given in Table 26.1

Table 26.1 Some Common Buffer Systems

STRONG ACID BUFFERS

 

A strong acid such as trioxonitrate(V) acid or hydrochloric acid can act as a buffer with a low pH. Strong acids are 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 pH of the solution.

For example, if 1.0 cm3 of 0.1 M hydrochloric acid is added to 100 cm3 of 0.01 M trioxonitrate(V) acid, the pH of the solution changes from 2.00 to 1.96, which is a negligible change. On the other hand if 1 cm3 of 0.1 M hydrochloric acid is added to 100 cm3 of pure water pH 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 pH. The addition of small amount of acid or base has negligible effect on pH of solution of such bases. For example, when 1 cm3 of 0.1 M HCl is added to 100 cm3 of 0.01 M NaOH solution, the pH changes fromĀ  12.00 to 11.96, which is negligible change.