Chemistry, Third edition

BUFFER SOLUTIONS 299

ethanoic acid is now ionized. Calculations show that in the presence of sodium

ethanoate, the ionization of ethanoic acid is so small that [CH 3 COO(aq)] may be

taken to be equal to the initial concentration of sodium ethanoate salt solution, Cs.

How does the buffer mixture resist changes in pH? The explanation is as follows.

●Suppose we add some hydrochloric acid to the mixture. The equilibrium

composition shifts, with ethanoate and hydronium ions combining together in

thereverseof equation (16.9). This mops up the added H 3 O(aq) ions, and

there is no change in pH. (The same conclusion is arrived at using Le Chatelier’s

principle.)

●If we add hydroxide ions (e.g. by adding sodium hydroxide solution), the tiny

amount of [H 3 O(aq)] at equilibrium in the buffer mixture reacts with OH(aq)

in the neutralisation reaction

H 3 O(aq)OH(aq)2H 2 O(l).

More ethanoic acid ionizes to restore the equilibrium concentration of

H 3 O(aq), and ionization continues until just enough extra H 3 O(aq) has been

made to convert all the OH(aq) to water. In this way, the addition of base does

not lead to a change in the pH of the buffer.

Calculating the pH of a buffer solution

We can easily obtain an expression with which to estimate the pH of a buffer, starting

with equation (16.10). Since virtually all the ethanoate ions in the buffer come from

the sodium ethanoate, we are justified in substituting Csfor [CH 3 COO(aq)]. Since

so little ethanoic acid is ionized in the buffer, we are also justified in taking the equi-

librium concentration of ethanoic acid, [CH 3 COOH(aq)], to be equal to the initial

concentration of acid, CA. Substitution of this information into equation (16.10) gives

Ka(T)

Cs[H 3 O(aq)]

CA

Rearranging,

[H 3 O(aq)]

CAKa(T)

(16.11)

Cs

This equation shows that dilution of the buffer mixture does not affect its pH

becauseCAandCsare affected equally. For example, if we add 50 cm^3 of water to

50 cm^3 of buffer, boththe acid (CA) and salt (Cs) concentrations in the buffer

solution are halved, and the pH predicted by equation (16.11) is unaffected by the

dilution.

Calculating acidity constants

The acidity constant of an acid may be calculated from the way that the pH of a buffer

changes as the concentration of acid and salt are changed. This involves using a graphical

method. Can you see from equation (16.11) how this might be done?

Exercise 16J