Chemistry, Third edition

BUFFER SOLUTIONS 297

Buffer solutions

Adding small amounts of acids or alkalis may lead to

drastic changes in solution pH

The addition of even one drop of dilute hydrochloric acid to water drastically

changes its pH (Box 16.2). Such changes in pH can be troublesome in the laboratory,

and catastrophic in living cells. Changes in pH due to trace contamination with acids

or bases can be prevented using a buffer solution(usually simply called a ‘buffer’

(Fig. 16.1)).

A buffer solution resists changes in pH when it is diluted or when acid or base

is added.

Figure 16.2 shows the pH values in separate experiments in which (a) 0.1 mol dm^3

HCl, (b) deionized water and (c) 0.1 mol dm^3 NaOH were added to a sodium

ethanoate–ethanoic acid buffer. In this case, adding 3 cm^3 or less of (a) or (c) pro-

duces only a slight change in pH. If we add more acid or alkali, the buffer becomes

exhausted and it can no longer resist changes in pH. However, dilution of the buffer

by adding water causes virtually no change in buffer pH.

16.4

(a)

pH 6 —→ 4 pH remains at 4

(b)

Buffer at

pH 4

Deionized water at

pH 6

Hydrochloric acid

Fig. 16.1A buffer in action. (a) When a drop of 2.0 mol dm^3 hydrochloric acid is added a

water, the pH falls drastically. (b) There is no change in pH when an acid is added to the

buffer solution.

Salt hydrolysis

Use Table 16.3 to complete this table:

Salt Formula Parent acid Acidic, basic

and base or neutral?

Iron(III) nitrate

Calcium chloride

Sodium sulfate

Ammonium benzoate

Exercise 16I