Chemistry, Third edition

VARIATION OF pH DURING AN ACID–BASE TITRATION 303

Variation of pH during an acid–base

titration

In an acid–base titration, base solution is added to the acid solution until the acid

has been exactly neutralized. This is known as the equivalence point(orstoichio-

metric point) of the reaction.

If we are using indicators in an acid–base titration, we are making the assumption

that the end pointof the indicator (the point at which it appears to suddenly change

colour) is the same as the equivalence point. In order to look at this assumption fur-

ther, we look at the way that the pH of a reaction mixture changes during an

acid–base titration.

Titration of a strong acid and a strong base (SA–SB)

Suppose that we slowly add 0.100 mol dm^3 NaOH to 25.00 cm^3 of 0.100 mol dm^3

HCl, and use a pH meter to record the pH of the mixture after each addition of base.

The reaction is

HCl(aq)NaOH(aq)NaCl(aq)H 2 O(l)

Table 16.5 shows the changes in pH during the titration. The initial pH of the acid is

1.00, and the pH of the mixture at the equivalence point (after addition of 25.00 cm^3

of NaOH, when only pure water and sodium chloride are present) is 7.00. When this

data is plotted, Fig. 16.3(a) is obtained – virtually identical curves are obtained using

any pair of strong acids and strong bases. Upon addition of NaOH, the pH rises

slowly and, suddenly, shoots up near the equivalence point. After this, the pH slowly

rises as the amount of excess NaOH increases.

Points to note:

●The very rapid change in pH near the equivalence point has a simple explanation.

We have already noted (page 298) that the addition of even one drop of acid to a

neutral solution causes a massive change in pH, and the same applies to the addi-

tion of base to a neutral solution. This is exactly what happens during the titration

just beforeandjust afterthe equivalence point. This is seen from the highlighted

data in Table 16.5, where the addition of only 0.10 cm^3 of base (about 2 drops)

causes the pH to change from 4 to 10.

16.6

Table 16.5pH changes

during the titration of

0.100 mol dm^3 NaOH

with 25.00 cm^3

0.100 mol dm^3 HCl

Vol of added pH

NaOH (cm^3 )

10.00 11.00

15.00 11.18

10.00 11.37

15.00 11.60

20.00 11.95

24.00 12.69

24.90 13.70

24.95 4.00

25.00 7.00

25.05 10.00

25.10 10.30

25.50 11.00

26.00 11.29

30.00 11.96

35.00 12.22

40.00 12.36

45.00 12.46

50.00 12.52

pH

14

10

6

2

0

Phenolphthalein

Methyl orange

0

Vol 0.100 mol dm–3NaOH

added/cm^3

10 20 30 40 50

(a)

pH

14

10

6

2

0

Phenolphthalein

Methyl orange

0

Vol 0.100 mol dm–3NaOH

added/cm^3

10 20 30 40 50

(b)

Fig. 16.3pH curves for titrations of (a) a strong acid and a strong base (HCl and NaOH) and (b)
a weak acid and a strong base (CH 3 COOH and NaOH). All initial concentrations are
0.100 mol dm^3. The initial volume of acid was 25.0 cm^3 in both cases.