304 16 · ACID–BASE EQUILIBRIA
●The approximate pH ranges of phenolphthalein (8–10) and methyl orange (3–4)
are also shown on Fig. 16.3(a). Methyl orange begins to change colour fractionally
before, and the phenolphthalein very fractionally after, the equivalence point. In
practice, because the pH changes so rapidly near the equivalence point, the colour
changes for phenolphthalein will start and finish over the addition of less than one
drop (0.05 cm^3 ) of added base. The error involved in assuming that the
indicated end point of phenolphthalein is the same as the equivalence point is
negligible. The use of methyl orange will produce a slightly greater titration error
with an endpoint at about 24.95 cm^3.Titration curve for a weak acid and a strong base
(WA–SB)
Figure 16.3(b) shows the pH curve for the titration of a strong base (NaOH) against
a weak acid (ethanoic acid):CH 3 COOH(aq)NaOH(aq)CH 3 COONa(aq)H 2 O(l)Similar pH curves are obtained with all titrations involving weak acids and strong
bases. Beyond the equivalence point (25.00 cm^3 of NaOH), the CH 3 COOH–NaOH
pH curve is identical to the HCl–NaOH curve, but before the equivalence point the
curve differs from Fig. 16.3(a) in two important respects:●The pH at the equivalence point is 8.7 (not 7) because the salt, sodium ethanoate,
hydrolyses in solution.●The pH of the acid at the beginning of the titration is higher than for the
HCl–NaOH case because CH 3 COOH(aq) is a weak acid and is only partially ion-
ized. A mixture of ethanoic acid and sodium ethanoate is also a buffer solution.
This has the effect of shortening the vertical part of the graph in the region of the
equivalence point. This, in turn, means that methyl orange is unsuitable for use
in this titration, since it changes colour well before the equivalence point. However,
phenolphthalein may still be used.Weak acid – strong base titration
(i)Citric acid, a weak acid, is a constituent of fizzy drinks. Would you use bromocresol green
indicator in titrations involving citric acid and potassium hydroxide?
(ii)Refer to the NaOH–CH 3 COOH titration shown in Fig. 16.3(b). What simplified form does the
Henderson–Hasselbalch equation take when 12.50 cm^3 of NaOH has been added? Use this
relationship to estimate Ka(CH 3 COOH) from Fig. 16.3(b).Exercise 16O
Strong acid–strong base titration
(i)Which of the indicators listed in Table 16.4 are unsuitable for use in a strong acid–strong
base titration?
(ii)Which of the following pairs will give a pH curve virtually identical to Fig. 16.3(a):
(a)HNO 3 –KOH, (b)HBr–NaOH (c)HCl–NH 3?Exercise 16N