To select an indicator for an acid-base titration it is necessary to know the pH of the end point before
using equation (5.5) or standard indicator tables. The end point pH may be calculated using equations
(3.27), (3.29) or (3.30). Alternatively, an experimentally determined titration curve may be used (see
next section). As an example, consider the titration of acetic acid (0.1 mol dm–^3 ), a weak acid, with
sodium hydroxide (0.1 mol dm–^3 ), a strong base. At the end point, a solution of sodium acetate (0.05
mol dm–^3 ) is obtained. Equation (3.28) then yields
thus phenolphthalein is a suitable indicator (Table 5.1).
Acid-base Titration Curves
If the pH of the titrand is monitored throughout a titration, a graph of pH against amount of titrant
added may be constructed. The characteristics of this curve are important in the selection of suitable
titration conditions and indicators. Of particular importance are the position of the point of inflexion
representing the neutralization point, the slope of the curve in the end point region, and the size of the
end point 'break'. The influences of concentration and the strength of the acid or base are summarized in
Figures 5.1, 5.2 and 5.3. Where both the acid and the base are strong electrolytes, the neutralization
point will be at pH = 7 and the end point break will be distinct unless the solutions are very dilute (<10–
3
Figure 5.1
The effect of concentration on titration curves and indicators
for a strong acid (HCl) and a strong base (NaOH). 100 cm^3 of^
HCl is being titrated with NaOH of the same molarity in
each case.