Adding 0.100 mol/L sodium hydroxide to 10.00 mL of 0.100 mol/L hydrochloric acid raises the pH only slowly at first, from 1.0 at the start to about 1.5 halfway.
Near the equivalence point the pH changes suddenly. It is about 3.3 at 9.90 mL and about 10.7 at 10.10 mL, so a few drops raise it by about 7. When almost no acid is left, each drop changes the hydrogen ion concentration many times over.
At the equivalence point itself, 10.00 mL, the solution holds only sodium chloride and water, and its pH is 7.0. All the curves in this article are calculated for 25 °C.
With 0.100 mol/L ethanoic acid, the curve starts higher, at about pH 2.9, because the weak acid ionizes only slightly.
As sodium hydroxide is added, ethanoate ions form alongside the remaining acid, and the pH changes little. At 5.00 mL, when half the acid has been neutralized, the pH is about 4.8 and the curve is at its flattest.
The equivalence point is at about pH 8.7, because sodium ethanoate solution is basic, and the jump is shorter, from about 6.8 at 9.90 mL to about 10.7 at 10.10 mL.
Titrating hydrochloric acid with 0.100 mol/L aqueous ammonia gives a curve that is the same as with sodium hydroxide at first, since the strong acid sets the pH.
The equivalence point lies at about pH 5.3, because ammonium chloride solution is acidic. The jump runs only from about 3.3 at 9.90 mL to about 7.3 at 10.10 mL.
After that, excess ammonia and ammonium ions keep the pH from rising much, and it levels off near 9.3 at 20.00 mL.
An indicator works only if its colour change falls within the jump. Methyl orange changes between about pH 3 and 4.5, and phenolphthalein between about pH 8 and 10.
For hydrochloric acid and sodium hydroxide, both ranges lie inside the long jump, so either indicator can be used.
For ethanoic acid with sodium hydroxide only phenolphthalein fits, and for hydrochloric acid with ammonia only methyl orange fits. The other indicator would change colour well before or after the equivalence point.
When ethanoic acid is titrated with aqueous ammonia, the acid and the base are both weak, and the curve has no sharp jump.
The pH is about 6.7 at 9.90 mL and about 7.4 at 10.10 mL, a change of only about 0.7, compared with about 7 for hydrochloric acid and sodium hydroxide.
An indicator would change colour gradually over several millilitres, so the end point cannot be seen clearly. Titrations are therefore planned so that at least one of the acid and the base is strong.