1. pH Curves Strong Acid/Strong Base
50 cm3 of 1.0M NaOH added to 25 cm3 of 1.0M HCl pH 7 14
Volume of alkali added / cm3 25
As both acid and alkali are strong, the “vertical section” is extensive and is symmetrical about pH 7. Because of this, almost any indicator can be used in a titration; specifically, either methyl orange (pKa 3.7) or phenolphthalein (pKa 9.3) are suitable.
Phenolphthalein pKa = 9.3
End Point
Methyl Orange pKa = 3.7

2. pH Curves Strong Acid/Strong Base
50 cm3 of 1.0M HNO3 added to 25 cm3 of 1.0M NaOH pH 7 14
Volume of acid added / cm3 25
Usually alkali is added to acid, but there is no reason why it cannot be done the other way round, as in this example. Again, any indicator may be used. This has appeared in exam questions, so read carefully.
Phenolphthalein pKa = 9.3
End Point
Methyl Orange pKa = 3.7

3. pH Curves Weak Acid/Strong Base
50 cm3 of 0.8 M NaOH added to 25 cm3 of 1.0M CH3COOH pH 7 14
Volume of alkali added / cm3
31.25
Because the acid is weak, its initial pH is much higher and as alkali is added a buffer is produced. Only phenolphthalein is suitable for this combination. Note the effect of different concentration on the end point volume.
Phenolphthalein pKa = 9.3
End Point
pKa of the acid

4. pH Curves Strong Acid/Weak Base
50 cm3 of 1.0 M NH3 added to 25 cm3 of 1.0M HCl pH 7 14
Volume of alkali added / cm3 25
Only methyl orange (or similar) is suitable for this combination. An alkaline buffer is produced after the end point.
End Point
Methyl Orange pKa = 3.7

5. pH Curves Weak Acid/Weak Base
50 cm3 of 1.0 M NH3 added to 25 cm3 of 1.0M CH3COOH pH 7 14
Volume of alkali added / cm3
Because both reagents are weak, there is no significant “vertical section”. There are no suitable indicators for this type of titration, although a pH meter could be used to find an approximate end point.