The titration curves that we have considered so far all involve a single proton and one sudden pH change. It is worth considering other, more complex examples, eg what is happening when a base such as sodium carbonate is titrated with strong acid, or when a diprotic weak acid is titrated with a strong alkali.

The titration curve for sodium carbonate and hydrochloric acid is shown below.

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Why are there two 'bumps' in the titration curve?

The first reaction is CO₃ ^2– + H⁺ ↓ HCO₃^–

The second reaction is  HCO₃^–  +H⁺ ↓   CO₂  + H₂O

Will methyl orange or phenolphthalein be able to show these changes?

Have a look at the changes on the curve below. Each shows one of the two different points.

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Why do I need to know this if it is not in our exam syllabus?

This could be an area of confusion for students. For example, imagine the titration experiment above carried out by a student using phenolphthalein indicator, and the same titration carried out by another student with methyl orange. Both will obtain valid titration results but the values will be very different. Both are valid.