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Why should colorimetric titration of weak acid with a weak base (or vice versa) not be done in laboratory?

How can I draw a pH graph for this?

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  • $\begingroup$ Here is a paper with titration curves of weak acids with weak bases. $\endgroup$ – Karsten Theis Mar 5 at 15:42
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When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the equivalence pH will be neutral. However, weak acids are not often titrated against weak bases because the colour change shown with the indicator is often quick, and therefore very difficult for the observer to see the change of colour.

Source: Wikipedia

Drawing the titration curve is hard, because you generate a buffer system which buffers against acids and bases at the same time. Around the equivalence point the buffer system works equally well against acids and bases (assuming their $\text{p}K_\text{a}$ values are the same), resulting in a slow rise of the pH. You don't get the usual "jump" in the curve at the equivalence point, which makes it hard to detect the actual equivalence point (your indicator should fulfill $\text{p}K_\text{a}^\text{Ind} = \text{pH}_\text{equiv.}$, which is highly unlikely).

For the reasons given above I disagree with the last part of the Wikipedia quote, but having never done the experiment myself I cannot be sure.

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