Question

Indicators are weak acids. Explain why we don’t see a buffer curve for them in the titration of the acids? What is it that makes methyl orange different from phenolphthalein (as an indicator)? How would one decide on an indicator to use for a given titration?

Answer 1

Buffer curve for any weak acid/conjugate base buffer can be observe only in presence of small amount of acid or base. However if the amaount of acid or base is large the buffering capability of the indicator/buffer will be lost and the the indicator will exist is one of the forms (either acid or conjugate base) in the solution . This is the reason you dont see a buffer curve.

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pKa of methyl organge is different from that of indicator. As a result the conversion of weak acid to conjugate base in case of methyl orange occurs in different pH range than phenolphthalein.

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Choice of indicator depends on the pH of the end point. If the pH of the end point lies in the basic region , we will choose phenolphthalein which shows color change in pH = 8-10.

However if the end point lies in the acidic region (weak base -strong acid titration) we will choose methyl orange which shows color change in acidic region (pH = 3-5)