Question

How does a strong acid/base titration change, if congo red is used instead of phenolphthalein?

For eg– We begin by creating 0.1 M NaOH solution. Then, we standardize NaOH and titrate w/KHP. Then, we use this to find the mass of KHP in the unknown. This is all done using phenolphthalein. What would happen if we used congo red instead?

Answer 1

Indicator	Range	Low pH color	High pH color
Phenolphtalein	8.0-9.6	Clear	Pink
Congo red	3.0-5.0	Purple	Red

When the titration congo red instead of phenolphthalein it is see that the range in which the indicator changes its color is less than that which makes phenolphthalein. Therefore ,the end point is reached much earlier using less amount of base for titrate the strong acid.

By the above if we find the mass of KHP using congo red , we get an incorrect value because the solution would still be KHP by titrate, so the use of congo red is not recommended.