Question

Why don we use methyl orange indicator firstinstead of phenolphthalein?

Answer 1

Indicator choice is based on the pH of the equivalence point for the acid-base titration you are doing. You should choose an indicator that changes colour at or near the equivvalence point. The titration curve is steep then, and you will get a good end point.

Phenolphthalein changes colour at a pH above 7. So it is quite good as an indicator for titrations of strong acids with strong bases. It is also suitable for titrations of weak acids and strong bases, which have an equivalence point at a pH above 7.

It is not a good choice for a titration between a strong acid and a weak base, as the equivalence point is below a pH of 7. Methyl orange changes colour at a suitable pH.