Theory of indicator: The main purpose of selecting a suitable indicator is to find out the end point as much nearer as possible to the equivalence point. This will minimize the titration error. As indicator indicates the end point by change in its color but all of them do not show at the same pH range. For example: Phenolphthalein changes its color at pH range of 8 to 9.5 and methyl orange at pH range 3-4.5. The pH at the end point of a titration depends upon the nature of titrand and titrant used. So the indicator should be selected properly so as to coincide the end point with equivalent point.
Selection of indicator:
- Strong acid and strong base titration: In titration of strong acid and strong base, the pH value changes before and after the equivalence point. Any indicator either methyl orange or phenolphthalein can be used for this titration.
- Strong acid and weak base titration: In this type of titration, salt is produced due to hydrolysis. The pH at the equivalent point becomes less than 7. So, methyl orange is used as indicator. It changes its color at pH 5-6.
- Weak acid and strong base: In this type of titration, there is very little change in pH until the neutral point is reached. When the pH value suddenly alters from 6.5 to 10, the solution becomes alkaline due to the hydrolysis of salt. So, phenolphthalein is used as an indicator.
- Weak acid and weak base: In this type of titration, no sharp changes in the value of pH take place. The pH value remains almost constant around the equivalence point. So, no indicator is required for the acid and weak base titration.