Question

Explain how to choose a proper indicator in an acid-base titration such as adding a strong acid (HCI) to a strong base (NaOH) , a strong acid (HCI) to a weak base (NH3) and s strong base (NaOH) to a weak acid (CH3COOH).

Answer 1

The pH of the solution at equivalence point is dependent on the strength of the acid and strength of the base used in the titration.

For strong acid-strong base titration, pH = 7 at equivalence point
For weak acid-strong base titration, pH > 7 at equivalence point
For strong acid-weak base titration, pH < 7 at equivalence point

https://www.khanacademy.org/test-prep/mcat/chemical-processes/titrations-and-solubility-equilibria/a...  

The indicator changes color when the pH changes at the endpoint of the titration. So, you need to determine what is present at the point and find the pH at the point. Then, you can reference a table of indicators to choose one whose color will change over the pH that includes your equivalency point.