Choice of Indicators:
All acid-base indicators are not suitable for the titration of any acid with any base. Depending on the nature of acids and bases, different indicators are used.
i. In the titration of a strong acid (HCL) with a strong base (NaOH), any indicator may be used because the salt formed doesn’t undergo hydrolysis and the solution remains neutral.
ii. If a weak acid (CH3COOH) is titrated with a strong base (NaOH), the resulting salt (CH3COONa) undergoes hydrolysis and the solution is alkaline having pH > 7 due to presence of strong base NaOH.
iii. If a strong acid (HCL) is titrated with a weak base (NH4)OH), the resulting salt (NH4Cl) undergoes hydrolysis and the solution becomes acidic due to the presence of strong acid in solution. Hence in the titration of a strong acid with a weak base, indicators whose colour change takes place in the lower range such as Methyl Orange (3.1 – 4.4) or Methyl red(4.2 – 6.3) should be used.
iv. To titrate a weak acid with a weak base, no indicator is suitable since such a solution on addition of alkali or acid during titration changes its pH very slowly like Buffer solution.
Indicators and Their Choice:
Indicators are substances that by their colour change indicate the end-points in titrations. Depending on the nature of reactions involved in titrations, three types of indicators are generally distinguished.
1. Acid-base indicators, such as – Phenophthalein, Methyl Orange, etc.
2. Oxidation-Reduction or Redox indicators, such as – Diphenyl Amine.
3. Adsorption indicators, such as Red Dye Eosin.
Indicators themselves are either weak acids or weak bases. They do not interfere with the reaction between an acid and an alkali.