Acid-base indicators are classified into 3 different groups: phthaleins and sulphophthaleins, azo indicators, and triphenylmethane indicators. The two most common indicators used in acid-base titration are phenolphthalein and methyl orange. Phenolphthalein can be used for strong acid-strong base titrations as well as weak acid-strong base titrations. Methyl orange can be used for strong acid-weak base titration. For weak acid-weak base, there is no indicator that is suitable for this titration.

Examples of indicators for each type of titration:

• Bromothymol blue and phenol red can be used for strong acid-strong base titrations.
• Cresol red or thymol blue can be used for strong acid-weak base titrations.
• Thymol blue can also be used for weak acid-strong base titrations.