Acid-base indicators

• Specific chemical indicators have to be used for specific reactions as they can only indicate a pH change within a certain range.
• Methyl orange indicator is used for reactions with a more acidic neutralisation point. It is red in acids and turns more yellow at the neutralisation point (as alkali is added).
• Phenolphthalein is used in reactions with a more basic neutralisation point. It is pink in alkalis and turns colourless at neutralisation point (as acid is added).
• Methyl orange changes in colour from red (acid) to yellow (alkali) between pH 3 and 5.
• Phenolphthalein changes in colour from colourless (acid) to pink (alkali) between pH 8 and 10.
• Litmus changes colour from red (acid) to blue (alkali) between pH 5 and 8.
• An acid-base indicator is either a weak acid or weak base. Most indicators are weak acids (HIn), so its dissociation can be represented as HIn(aq) ⇌ H+(aq) + In-(aq). Depending on the position of equilibrium, the colour of the solution will change in the presence of methyl orange.