3.1.15 (HL)—Indicator selection

Choose an indicator whose endpoint transition range lies within the steep pH change around the equivalence point. Use salt identity to predict whether that equivalence pH is acidic, neutral or alkaline.

The equivalence point is the stoichiometric condition; the endpoint is the observed indicator colour change. A good indicator makes them coincide closely.

Overlay the indicator transition range on the titration curve and require the whole visible change to fall inside the steep region. A weak acid–strong base equivalence is alkaline and favours an alkaline-range indicator; a strong acid–weak base equivalence is acidic. Endpoint proximity, not a memorized indicator name alone, is the criterion.

Representative question

[Maximum number: 1]

What is the best indicator to use in the titration of phenylamine with nitric acid?

Bromophenol blue, $\mathrm{p} K_{\mathrm{a}}=4.2$

Bromothymol blue, $\mathrm{pK}_{\mathrm{a}}=7.0$

Phenol red, $\mathrm{p} K_{\mathrm{a}}=7.9$

Phenolphthalein, $\mathrm{pK} \mathrm{K}_{\mathrm{a}}=9.6$

Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.

Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.

3.1.15 (HL)—Indicator selection

Choosing an Indicator

Matching Indicator Range to Equivalence pH

Assessment in practice

Proton Transfer Reactions Summary

Question 1