3.1.14 (HL)—Acid-base indicators

$HInd + H2O ⇌ H3O+ + Ind−$

HInd and Ind− have different colours. Changing pH shifts their ratio; the visible transition occurs around pH ≈ pKa. Universal indicator is a mixture of indicators with different transition ranges.

Added acid shifts HInd ⇌ H⁺ + Ind⁻ toward the HInd colour; added base favours Ind⁻. The visible transition spans a range around pKa because both colours must change in relative abundance. Universal indicator combines several such equilibria and is not one substance with every colour.

Representative question

[Maximum number: 2]

Explain how the indicator HInd, that is a weak acid, shows changes in pH using an equation.

HInd (aq) $+\mathrm{H}_{2} \mathrm{O}$ (I) ⇌ $\mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{Ind}^{-}(\mathrm{aq})$ AND HInd and Ind have different colours

OR HInd (aq) ⇌ $\mathrm{H}^{+}(\mathrm{aq})+\operatorname{Ind}^{-}(\mathrm{aq})$ AND HInd and Ind ${ }^{-}$ have different colours equilibrium shifts when acid or base is added «to give one of the colours» OR

HInd colour in acid/low pH AND Ind $^{-}$ colour in alkali/high pH

Marking guidance:

Accept equation for an ionic or
molecular reaction with a base AND HInd and Ind have different colours for
M1.

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.

3.1.14 (HL)—Acid-base indicators

Acid–Base Indicators

Explaining Indicator Colour Change

Assessment in practice

Proton Transfer Reactions Summary

Question 1