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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R3.1 - Proton transfer reactions

3.1.1 Bronsted-Lowry Acid-Base Theory 3.1.2 Conjugation Acid-Base Pairs 3.1.3 Acid-Base Behaviour and Oxides 3.1.4 pH and [H+] 3.1.5 Kw and pH of Water 3.1.6 Strong Vs. Weak Acids and Bases 3.1.7 Neutralization Reaction 3.1.8 pH Curves 3.1.9 pH and [OH-] (AHL) 3.1.10 Ka, Kb, pKa and pKb (AHL) 3.1.11 Ka x Kb = Kw (AHL) 3.1.12 pH of Salt Solutions (AHL) 3.1.13 pH Curves for Acid-Base Reaction (AHL) 3.1.14 Acid-Base Indicators (AHL) 3.1.15 Indicators and Titration Points (AHL) 3.1.16 Buffer Solutions (AHL) 3.1.17 pH of Buffer (AHL)

Acid–Base Indicators HL Only

Specification Reference R3.1.14

Quick Notes

  • Indicators are weak acids that partially dissociate in water.
  • General formula: HInd ⇌ H+ + Ind
  • Colour changes depend on the relative amounts of HInd and Ind.
  • At pH = pKa:
    • [HInd] ≈ [Ind] and the colour is a mix of both forms.
  • The endpoint of a titration is close to the pKa of the indicator.
  • Universal indicator is a mixture of indicators, giving a wide pH colour range.

Full Notes

Indicators as Weak Acids

Acid–base indicators function through an equilibrium system:

HInd (aq) ⇌ H+ (aq) + Ind (aq)

As pH changes, the position of this equilibrium shifts, causing the colour to change:

pKa and Endpoint

The pKa of the indicator tells you the pH at which the colour change occurs.

When pH = pKa, the concentration of weak acid and conjugate base are equal.

Meaning [HInd] = [Ind], giving an equal mix of both colours.

Indicators are chosen so their pKa matches the equivalence point of the titration as closely as possible.

Choosing an Appropriate Indicator

The indicator must change colour close to the equivalence point of the titration.

Examples

IB Chemistry diagram showing phenolphthalein indicator colour change between pH 8.3–10.0 from colourless to pink. IB Chemistry diagram showing methyl orange indicator colour change between pH 3.1–4.4 from red to yellow.

Universal Indicator

Universal indicator is a mixture of many indicators with different pKa values.

As each component changes colour at a different pH, a continuous colour gradient is produced from pH 1 to 14.

IB Chemistry diagram of the universal indicator colour chart showing pH 1 to 14.

Universal indicator is useful for estimating pH, but not suitable for precise titration endpoints.

Summary

Linked Course Question

Tool 1, Inquiry 2, Reactivity 3.2 — Linked Course Question

What are some of the similarities and differences between indicators used in acid–base titrations and in redox titrations?

Both give a colour change at the endpoint. Acid–base indicators rely on pH changes caused by changes in H+ ion concentration. Redox indicators rely on electron transfer and colour changes due to changes in oxidation state.