Choosing an Acid-Base Indicator (HL) (DP IB Chemistry): Revision Note

Choosing an acid-base indicator (HL)

Choosing a suitable indicator

• The equivalence point is where stoichiometrically equal amounts of acid and base have reacted

• The end point is when the indicator changes colour during a titration

• For accurate results, the pH range of the indicator’s colour change should match the pH at the equivalence point

• Indicators change colour over a narrow range centred around their pK_a

• Section 18 of the Data Booklet contains information about acid-base indicators

Common indicators and their colours table

Strong acid-strong base

• During a strong acid–strong base titration, the pH changes sharply from approximately 4 to 10 at the equivalence point

• A suitable indicator must change colour within this range

• Methyl red and phenolphthalein are both suitable indicators

• Methyl orange is not ideal but is often used because it shows a noticeable colour change near the endpoint and is widely available

Weak acid-strong base

• In a weak acid–strong base titration, the pH increases from around 7 to 10 at the equivalence point

• The indicator must change colour within this range

• Phenolphthalein is the only widely used suitable indicator for this type of titration

Strong acid-weak base

• In strong acid–weak base titrations, the pH changes from about 4 to 7 at the equivalence point

• The indicator should change colour within this pH window.

• Methyl red is the most appropriate choice

• However, methyl orange may also be used in practice due to its distinct colour change and availability, despite not being a perfect match

Weak acid-weak base

• For weak acid–weak base titrations, the pH change around the equivalence point is very gradual

• There is no sharp pH change, so no suitable indicator can accurately show the endpoint

• As a result, these titrations are rarely performed, and their equivalence points are difficult to detect

Weak acid-weak base titration curve including indicators