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

Choosing an Acid-Base Indicator (HL)

Choosing a suitable indicator

• Around the equivalence point of a titration, the pH changes very rapidly

• Indicators change colour over a narrow pH range, approximately centred around the pK_a of the indicator

• An indicator will be appropriate for a titration if the pH range of the indicator falls within the rapid pH change for that titration

• Section 18 of the data booklet contains information about acid-base indicators

Common Indicators and their colours table

Strong acid - strong base

• In strong acid – strong base titrations, the pH changes from 4 to 10 at the end-point so a suitable indicator must change colour within this range

• Methyl red and phenolphthalein are suitable indicators for these titrations

• Methyl orange is not ideal but it shows a significant enough colour change at the endpoint so is widely used

Weak acid - strong base

• In weak acid – strong base titrations, the pH changes from 7 to 10 at the endpoint so a suitable indicator must change colour within this range

• Phenolphthalein is the only suitable indicator for weak acid – strong base titrations that is widely available

Strong acid - weak base

• In strong acid – weak base titrations, the pH changes from 4 to 7 at the end-point so a suitable indicator must change colour within this range

• Methyl red is the most suitable indicator for these titrations

• However, methyl orange is often used since it shows a significant enough colour change at the endpoint and is more widely available than methyl red

Weak acid - weak base

• In weak acid - weak alkali titrations, there is no sudden pH change at the end-point and thus there are no suitable indicators for these titrations

• The endpoints of these titrations cannot be easily determined

Weak acid - weak base titration curve including indicators 

The overlay on the graph shows that both phenolphthalein and methyl orange would change colour outside the point of inflection in a weak acid-weak base titration so they would not be able to show the equivalence point of the titration