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

Revision Note

Philippa Platt

Last updated

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 pKa 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

Indicator Colour in acid Colour in alkali pKa pH range of colour change
Methyl orange red yellow 3.7 3.1 – 4.4
Bromophenol blue yellow blue 4.2 3.0 – 4.6
Methyl red red yellow 5.1 4.4 – 6.2
Phenolphthalein colourless pink 9.6 8.3 – 10.0

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 

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

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.