Learners identify the weak acid and conjugate base in an acid–base equilibrium reaction; explain why a particular indicator changes colour and sketch pH curves for acid–base titrations to identify the best indicator to use during a titration

This resource accompanies the Education in Chemistry article Acids in our oceans.

Learning objectives

  1. To understand that indicators are weak acids in which the colour of the aqueous solution of the acid is distinctly different from that of its conjugate base.
  2. To know that the pH range over which a colour change occurs can be estimated by pH = -pKIn ± 1.
  3. To be able to sketch the pH curves for different acid–base titrations and suggest suitable indicators based on data given.

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    Test your learners' understanding of indicators. MS Word and PDF available.

Questions

There are three questions on the student worksheet. Learners need to be able to identify the weak acid and conjugate base in an acid–base equilibrium reaction; explain why a particular indicator changes colour and sketch pH curves for acid–base titrations. Learners will demonstrate that they can identify the best indicator to use during a titration based on the pH range and circle the proton released during dissociation of a weak acid on a structural equation. 

The indicators in the resource include phenolphthalein, methyl-orange, phenol red and 4-nitrophenol.

  • Link to careers

    Meet Ben Fisher, a marine biogeochemist at the University of Edinburgh. Watch the video job profile where he explains his PhD, which focuses on how the oceans are responding to climate change.

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