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Acids and Bases

Indicators

  • 3 things to know
  • Papers 1 and 3

Overview

How an indicator works, and choosing one whose range falls on the vertical section of the curve.

What you need to know

Answer each one before you reveal it.

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Key term

What is an acid-base indicator in terms of its chemical nature?

Key term

How does an indicator function during a titration?

Key term

What determines whether an indicator is suitable for a particular acid-base titration?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is an acid-base indicator in terms of its chemical nature?

    Model answer

    A weak acid (or weak base) that has different colours in its protonated and deprotonated forms

  2. 2

    How does an indicator function during a titration?

    Model answer

    The indicator changes colour when the pH passes through its colour change range, signalling the end point

  3. 3

    Why must the pH range over which an indicator changes colour match the vertical section of a pH curve?

    Model answer

    So that the colour change occurs sharply at the equivalence point rather than gradually over a wide pH range

  4. 4

    What determines whether an indicator is suitable for a particular acid-base titration?

    Model answer

    The indicator's colour change range must fall within the steep vertical section of the pH curve for that titration

  5. 5

    For a strong acid-strong base titration, what type of indicator would be appropriate and why?

    Model answer

    Any indicator with a colour change range between approximately pH 3 and pH 11, because the vertical section spans this entire range

  6. 6

    For a weak acid-strong base titration, which indicator would be suitable?

    Model answer

    Phenolphthalein, because the equivalence point pH is above 7 and phenolphthalein changes colour in the appropriate range (approximately pH 8-10)

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