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Titration Curves

Chemistry: IB Diploma Higher Level, topic 6.9.5

Strong Acid/Base Titrations

Titrations of a strong acid with a strong base are the simplest to understand.

pH curve

pH curve

  • The pH curve pictured shows the change in pH when a strong acid is titrated with a strong base.
The beginning

The beginning

  • In the beginning, you can see that the pH is very low, and doesn’t change very quickly.
The middle

The middle

  • In the middle of the graph, there is a sharp change and the pH rises very rapidly. This is known as the equivalence point.
  • At the equivalence point, the acid is just about neutralized. Adding any more base causes a rapid shift to a basic solution.
The end

The end

  • The curve ends at a very high pH because the solution has excess base present.
Indicator

Indicator

  • When doing an acid/base titration, you need to use an indicator to find the equivalence point.
  • A good indicator will change color dramatically over a small pH range.
  • If the small pH range includes the range of the equivalence point, it’s a great indicator for the reaction.
Examples of good indicators

Examples of good indicators

  • The two main indicators are phenolphthalein and methyl orange.
  • Phenolphthalein changes from colorless to pink from pH 8.3 to pH 10.
  • Methyl orange changes color from red to yellow between pH 3.1 and pH 4.4.
  • Either of these are well suited for a strong acid/base titration.

Test yourself on Titration Curves

  • What do you need to use to find the equivalence point?

  • A basic solution would turn phenolphthalein what color?

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