Acid-base equilibria
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Why is phenolphthalein, but not methyl orange, a suitable indicator for a titration of a strong base against a weak acid?
Contrary to popular belief, indicators do not usually change their colour at pH 7. Indicators such as phenolphthalein change over the pH range from about 8 to 10, and methyl orange, for example, changes its colour between pH 3 and 5.
Another fallacy which needs to be addressed is that the pH of a solution obtained by neutralizing a base by an acid is 7. It can be – for example, when a strong base is neutralized by a strong acid [e.g. NaOH(aq) with HCl(aq)] – but often it is not.
When strong bases are titrated against weak acids, the pH of the solution at the end-point is in the region of pH 8-11, so, from what I have written in the previous paragraph you can see that phenolphthalein is the indicator of choice. However, if a weak base is titrated against a strong acid, the pH of the solution at the end-point is about 3-6, so methyl orange is an obvious indicator to choose.
You will find the pH ranges of some common indicators on page 342 of the Nuffield Students’ Book, and also some further explanation of the theory. On page 342 there is a brief explanation of the 'theory of indicators'.
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Risk assessment
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updated: 21 August 2003
