Acid-base equilibria
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What makes Phenolphthalein a good indicator?
If there is no change in an indicator, what conclusion can we draw?
Jenkin writes:
The best thing about phenolphthalein is that it has a really sharp colour change, in either direction, at a pH of around 9 (the range given in the Nuffield Book of Data is 8.2 to 10.0). This means it fine to use in titration of a strong base against either a strong acid or a weak acid. In the case of strong acid/strong base, there is very sharp pH change,around the end point, over the range 4 to 10. For a weak acid/strong base, this sharp change occurs over the pH range 7 to 10, again within the range of phenolphthalein.
However, phenolphthalein cannot be used to find the endpoint of a titration of a strong acid with a weak base, because this endpoint will occur at a pH in the 3 to 6 region.
Methyl orange, which changes over the range 2.9 to 4, could be used here. Neither phenolphthalein nor any other indicator is of any help if you are trying to titrate a weak acid with a weak base; here there is virtually no region of sharp pH change. There are, however, other methods of following this type of titration, such as direct pH measurement or conductance measurement.
If there is no change in an indicator, all it tells you is that the pH has not changed beyond the limits of the range. Say, for example, you added phenolphthalein to pure water. The pH is 7, so the indicator would show its acid colour and would not change unless you added something to take the pH up to around 9. But if it were added to an alkali it could show its acid colour when the pH fell to 9.
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Risk assessment
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updated: 07 April 2006
