Acid-base equilibria
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I am really confused on how you know which indicator to use before a titration.
Jenkin writes
I am assuming you are thinking of acid/alkali titrations. You will probably not yet have studied the theory behind this as it is normally A2 work and I won’t try to cover it all here.
Each indicator changes colour over a specific pH range of around 2 pH units, and this range has to coincide with the endpoint of the titration. Around the endpoint the pH changes very dramatically for the addition of a very small volume of the acid or alkali.
Essentially, these are the rules:
For the titration of a strong acid (e.g. HCl) with a strong alkali
(e.g.NaOH), you can use almost any indicator (phenolphthalein, methyl orange, bromophenol blue), but not normally litmus and never Universal Indicator, which is a mixture of several indicators.
If you are titrating a weak acid (e.g ethanoic or other organic acid) with a strong alkali, use phenolphthalein.
For a strong acid with a weak alkali (such as sodium carbonate or ammonia), use methyl orange or methyl red.
For a weak acid and weak base, no indicator is suitable and a different method has to be used to find the endpoint.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
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For further guidance see our tutorial on Risk Assessment.
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updated: 02 April 2007
