Practical investigations
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I'm carrying out an experiment on aspirin using a back titration. I've hydrolysed the aspirin and when I add the indicator to the hydrolysed solution it turns pink which shows that NaOH is present in it. But when I start to add my sulphuric acid solution from the burette, it went colourless. I don't know why it is going in a reverse order. Can you help me? Or have I got nothing to worry about?
Igloo writes ...
As you say, the solution went pink when you added the indicator (presumably phenolphthalein), and this is because of the excess sodium hydroxide present. On adding the sulphuric acid from the burette, the solution should become colourless when enough acid has been added to neutralise the excess alkali. This is to be expected, so you have nothing to worry about!!
However, if the solution turns colourless after the addition of only a few drops of acid then:
1 you haven’t used enough NaOH originally to provide a reasonable excess at the end of the hydrolysis, OR
2 the acid you are using is too concentrated and should be diluted by a known factor.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 17 January 2006
