Practical investigations
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The problem I'm having is that I don't know what back titration involves. I did try looking on your website to find a appropriate method for the back titration of aspirin but I could not find any. So is it possible for you to provide me a internet link maybe that tells me about the method of back titration or maybe just tell me in the email how to carry out a back titration of aspirin please?
The other problem I'm having is that the investigation is carried out using a ready-made aspirin tablets that I recrystallized and vacuum filtered to obtain the pure aspirin and what has happened is I ended up doing pH curves as my teacher suggested and now I'm stuck with how to find an equivalence point.
Corrie writes ....
To hydrolyse your aspirin you added an excess amount of NaOH solution to your sample. When the reactions (as described in the Aspirin Tutorial on React) are over, there will be some NaOH left. Back titration is simply titrating this mixture with acid of known concentration, using phenolphthalein indicator, to find out how much NaOH is left over.
Working back (hence 'back' titration), you can work the amount of NaOH used up during the hydrolysis itself by subtracting the amount left over from the original amount of NaOH added. You then use this to work out the number of moles of aspirin in your sample - using the equations and advice in the Tutorial.
As regards your pH curves from titrating your sample with NaOH solution, the equivalence point will be when the pH changes rapidly, giving a vertical, or near vertical, part of the titration curve.
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updated: 27 February 2008
