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I have been looking at the RSC method of analysis of paracetamol (titration with ammonium cerium(IV) sulfate). It says to "repeat the operation without the test material being present. The difference between the titration figures represents the amount of ammonium cerium(IV) sulfate required."
What does this actually mean and why do I need to know the amount of ammonium cerium(IV) sulfate? Am I testing the paracetamol I have made or a known tablet form? 260308
Igloo writes ...
The RSC publication doesn’t make it clear why you need to carry out the procedure once again, without the “test material”. In fact the procedure is asking you to carry out what is known as a “blank titration”. If you study the method in detail you will see that sulphuric acid, hydrochloric acid and ferroin indicator are being used in addition to the “paracetamol extract”, and the blank titration is needed to ensure/confirm that the ammonium cerium(IV) sulphate doesn’t actually interact with any of these as well. The blank titration (without the paracetamol extract test material) usually comes out with a titre of only 0.0 or 0.1 cm3 (i.e. one drop or so!), but you need to subtract this from the main titre in order to be sure that the net titre is due to oxidation of the paracetamol extract, and nothing else.
You may think that carrying out a blank titration is a waste of time, since the titre is often virtually negligible, but a careful analyst will often carry one out to ensure that his/her final results are as accurate as they can be.
I hope that this explains what is going on here.
By the way, if you intend to carry out this analysis please note that there is a significant misprint in the central equation on page 11 (the one with 2Ce4+ at the start).
There should be 2Ce3+ formed on the right-hand side (not 2Ce4+ again).
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updated: 01 April 2008
