Practical investigations
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Hi, I'm doing an investigation on testing the purity of aspirin. I have a back titration method, but it doesn't seem to work. Basically, I have to make up a standard solution of NaOH by taking 25 cm3 1.0 M NaOH, put it into a 250 cm3 volumetric flask and make up to the mark with water. The concentration of this solution is 0.1 M. Am I correct?
Then I take 1.5 g aspirin and 25 cm3 of this NaOH solution and 25 cm3 distilled water, heat for 10 mins, then cool and transfer to a volumetric flask and make up to the mark with water. I then take 25 cm3 of this hydrolysed aspirin solution, add phenolphthalein indicator and titrate with 0.1 M HCl.
However, when I add phenolphthalein the solution doesn't go pink. Why is that? Should I use a stronger concentration of NaOH, or more of it? Or maybe use a pH meter for the end point? Or use a different indicator? I am really stuck!
Igloo writes ...
You are correct. The operation with the volumetric flask gives a solution of NaOH of concentration 0.10M.
When you measure out 25 cm3 you are removing 25/1000 x 0.10 moles of NaOH = 0.0025 moles
This NaOH will be able to destroy half as many moles of aspirin (see our tutorial on aspirin - link is via 'Practical Investigations' on the Re:act Hoempage), i.e. 0.00125 mole of aspirin.
Since the molar mass of aspirin is 180 g mol-1, 0.00125 mole = 0.45 g
Your 1.5 g aspirin tablet will probably contain more than 0.45 g aspirin, so it appears that you haven’t used enough NaOH.
You must assume that the tablet is entirely aspirin, and use 1.5/0.45 times as much NaOH, i.e. over 3 times as much.
I suggest that you start all over again and dilute 50 cm3 of 1.0 M NaOH in the volumetric flask, and then use 50 cm3 of this diluted solution for the hydrolysis operation. This will give you 4 times as much NaOH overall, and will provide a generous excess. A pink colour with phenolphthalein is then assured!
Continue using phenolphthalein as your indicator. You cannot carry out this determination with a pH meter.
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Risk assessment
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updated: 29 November 2007
