Practical investigations
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I'm completing my individual investigation on methods of determining aspirin purity and have just completed a pH titration. Would you please be able to give me some information or instructions on how I can work out the purity of my aspirin sample from the titration curve I have completed from my results? Please could you give me details of a website or book where I could find information to help me? 100408
Igloo writes ...
I assume that you have carried out a “direct titration” with the aspirin solution in the flask and the sodium hydroxide solution in the burette.
Proceed as follows:
From your curve find the volume of NaOH which corresponds to the point of inflexion in your graph, i.e. the place where there is a sharp rise in pH. This is the neutralising volume of NaOH.
Knowing the concentration of the NaOH and the neutralising volume you can deduce how many moles of NaOH were used in the titration.
This quantity equals the number of moles of aspirin present in the flask (because one mole of aspirin is neutralised by one mole of NaOH).
You do not give me enough details about how you carried out the procedure, but now that you know the number of moles of aspirin in the volume used for the titration you can “scale up” to calculate what was present in the solution contained in the volumetric flask.
Multiplying this by the molar mass of aspirin will enable you to convert the quantity in moles to grams.
This number of grams needs to be divided by the mass of aspirin actually weighed out, and the answer multiplied by 100 to give the percentage purity of the aspirin in the tablet(s).
Since you gave me no practical details I can only guess as to what you did, so it is quite probable that you will need to adapt my method above to suit your needs.
Risk assessment
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updated: 11 April 2008
