Practical investigations
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I'm currently doing a practical investigation on the ASA content of a commercial brand of aspirin. I am doing an acid-base titration, using 1.0 M NaOH against my aspirin solution made up of one tablet, distilled water and ethanol. However, when I titrated with aspirin in the burette, no change occurred, and the second attempt with NaOH in the burette took less than 0.1 ml to reach equivalence. Also how am I to know that the only acid I am finding is ASA!
Igloo writes ...
Commercial aspirin tablets contain aspirin itself (which is of course an acid) as well as trace amounts of ASA (formed by slow hydrolysis of the aspirin). Both of these will react with alkali, so until you modify your technique as explained in our aspirin tutorial on the React website, there is no way of analysing for the ASA separately.
In your second practical, with the alkali run in from the burette, such a low titration reading can mean one or more of the following:
1 The 1 M NaOH solution is much more concentrated than the concentration of the acids present in your “aspirin solution”. 1 ml of 1 M NaOH contains 0.001 mol of OH- ions. A 200 mg tablet of aspirin could contain up to 0.0011 mol aspirin. So 1 ml NaOH would be almost sufficient to neutralise the aspirin!
2 You are using NaOH of even greater concentration than 1.0 M by mistake.
3 You are not using aspirin at all, but “soluble aspirin” instead. This is the sodium salt of aspirin, so is already neutralised.
If you were intending to use “soluble aspirin” in the first place, then a low titration reading simply means that there is very little ASA in the tablet, which is what I would expect.
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: 31 May 2007
