Practical investigations
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I am preparing aspirin as my Advanced Higher chemistry investigation. When preparing the 2-hydroxybenzoic acid there was no product formed after filtering with Buchner funnel and suction apparatus. What may I have done wrong?
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Igloo writes ...
In your second sentence I presume that you are referring to the aspirin product, rather than the 2-hydroxybenzoic acid, which should have been one of the reagents!
Here are some possible causes for your failure to collect any product.
1 Did you use the correct quantities of ethanoic anhydride, 2-hydroxybenzoic acid and remember to add the catalyst (usually phosphoric acid)?
2 Did you reflux the mixture for long enough? A minimum of 5 minutes and preferably 15 minutes is advised.
3 Perhaps the ethanoic anhydride was not a fresh sample and had been partially hydrolysed to ethanoic acid. If other students are carrying out the same preparation and have had no problems with the same sample of ethanoic anhydride, then this is obviously not the reason.
4 Did you use an ice bath to cool down the reaction mixture sufficiently, and did you wait long enough for crystals to be formed?
5 I haven’t the space to include a description here, but “supersaturation” sometimes occurs, and it is advisable to use a sharp glass rod to scratch on the edge of the glass vessel in the ice cold mixture to encourage crystallisation to occur.
The good thing to know is that this preparation does work! So if you have had problems the cause is bound to be covered by one or more of the points mentioned above.
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: 13 April 2008
