Practical investigations
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I am currently writing up a practical on the analysis of aspirin tablets by the back titration method. I have already found the percentage of the 2-ethanoylhydroxybenzoic acid in one tablet. Using this site I found a lot of useful information about the back titration method, however I’m still confused as to what is/are the other constituent(s) of the tablet. If you could tell me what they are as well as the purpose for their inclusion, I would be very grateful. Also, after comparing results I would like to know as well why overheating (vigorous boiling) during hydrolysis of aspirin would have an effect on the results. Thank you in advance.
Igloo replies ...
I am pleased you have found some useful information on the React website.
Apart from 2-ethanoylhydroxybenzoic acid, there is bound to be a little 2-hydroxybenzoic acid present due to slow hydrolysis of the aspirin tablets. This constituent will increase as the tablets “age”. However, the main “impurity” present in aspirin and most other medicinal tablets is some sort of insoluble filler. Fillers are used to bulk out the tablet so that they are a convenient size to hold and to swallow. They are totally harmless and include such substances as chalk (CaCO3) and silica (SiO2).
“Overheating”, as you put it, should have no adverse effect on the results. Prolonged boiling is essential to ensure that the ester linkage in all of the molecules has been hydrolysed. However, there is always a danger that boiling for too long will cause solid to crystallise and to “spit” out of the mixture, and this would be hazardous for those nearby, especially if the solid is excess (hot and corrosive) sodium hydroxide.
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: 02 April 2007
