Practical investigations
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What would the ideal concentration of NaOH be if I were to hydrolyse aspirin before using back titration to find its purity?
Chris Otter of Salters Advanced Chemistry replies
This is a bit of a 'how long is a piece of string' one. It really depends on how many tablets are hydrolysed.
One method, given in Chemistry in context, Laboratory manual and study guide by G. Hill and J. Holman, recommends 5 tablets hydrolysed in 25 cm3 of 1.0M NaOH. Of course the caustic solution needs to be standardised. The hydrolysed mixture is made up to 250 cm3 and a 25 cm3 aliquot titrated with 0.1M HCl.
You should be able to calculate the numbers of moles involved and therefore determine appropriate concentrations and volumes.
Incidentally, the three main quantitative methods of analysis are titration (in the What's in a Medicine? unit of Salters Advanced Chemistry), back titration (see Chemistry in context) and colorimetric using FeCl3 on hydrolysed aspirin.
See if you can find the RSC booklet Aspirin, which gives some good background information and covers analysis by tlc and melting point.
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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.
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updated: 20 February 2005
