Practical investigations
Read our general notes on Risk Assessment
With regard to aspirin analysis, I have a question about a back titration that I wish to carry out. Will any impurities (like salicylic acid) interfere when I warm my aspirin sample with excess NaOH (my synthesised aspirin is bound to contain some)? If so how will I work out how much aspirin is in my sample? If the NaOH has reacted with another compound then there will be less of the excess to neutralise and therefore calculations will show a greater amount of aspirin.
Igloo writes
You are absolutely correct in saying that any salicylic acid present will interfere with your analysis. I have answered a similar question before and maybe if you search through our FAQs on React you will come across my previous detailed answer. In summary, what you need to do is to obtain pure samples of salicylic acid (this shouldn’t be a problem) and pure acetylsalicylic acid (aspirin) (this might be harder to come by). Then you need to carry out a direct titration with a standardised solution of sodium hydroxide three times, using equal masses of pure salicylic acid first, then pure aspirin and finally your sample.
Remember to use ice-cold conditions so that the ester linkage in the aspirin isn’t hydrolysed during the titration. By plotting a graph of composition against titration you will be able to deduce the composition of your mixture, which, as you say, is likely to be a mixture of SA and ASA. A similar procedure can be carried out for the back titration, but I suggest that you use the direct titration first. Please also refer to our React tutorial on aspirin.
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.
back to Practical investigations
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 18 December 2007
