Practical investigations
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I am carrying out the colorimetry of aspirin to determine the percentage of 2-ethanoylhydroxybenzoic acid that is present in aspirin tablets. I have researched this method and have found that FeCl3, KCl and HCl are used to make up a solution. Could you please tell me why they are each used to make up the solution and how do you know what quantity of each to use. Also could you tell me which ones are solids and which are liquids for when using in the experiment.
Igloo writes ..........
I strongly recommend that you read our tutorial on aspirin on our web site and look up some of the FAQs relevant to this particular method, using the Search facility on Re:act.
Briefly, FeCl3 is used to form the iron(III)-2-hydroxybenzoic acid complex. HCl is used to keep the pH low (in the region of 1.5) so that the complex is stabilised.
KCl : I am not sure of the function of this salt. Many references will tell you that it forms a buffer with the HCl in order to keep the pH constant, but this cannot be correct. HCl is a strong acid and therefore cannot form a buffer with one of its salts.
The solution mixture is made up using solid KCl and solid FeCl3 in 0.02 M aqueous HCl.
I suggest that you use Google to research a good laboratory procedure. There is a good description of the method as given by Kerber and Akhtar. Try using these names in your search phrase together with “aspirin”.
If you really cannot find the detail you need get back to me and I’ll give you a workable method.
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: 08 November 2007
