Practical investigations
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My coursework is completely ruined! I have conducted my back titration method to determine the amount of aspirin in the sample. I used ethanol to dissolve the tablet contents. Then I used phenolphthalein indicator to determine the amount of 0.1 M NaOH needed. I doubled the amount as aspirin is hydrolysed twice. Then I heated the solution for about 15 min. Afterwards the solution was titrated with 0.1 M HCl. Calculations showed that all of the tablet was aspirin (0.33 g). After research I found that it contained 0.30 g aspirin and 0.03 g maize starch. Could this have affected my results and if so how do I avoid this?
Is there anyway to maybe eliminate the starch from the aspirin solution or to measure the amount present, so that my results would be more accurate?
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Igloo says
Presumably you carried out an investigation to find out whether the aspirin tablet was contaminated with salicylic acid. However, commercial aspirin tablets always contain “filler” material of some kind or another, and this is usually starch, as your research has shown.
The problem is that in theory you could be dealing with three unknowns: aspirin, salicylic acid and starch, and a single investigative method such as yours cannot cope with this, since the back titration method assumes that there are only two such unknowns: aspirin and starch. In other words, the assumption is made that a commercial aspirin tablet contains only aspirin and starch (with no salicylic acid, unless it is “old” and has partly hydrolysed).
Alternatively, if the sample is laboratory prepared, then starch and other insoluble matter is unlikely to be present, and therefore a fair assumption can be made that the only contaminant with the aspirin is unreacted salicylic acid. Analytical methods can be devised to find out how much of each is present.
You imply by your question that you have three unknowns in mind, and, as I have explained, your back titration is not going to give you the answer you are seeking. The solution is to assume that there is no salicylic acid present, carry out your calculations, which will tell you the mass of aspirin present, and then by subtraction from the weighed mass of tablet this will give you the mass of starch present. Most students will interpret their results in this manner.
Unfortunately, there is no easy way of separating off the starch before your start the analytical procedure, since it forms a colloidal mixture when mixed with water, and cannot therefore be filtered off. Also there is no straightforward method of analysing for the starch – the quantity can only be deduced by subtraction, as described above.
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updated: 14 April 2008
