Practical investigations
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I'm doing an investigation on aspirin and testing its purity. I've used the direct titration and back titration methods. I was just wondering, although back titration is more accurate/reliable, wouldn't the results of direct titration give a higher percentage of aspirin because everything in the solution - all the aspirin and salicylic acid are being neutralised by sodium hydroxide? Or should back titration give a higher percentage of aspirin in the sample, or should they both show equal results?
Igloo writes ...
I am assuming that you are using commercial aspirin samples in which it can be assumed that the only impurity in the aspirin is insoluble “filler”.
Both methods give perfectly valid results, though with the “direct method” the titration needs to be carried out in ice-cold conditions so the ester linkage is unaffected.
The back titration method is preferable in that there are no “chemical” complications, i.e. both the ester and carboxylic acid groups are affected by design. However, the procedure is more complex involving several stages, and therefore more open to overall experimental error. Within these constraints both methods should give comparable results.
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updated: 21 January 2008
