Practical investigations
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I’ve done a back titration to find the aspirin content in two different manufacturers’ aspirin tablets. One of these tablets contains lactose, which has given a higher burette reading to the other tablet that just contains starch, why is this? The aspirin was hydrolysed using sodium hydroxide, and then back-titrated with sulphuric acid, using a phenyl red indicator. Please help!
Igloo writes ...
I am assuming that you used the same mass of tablets in each case, and that the two brands of aspirin claim to contain the same mass of active ingredient (aspirin).
Unless one of the manufacturers is cheating (unlikely), the only possible interpretation is that the lactose is in some way interfering with the analysis. Being a disaccharide I would expect it to be hydrolysed by acid into its monosaccharide components, and this would of course explain why your back-titration reading is higher, since some of the acid is used to neutralise the excess alkali whilst the rest is needed for the hydrolysis. However, disaccharide hydrolysis is not normally a very fast process and it would be surprising that much hydrolysis could take place in the time taken to carry out a titration.
One way to test this theory would be to repeat the determination, but to carry out the back titration very much more slowly than usual. If the titre is even greater under these circumstances then this would confirm the lactose hydrolysis theory.
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Risk assessment
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updated: 17 December 2006
