Practical investigations
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I am trying to test the purity of my aspirin but when I refluxed my aspirin sample with a known quantity of sodium hydroxide and then, after it had cooled down, I titrated a known amount of sulphuric acid but nothing happened. I don’t understand why the sulphuric acid didn’t react with the excess sodium hydroxide. I did add an indicator. Is it possible that the ethanoic acid interferes with the titration?
Igloo replies
I am not sure what you mean by “nothing happened”. If there is excess sodium hydroxide remaining after the refluxing then this is bound to react with the sulphuric acid.
I suspect that you didn’t use an excess of sodium hydroxide in the first place. As a result all of it would have been used up in the hydrolysis process and none would have remained afterwards. Consequently when you added an indicator, probably phenolphthalein, the solution would have stayed colourless, and the addition of sulphuric acid from a burette would have had no effect on the indicator, since the solution was already acidic!
What you need to do is to use more sodium hydroxide to begin with, so that an excess is guaranteed to remain after the hydrolysis process is finished.
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updated: 21 March 2005
