Why is titration method applicable in aspirin analysis?

When I am using back titration and direct titration as my two methods for determining the purity of aspirin tablets. doesn't the direct titration not take into consideration of the excess sodium hydroxide added to the aspirin?

In both these titrations when I reach the end-point it turns pink but I kept these solution the next day and they turned back to colourless. Is this meant to happen and what chemical reaction has happened here?

I titrated this again with more sodium hydroxide till the end-point then added this to the overall amount of NaOH i added in the flask.

Is the only difference between back titration and direct titration not addding the HCl at the end to determine the excess amount of NaOH?

Are there any methods for direct titration.

Would really appreciate any help.

The best you can obtain is an end point lasting 20-30 seconds. Any excess base will slowly hydrolyze the ester and liberate acetate ion from the act's because aspirin is such a weak acid that it reacts slowly with the NaOH, making it difficult to accurately get a good endpoint in a reasonable time.

With back titration, you react with an excess of NaOH (known amount), heat it to make the reaction go to completion quickly, then use HCl to determine the amount of NaOH that is remaining. This reaction will go quickly, and is much easier to measure.elylsalicylic acid,aspirin.