Practical investigations
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I have done an experiment to analyse the aspirin content of aspirin tablets. I have used the method of hydrolysis with excess NaOH, and have then carried out a back titration with sulphuric acid. The equation for hydrolysis is:
CH3COOC6H4COOH + 2NaOH -> CH3COONa + HOC6H4COONa + H2O
I would like to know whether CH3COONa and HOC6H4COONa would act as buffers. If yes, would it affect the pH of the equivalence point during titration between sulphuric acid and sodium hydroxide in the latter part of the experiment? (as the sodium hydroxide is mixed with CH3COONa and HOC6H4COONa in the solution used for titration) What would the pH of the equivalence point of the above titration be?
Igloo says
A buffer would be formed between the two salts you have mentioned in your question and the parent acids, i.e. CH3COOH and HOC6H4COOH, but neither of these is present during your back titration (only an excess of NaOH), so no buffer is formed with the anions of these salts.
However, both of these anions undergo hydrolysis with water to form OH- ions, so, at equivalence, the solution will still have a pH above 7, maybe in the region of 8-10. Therefore phenolphthalein is the indicator of choice, rather than, for example, methyl orange.
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updated: 26 February 2007
