Practical investigations
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I am currently working on my formal report on the quantitative determination of ASA in aspirin tablets using a back titration. Can you tell me what is the rationale behind the dilution and aliquoting of the aspirin sample? Also, what are the principles behind the use of back titration in the analysis of aspirin tablets? Lastly, why are we using a more dilute solution of NaOH for standardization?
Igloo writes ...
The reason why the solution is diluted is to make it up to a known volume. When portions (aliquots) of smaller known volume are removed, it is easy to calculate what proportion of the total mixture is being titrated. For example, if the solution is diluted (made up) to 250 cm3 and if 25 cm3 portions (aliquots) are removed and analysed, the calculated ASA value only needs to be multiplied up by 10 in order to determine the quantity in the entire sample (the tablets).
The reason why we take aliquots is so that the titration can be carried out several times to obtain consistent results.
As for the theory and principles involved please refer to our aspirin tutorial where you should find all you need.
Finally, the acid used for the back titration is more dilute than the NaOH used to carry out the hydrolysis, so, in order to standardise this acid we need to use an NaOH solution of approximately the same concentration to obtain “sensible” titration readings.
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Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
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updated: 14 December 2006
