Determination of Aspirin Concentration by Back Titration

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Back titration was the method used to determine the concentration of aspirin by reacting it with known amount of excess base. Aspirin- chemically known as acetyl salicylic acid is a weak acid undergoing slow hydrolysis where one molecule of aspirin reacting with two hydroxide ions, to avoid this step, a known excess amount of base was added to the sample and titrated with sulfuric acid and phenolphthalein indicator to determine the amount of excess base. known sample of aspirin yield was obtained as 1.171g and unknown sample is 1.040g. Errors can be determinate and indeterminate. Error expected not more than 5%.


To find the concentration of aspirin from known sample of aspirin tablets and unknown sample of aspirin by back titration.

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What is Aspirin? – Introduction

Aspirin is common drug taken orally which is non-steroidal anti-inflammatory drug mostly used as analgesic (pain killer) and anti-pyretic (to reduce increase in temperature) with anticoagulant properties. Chemically, aspirin is acetyl salicylic acid that acts as an inhibitor of cyclooxygenase which inhibits the biosynthesis of prostaglandins, platelet aggregation and inflammation. (PubChemDatabase.Aspirin, CID=2244, here, back titration was done to obtain the concentration of aspirin, from known aspirin tablets, amount of product expected was 0.5g. measured yield obtained was 0.335g which shows there was an error In the determination. Errors might be determinate and in-determinate, it depends on how large the error is made.


1.5g of given aspirin tablets were weighed and noted number of tablets used, transferred to 250ml conical flask where 25ml of NAOH and 25ml of water were added with magnetic stirring bar and this mixture was placed on hotplate and bought it to simmer and was held there for 30 minutes. Then flask was removed and cooled and then solution was transferred in to 250ml volumetric flask and volume filled up to calibration mark with water and closed with stopper and shaking this upside down in order to make sure the solution is thoroughly mixed. 50ml of this solution was taken in to a beaker and 25ml of this 50ml is taken in to 100ml conical flask with Pasteur pipette, 4 drops of phenolphthalein indicator was added, titrated this mixture drop wise with H2SO4 solution which was filled in the burette up to the calibration mark. Endpoint where pink solution turns clear was observed. Titrations were repeated 4 times in order to get concordant values.


Aspirin obtained from the 0.5g sample of tablets was 0.335g. Accuracy is always expressed in terms of error. Error is defined as the difference between the true value (0.5g) and measure value (0.335g) known as absolute error (0.165g). Alternatively, relative error can also be calculated, which is the error expressed as a percentage of the measured value or in “parts per thousand” was obtained as 33% for aspirin tablets (0.5g). however, 0.5g of aspirin was expected and 0.335g was determined which shows less amount of aspirin was determined.

This might be due to errors which can be determinate or indeterminate, it also depends on how large the error is made. Determinate errors might be constant or proportional having measurable and fixed value which can be corrected and as the experiment proceeds, error increases with the magnitude of the measurement. Determinate errors usually originate from operator error, equipment error, method errors. (D. Kealey & P. J. Haines) Instrument error might be the major error as there was common glass ware used and shared among everyone such as bulbs of the Pasteur pipettes which were wet, also common flasks and funnels were used which might had led to the error. While refilling the burettes with H2SO4 and diluting the mixture in 250mL volumetric flask for concordant titrations, if not properly filled up to calibration mark might also lead to the operator error. however, proper measures were taken and accurately filled up to the mark. Indeterminate errors arise from unpredictable individual manipulations in a procedure (F.W. Fifield & D. Kealey) error might be uncertain that can be introduced in to the final result. However, proper measures were taken and manipulation was not done in the procedure. The titre values were noted when the end point was determined that is when the solution turned from pink to clear. All the four concordant values obtained were precise values where average value was taken out of them to obtain the volume of H2so4 consumed by aspirin in the experiment.


Сonclusion is made by the results which were not as accurate as expected, purity and determination of aspirin concentration widely relies on accuracy for quality of results in analytical science. Error is particularly high. More precautions should be taken while performing the experiment in order to minimise the quality of error. Therefore, amount of aspirin determined from tablets is 0.335g out of 0.5g. Percentage error is 33% and percentage of aspirin determined is 78.06%. The amount of aspirin obtained from unknown sample is 1.040g.

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