Aspirin Tablets Lab
Analysis of Aspirin Tablets
The objective of this experiment was to determine the percentage of 2-ethanoylhydroxybenzoic acid in Aspirin tablets.
2-ethanoylhydroxybenzoic acid, also known as acetylsalicylic acid, is an analgesic (pain-killer) drug available under a commercial name of Aspirin. It is an ethanoate ester of salicylic acid, which is obtained from willow bark.
Aspirin inhibits the production of prostaglandin synthase, an enzyme essential for production of prostaglandins, that cause swelling, fever, and are responsible for pain transmission to the brain .
Figure 1. Structural formula of 2-ethanoylhydroxybenzoic acid.
The acetylsalicylic acid makes up only a percentage of Aspirin tablets. The aim of this lab session is to determine the percentage. The method chosen involves an analytical technique of volumetric analysis called back titration.
- weighing bottle
- semi-analytical, electronic balance
- four 250cm3-conical flasks
- magnetic stirrer
- 250cm3-standard flask
- burette with a stand
Table 1. Reagents used.
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Firstly, one aspirin tablet was weighed on a semi-analytical, electronic balance, its mass being recorded. Subsequently, the tablet was hydrolyzed using an excess (25cm3) of solution of 1.0 moldm-3 sodium hydroxide. 25cm3 of distilled water were added. The obtained mixture was then gently heated on a magnetic stirrer for a period of 10 minutes to ensure that all the acetylsalicylic acid was hydrolyzed. After cooling, the mixture (50cm3) was transferred to a standard flask and made up with distilled water to the 250cm3 mark. Finally, a series of three titrations was conducted, in each one 25cm3 of hydrolyzed solution were transferred to the conical flask and titrated against 0.10 moldm-3 hydrochloric acid from a burette. Phenolphthalein was used as an indicator in the titrations. Few drops were added each time to a hydrolyzed solution and a change in its color from colorless to light pink (persisting for 20s) marked the end point of the titration.
Appropriate calculations were then carried out to compute the amount of acetylsalicylic acid.
Table 2. Recorded masses.
Table 3. Results of titration of hydrolyzed acetylsalicylic acid with hydrochloric acid.
Data Processing & Presentation
In the reaction of hydrolysis of the acetylsalicylic acid with sodium hydroxide the ratio in which the two substrates react is 1:2.
However, an excess of sodium hydroxide was used in the hydrolysis. The excess of the sodium hydroxide was then titrated against hydrochloric acid of known concentration.
Three trials were conducted and the average volume of HCl that was needed to neutralize NaOH was found to be 19.6cm3.
Knowing the volume and the concentration of HCl (0.10 moldm-3), number of moles that reacted can be calculated.
As the ratio of NaOH to HCl in the reaction was 1:1, the number of moles calculated was also a number of moles in of NaOH that took part in the reaction. For the titration only 25cm3 of NaOH were used, taken from the standard flask containing 250cm3 of a hydrolyzed solution. Therefore the number of moles of NaOH in the original, 250cm3 flask is ten times higher than the number of moles of NaOH in 25cm3.
It was said that in hydrolysis 25cm3 (0.025dm3) of 1.0moldm-3 NaOH were used. The initial number of moles can be calculated.
Subtracting the number of moles of excess of NaOH from the initial number of moles we obtain the number of moles that reacted in the reaction of hydrolysis.
It was already mentioned that in the reaction of hydrolysis of the acetylsalicylic acid with sodium hydroxide the ratio in which the two substrates react is 1:2. So, the number of moles of acetylsalicylic acid that reacted was half a number of moles of NaOH.
The molar mass of acetylsalicylic acid is 180 g mol-1 (from the IBO Data Booklet). Knowing the molecular mass and the number of moles, the mass that reacted can be calculated.
The recorded mass of the Aspirin tablet was 0.6g, therefore the percentage of the acetylsalicylic acid in the Aspirin tablet is 81%.
Table 4. Percentage errors.
Conclusion & Evaluation
The objective of this experiment has been successfully achieved, the percentage of acetylsalicylic acid in the Aspirin tablet was determined. The value found was 81%. The calculated value is very similar to the percentage calculated using the masses of acetylsalicylic acid (500mg = 0.5g) and Aspirin tablet (0.6g) provided by the manufacturer :
The percentage difference between the two values is
Sources of Error
- The main source of error in the experiment was the assumption that the additional substances in the tablet did not react in the reaction of hydrolysis nor in titration. In fact, we do not know what substances were there, and we cannot assume that only acetylsalicylic acid was reacting.
- Error could arise from the uncertainties of the measuring devices: pipettes, burettes and volumetric flask. (the error calculated to be 8.8%)
- Error can also be caused by inaccurate determination of the end point in titration. The method used was based on the observation of the change in color of the indicator.
- the human reaction time when turning the tap of the burette decreased the accuracy of the volume titrated
Suggestions for Improvement
- pH meter should be used to determine the end point of titration
- mechanical pipettes can be used to decrease the error, because they usually have smaller uncertainties
- more titrations should be carried out to pinpoint the most accurate average value and to minimize the possibility of random errors.
- to decrease the uncertainty of the balance analytical one should be used instead of a semi-analytical one.
- Chemistry for the IB Diploma, Oxford University Press, 2004
- Bayer Health Care Aspirin leaflet, revised 2004.