Preparation and recrystallisation of aspirin

Synopsis This report is based on preparation and recrystallization of aspirin. The objective of the experiment is to conduct the synthesis of aspirin and reinforce the skills of recrystallization and technique of melting point determination. Both experimental successes and errors were analyzed. The mass of recrystallized aspirin recorded was 0. 85g, which led to a low percent yield of 27. 1%. The melting point was 135. 5 – 138. 3 ? C, but had a quite a wide range which could be due to impurities present. The aspirin had an appearance of a white, needle-like crystalline solid.

If time permitted, the experiment could be repeated to improve the results, by reducing the errors which was analyzed. Overall, the experiment can be deemed as quite successful.



The objective of this experiment is to learn to conduct the synthesis of aspirin, reinforce the skills of recrystallization and determination of melting point.


The active ingredient in aspirin, acetylsalicylic acid, is a synthetic derivative of a compound, salicin found commonly in willow trees. Ancient Greeks had noted the medicinal effect of willow tree extracts, yet its active ingredient salicylic acid was only isolated until the 1800s. However, it was strongly acidic making it hard to swallow and caused irritation to the lining of the mouth and stomach. Only later in 1897, the process to synthesize an ester of salicylic acid, acetylsalicylic acid was discovered. Thought it was not as strong as salicylic acid, they had the same medicinal properties.

Aspirin is among the most effective, fairly non-toxic, pain killers, commonly used to treat mild to moderate pain, to reduce fever or inflammation, and sometimes used to treat or prevent heart attacks, strokes and angina. It reduces substances in the body that causes pain, fever and inflammation. It also reduces blot clots and small doses can be used to prevent heart attack and stroke. However, some people might get irritation to the stomach membranes or lose blood in the stomach lining from the consumption of aspirin. 2Theory Aspirin can be made from the reaction between salicylic acid and several different acetylating agents.

In this experiment, acetic anhydride was used as it is cheap, and forms a non-corrosive by-product, acetic acid which can be reused to make more acetic anhydride. Recrystallization was used to obtain a pure compound of acetylsalicylic acid since the ester is in a solid state and impurities may remain in the mixture. It is important to recrystallize as the impurities will stay in the solvent, producing the purest final product which will be safe for human consumption.

A mixture of ethanol and water is a suitable solvent for this experiment. Concentrated sulfuric acid was added as a catalyst to speed up the reaction. The theoretical number of moles of aspirin is calculated based on the molecular equation. Salicylic acid is used as a limiting reactant hence giving a mole ratio of 1:1of salicylic acid to acetylsalicylic acid produced. Therefore, the theoretical mass of acetylsalicylic acid is equivalent to the mass of salicylic acid used. Percent yield can be calculated by dividing the actual mass of aspirin over the theoretical mass and multiplying it by 100%.



The materials used in this experiment were: salicylic acid, acetic anhydride, concentrated sulfuric acid and ethanol. The equipment used were: conical flask, water bath, hot plate, dropper, vacuum filtration set up, watch glass, filter paper, electronic balance, automatic melting point apparatus and desiccator.


Firstly, 2. 41g of salicylic acid was weighed in a dry 100ml conical flask. In the fume hood, 6ml of acetic anhydride was added to the salicylic acid in the flask. Drops of concentrated sulfuric acid was then added to the mixture and swirled to mix.

The mixture was heated in a water bath for 15 minutes to complete the reaction and removed after 15 minutes. While the mixture was still hot, about 1ml of distilled water was added from a dropper to the mixture to decompose the excess acetic anhydride. An additional 50ml of cold distilled water was added and the mixture was stirred with a stirring rod to induce crystallization. The crude product was collected by suction filtration and washed with cold water.


The crude product aspirin was dissolved in approximately 5ml of ethanol in a 100ml conical flask and then warmed on a hot plate. Approximately 30ml of hot distilled water was added to the solution. The solution was heated until all the solid was completely dissolved, then allowed to cool for crystals to form. A clean, dry watch glass together with a filter paper was weighed and recorded (32. 85g). The weighed filter paper was used to obtain the recrystallized sample by suction filtration.

Next, the crystals and filter paper was transferred onto the weighed watch glass and then dried in the oven (100? C) for 20 minutes, subsequently placed in a desiccator for 10 minutes. The dried crystals was weighed and recorded together with the filter paper and watch glass. The weight of dried, recrystallized aspirin was calculated. The expected yield of aspirin from the amount of salicylic acid that were used and the percentage yield of dried recrystallized aspirin was calculated.

Determination of Melting Point

An automatic melting point apparatus was used to determine the melting point of the aspirin product.

Results and Calculation

4. 1 Mass Mass of salicylic acid = 2. 41 g Mass of filter paper & watch glass = 32. 85 g Mass of dried, recrystall.ized aspirin, filter paper & watch glass = 33. 70 g Mass of dried, recrystallized aspirin= 33. 70 g – 32. 85 g = 0. 85 g.

4.2 Percent yield No. of moles of salicylic acid used= 0. 0175 mol Expected no. of moles of aspirin= 0. 0175 mol Expected mass of aspirin = 3. 14 g Percentage yield= 0. 853. 14 x 100% = 27. 1 % yield

4.3 Melting Point Temperature range135. 5 – 138. 2 ? C

4.4 Appearance The aspirin product had an appearance of a white, needle-like crystalline solid.


The theoretical mass of aspirin was calculated to be 3. 14 g, however only 0. 85 g was obtained. It is expected for the actual mass to be lower than its theoretical mass as impurities were removed. However, our actual mass is much lower and this could be due to experimental errors that occurred during the experiment. Firstly, the purpose of washing is to remove excess acetic anhydride to obtain a better yield, and cold water was used to minimize the amount of acetylsalicylic acid removed while washing. This leads to the suspicion that the cold water which was used during the preparation was not cold enough, or the solution had increased in temperature while we waited for our turn to use the suction filtration, leading to the acetylsalicylic acid dissolving in that process, subsequently being filtered away.

Therefore, the volume and mass had decreased leading to a low percent yield. Another reason could be that the reaction was not given enough time to reach completion during recrystallization. It could be that the solution was not cooled enough and the acetylsalicylic acid was still in liquid form, hence being filtered away through suction filtration. The volume decreased and so mass decreased too. This again led to a low percent yield of 27. 1%. To fix this in order to have a higher percent yield, phosphoric acid could be used as a catalyst instead.

Sulfuric acid being a strong oxidizer reacts more readily with molecules involved in the reaction than with acetic acid, leading to a low percent yield. According to the Royal Society of Chemistry, the melting point of aspirin is 138 – 140 ? C. Hence our results of 135. 5 – 138. 2 ? C is considerably close but the range is quite large. The lower melting point could be due the presence of impurities. During crystallization, cold water was added to cool the mixture, while the solution was allowed to cool down during recrystallization.

The addition of cold water forced the mixture to cool down unnaturally and the aspirin may not be able to not crystallize properly. Hence, this could have affected the purity of our sample. Additionally, when the experiment was performed, impurities from the air or equipment could have entered and interfered with the reaction or reactants at any time, since the containers used in the experiment was left exposed. This would have contributed to the large range in our melting point. 6Conclusion.

Throughout the experiment, the procedure of synthesis of aspirin was learnt. The skills of recrystallization and technique to determine melting point was also reinforced. The interpretations of results were acquired. Both experimental successes and errors were analyzed. The percent yield was not successful as the mass was quite low. If the melting point of aspirin is assumed to be 138? C, our results (135. 5 – 138. 2 ? C) can be deemed quite successful. However, the temperature range is quite large.

By crystallizing the aspirin slowly, large crystal with a rigorous structure could be obtained as the molecules are allowed to join together in a precise manner. This makes it more difficult for impurities to be included, reducing many impurities present in the final product. Thus, if time permitted, the experiment could be repeated to correct the errors in technique made and the results could be improved.


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