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Synthesis and BP Assay of Aspirin: Analytical Essay

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Presently, Aspirin is the most prescribed drug round the earth and producing it is known to have been one of the greatest achievements of the last hundred years in the area of medicinal production (1).

The tale of this medicine started more than three thousand years ago (3000 years) when it was known as Salicyline (located in barks of willow trees) (1). It was used to cure Inflammatory infections and was known already by the earliest Egyptians (1). This antidote was used to relieve discomforts in their bodies (1). Many years before the Egyptians, it was used to increase the sense of hearing for people who had hearing difficulties as well as to heal individuals who had joint agonies (1). Places in Africa also used the pieces from these trees to cure discomforts and by the 18th century, vegetation containing Salicylates were used to heal open sores that can develop on the lining of the stomach and other infections (1).

An English reverend named Edward Stone investigated the characteristics of willow(1). While doing this, he gave an uproot of Salix Alba to victims with fever and found out that the uproots had an action that prevents fever from occurring every four hours (1). He was known as the first writer to show the importance of the bark in curing fever (1). Two Italian chemists tried fiding the effective part of the bark when one of them named this bark according to its characteristics while the other chemist changed the name later to ‘Salicin’ and this recognised by everyone (1). A good example of an alcoholic glucoside is Salicin, and when taken, the ether link was broken down while the two divisions of this drug were glucose and salicyl alcohol which both were processed independently (1). Salicylic acid was used mainly for making Aspirin as this was done and explained by a European Scientist known as Hermann Kolbe (1). The process was brought to an end by his partner, Rudolf Schmidt and was named after them (1).

By the end of the 18th century, a medical practitioner known as Franz Stricker showed the potency of Sodium Salicylate in relieving pains as well as another practitioner who showed that Sodium Salicylate could be used to cure other severe pains and swelling (1). A firm was planning to make another type of drug that was related to salicylates which did not possess any danger(1). This job was for John Hoffmann and he tried reacting the aromatic alcohol group with acetic anhydride this was successful, he was able to get acetylsalicylic acid which was in its clear state and a trusted way (1). The date at which he did this was the 10th day of August in the year 1897 and this is known as the anniversary of Aspirin (1). Finally, this medicine was named ‘Aspirin’ (1).

1.2 Uses

Aspirin has several uses while some of them are;

  1. It can be used in conjunction with Rivaroxaban to treat heart and renal infections (2). The use of these two drugs helps to reduce the rate at which people die from heart diseases as well as Kidney diseases (2).
  2. This drug molecule can be used to stop other cardiac diseases like stroke even though its efficiency may differ among individuals as this would also reduce the rate at which people die (3).
  3. It can be used to cure or inhibit any form of fever (1).

1.3 Structure

The structure of Aspirin is shown in figure 1 below.

Fig.1 Structure of Aspirin

Aspirin is made up of a Carboxylic acid group, ester group and also an aromatic group (benzene ring) as seen in the structure above.

1.4 Synthesis

Aspirin is made by reacting Salicylic acid with Acetic anhydride with the presence of a very strong catalyst. The reaction is shown in figure 2 below:

Salicylic acid Acetic anhydride Aspirin Acetic acid

Fig.2 Synthesis of Aspirin

The other product formed from the above reaction was Acetic acid.

1.5 Aims and objectives

At the end of the experiment, Aspirin would be produced, a BP assay would be performed and tests would be performed to check the purity of samples. The tests and techniques used would also be discussed and analysed.

2. Results and discussion

2.1 Synthesis

In this experiment, Aspirin synthesised from Salicylic acid, BP assay was carried out and Thin Layer Chromatography of aspirin was made on a TLC paper.

A known mass of Salicylic acid dissolved in a known volume acetic anhydride with the addition of Phosphoric acid (catalyst) Phosphoric acid acted as a catalyst to make this reaction occur at a faster rate (6). Without this catalyst, the reactions can take long before it would occur and another unique thing about the catalyst is that it does not end up being used up even after the reaction (6). The table below shows the masses of some compounds used to make the crude product as weighed through a balance.

  • Mass of salicylic acid 5.01g
  • Weight of watch glass and crude Aspirin 41.06g
  • Weight of watch glass 34.57g
  • Mass of crude Aspirin 6.49g

Table 1 Mass of Salicylic Acid, watch glass and Crude product.

2.2 Recrystallisation

Recrystallisation was done to remove extra impurities that may have been present. The new product formed was known as the purified aspirin (purified product). The purified product was also preserved to determine its melting point and to perform ferric chloride test. The table below shows the mass of the watch glass as well as the mass of the recrystallised product.

  • Weight of watch glass and Purified Aspirin 48.02g
  • Weight of watch glass 45.28g
  • Mass of purified aspirin 2.74g

Table 2. Mass of Watch glass and Purified product.

The way of measuring the mass of Aspirin in this part of the experiment is related to the first part. The mass of the empty watch glass was known to be 45.28g while the mass of the watch glass and purified watch glass was determined to be 48.02g. The difference was known to be the mass of the purified aspirin.

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2.3 Melting Point determination

The table below shows the literature melting points and the theoretical melting points of Salicylic acid and aspirin.

  • Melting Point
  • Salicylic acid 157oC
  • Salicylic acid (Literature) 157oC – 159oC
  • Crude Aspirin 121oC
  • Purified Aspirin 134oC
  • Aspirin (Literature) 135oC

Table 3. Melting Points of Salicylic Acid and Aspirin.

As shown in the table above, the melting point of Salicylic Acid provided was 157oC while the literature melting point was known to be between 157-1590C when it was looked up in books (7). The melting point of the crude Aspirin was 1210C which was by far different from its literature melting point (1350C) and this was due to the presence of impurities as impurities decrease the melting points of substances (8). For the recrystallised Aspirin, it turned liquid at 1340C which was very close to the literature melting point (7). Hence, this shows that while recrystallising the crude product, a lot of impurities were removed making the melting temperature closer to what studies show.

2.4 Ferric Chloride test

Ferric Chloride tests were performed to show the compounds are pure. This test is also used to test the presence of phenols in various compounds (9). The compounds this test was performed on were Aspirin (both the crude and recrystallised product), Salicylic Acid and Phenol. The substance used for this test was ferric chloride (FeCl3).

The figure below shows the structure of Phenol.

Fig.3 Structure of Phenol

The table below shows the results of the chemical tests performed on the substances already listed for the ferric chloride test. Colour changes were observed and written in the table below.

  • Substance Initial colour of aqueous solution Colour after adding FeCl3 solution
  • Phenol Colourless Purple
  • Salicylic Acid Colourless Dark Purple
  • Crude Aspirin Colourless Light Purple (Pink)
  • Recrystallised Aspirin Colourless Colourless

Table 4. Ferric chloride tests on compounds

Phenol which was colourless turned purple after adding ferric chloride showing the presence of a phenol group (itself). As seen in figure 2, the structure of salicylic acid contains a phenol group which explains the reason why salicylic acid turned purple while adding the solution of ferric chloride. The crude aspirin turned light purple in this test even though it does not have a phenol (aromatic alcohol) group. In this case, the colour change was due to the presence of impurities present. In the recrystallised product, colour changes were not observed as this structure does not have an aromatic alcohol group and was pure.

2.5 Percentage yield

Shown below is the percentage yield for crude and purified aspirin.

  1. Mole ratio of salicylic acid to aspirin is 1:1
  2. Mass of salicylic acid = 5.01g
  3. Molar mass of salicylic acid = 138.1g/mol
  4. No. of moles = mass/molar mass =5.01/138.1 =0.036 moles.
  5. Due to equal mole ratio, no. of moles of salicylic acid used was identical to the number of moles of aspirin which is 0.036 moles.
  6. Molar mass of Aspirin =180 g/mol
  7. 0.036 x 180= 6.53g (expected or theoretical mass of aspirin).
  8. The theoretical mass is the mass gotten given the number of moles of aspirin.
  9. Percentage yield of crude aspirin= (mass of crude aspirin/ theoretical mass) x 100% = (6.49/6.53) x 100
  10. Percentage yield of crude aspirin= 99.3%
  11. Yield of purified Aspirin= (mass of purified aspirin/ theoretical mass) x 100% = (2.74/6.53) x 100
  12. Percentage yield of Purified Aspirin =41.9%

Aspirin was lost during the recrystallisation process which was a significant error deduced as it dissolved in water resulting in the low percentage yield (41.9%). The mass of the crude product was almost equal to the theoretical mass (99.3%) as it the aspirin that dissolved in water in that process was insignificant.

2.6 BP Assay for aspirin

The table below shows the masses of the samples;

  1. Sample 1 Mass of weighing bottle + aspirin 11.20g
  • Mass of weighing bottle 10.21g
  • Mass of aspirin taken 0.99g
  • Sample 2 Mass of weighing bottle + aspirin 11.25g
  • Mass of weighing bottle 10.25g
  • Mass of aspirin taken 1.00g

Table 5. Mass of Aspirin samples

  1. Sample 1 Final burette reading 31.00cm3
  • Initial burette reading 0.00cm3
  • Titre 31.00cm3
  • Sample 2 Final burette reading 30.10cm3
  • Initial burette reading 0.00cm3
  • Titre 30.10cm3
  • Blank Titre 49.40cm

Table 6; Volume of acid used

In the titration, the burette was filled to the top (0.00cm3) as seen in the initial burette readings in table 6 above. For sample one, the volume of acid that was used before a colour change was observed was 31.00cm3. In the second flask, the volume used was 30.10cm3. Before adding the acid, the three flasks were pipetted with the base (NaOH) and an indicator called phenolphthalein was added. In the third flask (sample), aspirin was not combined with sodium hydroxide resulting in the high titre value as seen in the table above. This was called the blank.

2.7 Percentage purity of samples

  1. For sample 1,
  • Mole ratio of aspirin to HCl to NaOH = 1:1:2
  • Number of moles of aspirin= (b x a) x molarity of HCl x 0.5
  • 1000
  • b= blank titre, a = analytical titre= (49.40 – 31.00) x 0.5 x 0.5
  • 1000
  • = 4.6 x 10-3 moles.
  • Mass= molar mass x number of moles
  • Molar mass= 180.2g/mol= (4.63 x 10-3 x 180.2) g
  • Mass= 0.83g
  • Purity= (calculated mass/ mass taken) x 100 (0.83 x 0.99) x 100
  • Purity= 83.83%
  1. In sample 2,
  • n = (49.40-30.10) x 0.5 x 0.5
  • 1000
  • n = 4.83 x 10 -3 moles
  • Mass = (4.83 x 10 -3 x 180.2)
  • Mass= 0.87g
  • % purity= 0.87 x 100
  • 100
  • Percentage purity= 87 %

In the two samples of aspirin, the purity of aspirin (83% and 87%) is less than the expected purity (99.5% -101.0%) thus the samples are not of acceptable purities for use.

2.8 Thin layer chromatography

The diagram of the TLC of Pure Aspirin, Synthesised Aspirin and Aspirin is shown below.

The distance travelled by the solvent (dichloromethane mixed with acetic acid, toluene, ether and methanol) was 4.3cm. During the experiment, the spots of the samples could not be visualized by the eyes and then the spots became visible when the paper was placed in the ultra-violet light. Ferric chloride was also prayed on the paper forming a purple colour due to the presence of a phenol functional group in salicylic acid. The distances travelled by pure aspirin, salicylic acid and synthesised aspirin respectively were 2.8cm, 2.9cm and 2.5cm and their Rf values were 0.65,0.67 and 0.58. All the samples used as well as the synthesised aspirin were pure as they formed one spot on the TLC paper.


Aspirin was synthesised by reacting salicylic acid with acetic anhydride. The crude product obtained was purified by recrystallisation with a percentage yield of 41.9% and the purity of the aspirin samples were ascertained by TLC, ferric chloride tests and their melting points.

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