Sodium Salicylate The Salicylic Acid Biology Essay

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To synthesize the drug aspirin, to use a chemical test (1% iron (III) chloride) to determine the purity of preparation of aspirin and to determine the % yield of aspirin synthesized.


Almost all of us have used aspirin in our lives, yet not many of us know the background of it and it is actually being used 2400 years ago. A Greek physician Hippocrates prescribed the active extract of the willow bark and leaves can ease pains and reduced fever. In 1763, observations on the effect of willow bark on the relief of fever due to malaria were collected by Edward Stone. A Scottish physician then found that the extracts of willow bark had actually relieved the symptoms of acute rheumatism in 1830. After 10 years, the active ingredient in willow bark was identified as salicin by working willow bark with spirea.

Starting around 1850, salicylic acid was prescribed to patients with fever by physicians. Their symptoms were relieved. In 1870, it was demonstrated that salicin was converted into salicylic acid in our body. Salicin was extracted into crystalline form by Henri Leroux while Raffaele Piria had obtained salicylic acid in its pure state from willow extract. However, salicylic acid had unpleasant side-effects. It was bitter and very harsh which caused irritation on the lining of stomach, esophagus and mouth so many people cannot tolerate its use. Hence, in 1853, a French chemist, Charles Frederic Gerhardt used buffering method to neutralize the salicylic acid. Sodium salicylate and acetyl chloride were used as buffer to form acetylsalicylic acid. Still, when large doses are used to treat rheumatism, sodium salicylate often caused patient to vomit.

Sodium Salicylate Salicylic Acid (2-Hydroxybenzoic acid)

In 1897, a German chemist, Felix Hoffmann of the Bayer Company was seeking an effective treatment to cure his father's arthritic pain. Then, he began to research on acetylsalicylic acid and he found that it actually had worked well. His discovery leads to the development of Aspirin. He rediscovered Gerhardt's formula by synthesizing an ester derivative of salicylic acid which is acetylsalicylic acid. This is done by adding an acetyl group to the phenol group so the phenol group is replaced with as ester group thus it caused low membrane irritation as acetylsalicylic acid is a weaker acid than salicylic acid so it would not have the objectionable taste too. In fact, during ingestion of acetylsalicylic acid, the acetyl group masks the acidity of the drug and then it is broken down to salicylic acid in the small intestine where it can be absorbed into the bloodstream and pain relieving action started. To prepare aspirin, salicylic acid is reacted with the excess of acetic anhydride and a small amount of strong acid is used as catalyst to speed up the reaction. Reaction of salicylic acid with acetic anhydride is shown as below:


Thus, acetylsalicylic acid was used as a less irritating medicine to replace the commonly used salicylate medicines. Later, it becomes the active ingredient in aspirin, one of the top pain reliever today. The name Aspirin comes from the 'A' for acetyl, the 'spir' for spiraea which is the meadowsweet plant that yield salicin and the 'in' is a common suffix for medications.


(2-Ethanoyloxybenzenecarboxylic acid)

By 1899, The Bayer Company manufactured aspirin and marketed it worldwide. At first, aspirin was sold in powder form. In 1900, the first aspirin was introduced in water-soluble tablets and then manufactured in tablet form to treat symptoms of pain related to neuralgia, rheumatism and so on.

Today, Aspirin with the common name of acetylsalicylic acid had become the very commonly used drugs in the world. It is an effective analgesic, anti-inflammatory and antipyretic agent which is widely used as pain killer and fever reducer. It is also proved that aspirin is very effective in combating headaches and muscle pains which frequently resulted from long periods of immobility. Furthermore, aspirin is effective in reducing the incidence of heart diseases and severity of heart attacks. Thus, people with heart conditions usually put on a daily aspirin regiment. People who believe may have heart attack also recommended to take aspirin immediately to reduce damage. Nowadays, the use of aspirin had expanded beyond pain relief to that of a potential lifesaver.


Preparation of Aspirin

A bath using a 400-mL beaker filled about half way with water is prepared. A few boiling stones is added into the breaker. The water is heated to warm.

2.00g of salicylic acid (weighed accurately) is placed in a 125-mL dry conical flask. The theoretical or expected yield of aspirin is calculated with this quantity of salicylic acid.

3mL of acetic anhydride is added carefully to the flask while swirling, follow by 3 drops of concentrated sulphuric acid is added. (Perform this inside a fumehood)

The flask is swirled gently to mix the solution. The flask is placed in a beaker of warm water for 15 minutes and a plastic stopper is used to cover the flask.

The reagents are mixed and the flask is placed in the boiling-water bath; heated for 30 minutes. The solid is dissolved completely. The solution is swirled occasionally (the flask can be rotated alternately for mixing purposes.)

The conical flask is removed from the water bath and it is cooled approximately to room temperature.

The solution is poured slowly into a 150-mL beaker containing 20mL of ice water, thoroughly mixed, and the beaker is placed in an ice bath.

A glass rod is used to mix the solution while in the water bath, the glass rod was rubbing vigorously (called scratching) along the bottom of the beaker (be careful not to poke a hole through the beaker).

The crystal is collected by filtering under suction with a Buchner funnel.

The crystals are washed with two 5-mL portions of cold water, followed by one 10-mL portion of cold ethyl acetate.

The suction is continued through the crystals for at least 5 minutes, to help in drying them. The rubber tubing is disconnected from the filter flask before turning off the water aspirator.

A re-crystallization process is performed to purify the product. The crystals are transferred by a spatula to a 250mL beaker. 5mL of ethanol is added; the beaker inside a water bath is stirred and warmed to dissolve the crystals.

After the crystals have dissolved, 15mL of warm water is added to the alcohol solution and is covered. Crystals are reformed as the solution cooled. The crystallization is started, the beaker is set in an ice bath to complete the re-crystallization.

The contents of the beaker are poured into a Buchner funnel and suction filtration is applied.

If the crystals are still wet. The crystals are placed between several sheets of filter paper and the solid is pressed to dry.

A petri dish is weighed. The crystals are added and reweighed. The weight of the purified aspirin is calculated. The percent yield is determined (the calculation is included in the result section of the report).

Analysis of Aspirin

The quality of the synthesized aspirin with commercial aspirin and salicylic acid are being compared.

One drop of 1% iron III chloride is added to separate test tubes containing a few crystals of each substance. The colour is observed: Pure aspirin would show no colour, while salicylic acid or trace of it in impure aspirin will show a purple colour. The intensity of the colour qualitatively tells how much salicylic acid is present.

The aspirin crystals are examined under a microscope. The morphology of the crystals is recorded.

The purity of sample is tested by determining its melting point range. A mortar and pestle are used to reduce the sample to a fine powder. A capillary melting point tube with a 1-2 depth of fine crystals is loaded. The melting point of pure Aspirin varies between 130-135 °C (because of partial decomposition). The temperature at which the Aspirin begins to melt and when the sample has completely melted is noted.


Theoretical yield of aspirin:

Table below shows the data used to calculate the theoretical yield of aspirin.


Mass of salicylic acid used (g)



Relative molecular mass of salicylic acid (g/mol)



Moles of salicylic acid used (mol)



Moles of aspirin formed (mol)



Relative molecular mass of aspirin (g/mol)



Expected mass of aspirin yield (g)


So, theoretical yield of aspirin = 2.61000 g

Experimental yield of aspirin:

The table below displays the results obtained after the re-crystallisation process.


Mass of empty Petri dish (g)



Mass of Petri dish + Crystals (g)



Mass of aspirin crystals (g)


Thus, experimental yield of aspirin =

Comparison on melting point of aspirin:


Melting point (°C)

Commercial aspirin


Prepared aspirin


Iron (III) chloride test



Salicylic Acid

Deep purple

Commercial aqueous

Lighter purple

Prepared Aspirin

Slightly yellowish

Morphology of Prepared Aspirin:

Total magnification= 40x


During the experiment, 3 drops of concentrated sulfuric acid is added to aid the reaction because the reaction in pure acetic anhydride is slow. Concentrated sulfuric acid is a strong acid which acts as catalyst to speed up the rate of reaction. According to le Chatelier's principle, acetic anhydride which is in excess forces the equilibrium towards aspirin and heat is required to reach the equilibrium faster. Since strong acid has many available protons, the oxygen of the carbonyl of acetic anhydride is protonated and the electrophilicity of the carbon of the carbonyl increases. In salicylic acid, the oxygen of phenol acts as nucleophile which attack the carbon of the carbonyl within acetic anhydride. This silmultaneously breaks the pi bond to oxygen. A tetrahedral intermediate is formed. Here occurs a series of protons transfer. The electrons on the oxygen of the hydroxyl group on the tetrahedral intermediate, ultimately come down and reform a carbonyl group, this simultaneously induces an acetic acid to leave. After one additional proton transfer, the final aspirin is formed. If the sulfuric acid were omitted, H+ cation are absent and thus acetic anhydride will not split and acetic acid is formed. Yet, phosphoric acid can be used in place of sulpfuric acid but may give higher yields.

Based on the calculation that can be seen in the result section, salicylic acid is the limiting reactant while acetic anhydride is in excess. The yield percentage obtained is extremely low, it shows that there is some errors occurred during the experiment. Low experimental yield also can be influenced by when salicylic acid is not fully reacted. Since the esterification is a reversible reaction, there may be possible that some acetylsalicylic acid convert back to salicylic acid, acetic anhydride and acetic acid.

Unreacted acetic anhydride was destroyed by the addition of the addition of ice water to the mixture. Reaction between water and acetic anhydride forming 2 molecules of acetic acid as below:

Ice water is used during the work up to allow complete crystallization. Ice water is used to cool the surrounding of the flask and this allows the crystal to form to reduce the solubility of the product in the water further and increase the percent yield. Also, ice water is used instead of room temperature water because aspirin has low solubility in cold water, yet acetic anhydride decomposes to water soluble acetic acid at any temperature water. Hence, impurities like acetic acid and sulfuric acid which are soluble in water are washed away by the cold water which then leaves the undissolved aspirin free of any excess acid.

In the synthesized of acetylsalicylic acid, it had mixed with the impurities like acetic acid and other side products. Thus, acetylsalicylic acid collected need to be purified by recrystallization. Acetylsalicylic acid collected is in solid form and it is dissolved in warm ethanol before water is added and left to cool. It is now recrystallized which left the solid impurities remain dissolved in the solution. Vacuum filtration will then be used to collect the solid aspirin. Thus, a pure aspirin without impurities is obtained by recrystallization. Besides, a good solvent has to be used to perform successful recrystallization. Water is chosen to be the solvent in the process of recrystallisation as it is cheap and easily available.

The purity of the synthesized acetylsalicylic acid is quite pure because:

When iron (III) chloride is added to the synthesized acetylsalicylic acid, it produces a pale yellow colour solution. It indicates the absence of phenol group in the synthesized aspirin. This in turns shows no remaining of any composition of unreacted salicylic acid in the synthesised aspirin as a purple colour solution will be produced if an impure aspirin is tested.

Pure substance has its own unique melting point. In this experiment, the melting point of synthesized aspirin varies between 132-136 which is very near to the melting point of the pure aspirin that is varies between 130-135.

A pure aspirin is a white and fine crystal. In this experiment, a white irregular shape of crystal is obtained. The irregularly shape of the crystal might due to the disruption of impurities.


Acetylsalicylic acid can be synthesised via esterification reaction between acetic anhydride and salicylic acid with the required of strong acid as catalyst to aid the reaction. The pure aspirin was obtained by recrystallization and its purity was tested with Iron(III) Chloride solution. The experimental yield was much lower than the theoretical yield due to the types of strong acid used or the occurred of some errors. The final product was found to be quite pure compared to that of the commercial aspirin by its characteristic shape and color and its unique melting point which falls nearly to the range.