Aspirin Melting Point

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This experiment involved three steps: synthesis of aspirin, isolation and purification, and the estimation of purity of the final product. The synthesis involved the reaction of salicylic acid and acetic anhydride in the presence of a catalyst, phosphoric acid, H3PO4. When the aspirin was prepared, it was isolated and filtered. The percentage yield of the synthesis was calculated to be 78. 42%. The experimental melting point range of aspirin was determined to be 122 -132°C. Due to its wide range, and lower value than that of the theoretical melting point of 136°C, it was assessed that the prepared aspirin was impure.


Aspirin is a most widely sold over-the-counter drug. It has the ability to reduce fever (an antipyretic), to reduce pain (an analgesic), and to reduce swelling, soreness, and redness (an anti-inflammatory agent). One of the first recorded accounts for the discovery of aspirin appeared in England, in 1763, crediting the bark of willow trees with a beneficial effect in alleviating distress due to fevers, aches, and pains. Later, the compound salicylic acid (named for the Latin word for willow, salix) was isolated from willow bark; it proved to be the active ingredient.

By 1860, organic chemists were able to synthesize salicylic acid from basic starting materials, this furthered the therapeutic use of the substance, but there were problems. Salicylic acid proved to be irritating to the membranes of the throat, mouth, and stomach. These problems are directly associated with the high acidity of the compound, but a simple remedy was discovered, namely, replacement of the acidic phenolic hydrogen atom with an acetyl group. A useful synthesis of acetylsalicylic acid was developed in 1893, patented in 1899, marketed under the trade name of “aspirin” by the Bayer Company in Germany.

The name aspirin was invented by the chemist, Felix Hofmann, who originally synthesized acetylsalicylic acid for Bayer. More than 50 million 5-grain tablets of aspirin are consumed daily in the United States. Aspirin still has its side effects. This may result in hemorrhaging of the stomach walls even with normal dosages. The acidic irritation can be reduced through the use of buffering agents, like antacids, in the form of magnesium hydroxide, magnesium carbonate, and aluminum glycinate when mixed with aspirin (Bufferin).

While the ester can be formed from acetic acid and salicylic acid, a better preparative method uses acetic anhydrides in the reaction instead of acetic acid. An acid catalyst, like sulfuric acid or phosphoric acid, is used to speed up the process. [1]


Aspirin is used to relieve fevers, minor aches and pains. It also can be used as an anti-inflammatory medication. Low-doses of aspirin can be used over a long period of time to help prevent strokes, heart attacks and blood clots when recommended by your physician.

Too much aspirin can be bad for your body, causing ulcers and stomach bleeding. Salicylic Acid Salicylic acid has been used for thousands of years to treat fever and inflammation. Salicylic acid is a plant hormone found in willow bark or willow extract. Highly concentrated levels of salicylic acid can produce a strong vinegar smell. Aspirin or Acetylsalicylic Acid (ASA) Aspirin is made through a chemical reaction called esterification, when salicylic acid and acetyl chloride are combined. Felix Hoffman was the first to perform this experiment.

During this chemical reaction, these substances melt together and change to a solid, which is aspirin. In this process, the phenol group of salicylic acid is changed into an acetyl group. Other Salicylic Treatments Salicylic acid is a type of beta hydroxy acid that is the main ingredient of several skin care products used to nourish and hydrate the skin. When salicylic acid is combined with bismuth, it produces Pepto-Bismol, which relieves symptoms of nausea, vomiting, heartburn and gas. Salicylic acid is also used in a variety of other personal care items such as dandruff shampoo, antiseptic and toothpaste. [2]


3. 0g of salicylic acid was weighed then 3. 0mL of acetic anhydride and 6 drops of 85% H3PO4 were added to it. The mixture was warmed over a water bath for 5 minutes while stirring. After warming, 20 drops of distilled water was slowly added. 15mL of water was added then the solution was heated until it became clear. It was allowed to cool and was placed in an ice bath until the solution becomes cloudy. Using pre-weighed filter paper, the mixture was filtered and was allowed to dry in the filter paper.

In determining the melting point range of the aspirin, a capillary tube (sealed at one end) was one-third-filled with the dried aspirin. The capillary tube and a thermometer were immersed in an oil bath. The temperature at which the solid started to melt and the temperature when the entire sample was completely liquefied were recorded as the melting point temperature range.


Aspirin was synthesized by reacting salicylic acid with acetic anhydride in the presence of phosphoric acid, H3PO4, as a catalyst: The aspirin was synthesized when it was placed in a cold bath. Purification was necessary to remove any unreacted salicylic acid and acetic anhydride as well as the acetic product and phosphoric acid. Acetic acid and phosphoric acid are water soluble so they were removed by washing the aspirin with chilled water. Table 1 shows the data obtained after the synthesis of aspirin. A high percentage yield means a successful synthesis [3]. Table 1: Preparation of Aspirin Weight of Filter paper 1. 4782 g Weight of Filter paper w/ aspirin4. 5468 g Weight of Aspirin (Actual Yield)3. 0686 g Theoretical Yield of Aspirin3. 9130 g % Yield78.

42 % One way to know the purity of the aspirin was by determining its melting point. The experiment required a small amount of the sample to be firmly packed in a capillary tube. It was then immersed in an oil bath and was heated. Table 2 shows the data obtained after melting the aspirin. When a melting point is determined, it is therefore important that the melting range be recorded. The bottom of the melting range is the temperature at which the first signs of liquid can be seen.

The top of the melting range is the temperature at which the last of the solid fuses, i.e. turns into liquid. The compound is generally regarded as pure enough for most purposes if the melting range is no greater than 2oC. A wide melting range signals the need for further purification. [4] Table 2: Determination of Melting Point Experimental Melting Point122 – 132°C Theoretical Melting Point136°C The temperature range at which melting began and was completed is the melting point range. In the experiment, the melting point obtained was 122-132 °C. The theoretical melting point of aspirin is 136 °C.


The main purpose of this laboratory is to obtain aspirin through the chemical synthesis of salicylic acid by acetylation with acetic anhydride and crystallization. Crystallization is the process of arranging atoms or molecules in a liquid state into an ordered solid state. During this process, the sample that is composed of more than one substance is transformed into new samples, each of which consists of a single substance. Thus a pure sample of the compound is obtained. [5] Impurities will always lower the melting point of a sample.

In our experiment, the melting point range of the product was observed to be 122°C to 132°C. The percentage yield of the final product was calculated to be 78. 42%. It can be concluded that the aspirin that was obtained was not pure because it has a wide melting range. This signals the need for further purification. It is recommended to recrystallize the crude aspirin, though it will decrease the percentage yield, a significantly purer aspirin will be produced.


  • “Synthesis and Analysis of Aspirin”. (n. d. ).
    • Retrieved 23 April 2011: “Preparation of Aspirin from Salicylic Acid”. 1999-2011.
    • Retrieved 23 April 2011: Theoretical Yield and Useful Nomenclature. (n. d. ).
    • Retrieved March 1, 2010, from CU Boulder Organic Chemistry Undergraduate Courses : “Determination of Melting Point”. (n. d. ).
    • Retrieved 23 April 2011: “The Synthesis of Aspirin”. (n. d. ).
    • Retrieved 23 April 2011: APPENDIX Calculations: “wt. of aspirin fr. SA = 3g ? “

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