Making Aspirin Safely in a Teaching Laboratory
Aspirin, or acetylsalicylic acid, is a useful teaching-lab product because its preparation demonstrates esterification, purification, yield calculations and melting-point analysis in one compact experiment. Students convert salicylic acid into a familiar pharmaceutical compound while learning why laboratory products require testing before they can be considered pure.
The reaction uses salicylic acid, acetic anhydride and a small amount of acid catalyst. Acetic anhydride supplies the acetyl group that reacts with the phenolic hydroxyl group of salicylic acid. The main product is aspirin, while ethanoic acid is formed as a by-product.
This activity suits senior secondary and undergraduate chemistry classes in Australia, including laboratories in Sydney, Melbourne, Brisbane and regional campuses. It must be conducted under the institution’s Work Health and Safety procedures, with a current safety data sheet (SDS), suitable supervision and access to a functioning fume cupboard.
The product is for analysis only. A student-prepared sample must never be swallowed or treated as medicine, even if it resembles tablets sold in Australian pharmacies. The experiment demonstrates chemical preparation, not pharmaceutical manufacture.
The reaction and its purpose
The overall reaction can be represented as:
salicylic acid + ethanoic anhydride → acetylsalicylic acid + ethanoic acid
A typical microscale or small-scale class procedure begins with about 2.0 g of salicylic acid in a dry flask. Students add approximately 5 mL of acetic anhydride followed by a few drops of concentrated phosphoric acid as catalyst. Exact quantities should follow the laboratory’s approved method because glassware size, heating equipment and class experience affect the safe scale.
The mixture is warmed in a water bath, commonly around 70–80 °C, rather than with a naked flame. Gentle swirling helps the solid dissolve and allows the reaction mixture to become uniform. The catalyst accelerates the reaction but is not consumed overall, so it should be handled carefully and added only with the equipment specified by the supervisor.
Students can find broader chemistry resources and community-focused material through NepaChem’s chemistry resources, especially when connecting a practical exercise with analytical chemistry or the work of chemists from Nepal and Australia.
Equipment, chemicals and preparation
Essential equipment includes a balance, weighing boat, dry conical flask, measuring cylinder or pipette, water bath, glass rod, ice bath, Büchner funnel, filter paper, side-arm flask and vacuum source. A melting-point apparatus or digital melting-point instrument is valuable for assessing the dried product.
Salicylic acid is an irritant, acetic anhydride is corrosive and lachrymatory, and concentrated phosphoric acid can cause serious chemical burns. Students should wear a buttoned laboratory coat, splash-resistant safety glasses and suitable enclosed footwear. Gloves may be required by the local risk assessment, but they do not replace careful technique or fume-cupboard use.
Before starting, check that the flask is dry, label all containers and confirm where acidic liquid waste and contaminated solids will go. In Australia, the correct disposal route is determined by the school, university or research facility’s chemical-waste system; substances should not be poured into a sink simply because the volumes are small.
Crystallisation and isolation
After the heated reaction mixture has cooled slightly, a measured amount of water is added cautiously. This decomposes excess acetic anhydride and reduces the solubility of aspirin, allowing crystals to form. The mixture is then cooled in an ice bath to maximise crystallisation.
The solid is collected by vacuum filtration and rinsed with a small quantity of cold water. Excessive washing can dissolve some aspirin and lower the recovered mass. The crystals should be spread on a labelled watch glass or filter paper and dried thoroughly before weighing.
A useful teaching discussion compares crude and recrystallised material. Recrystallisation from a suitable solvent can improve purity, but every extra transfer may reduce percentage yield. Students should record observations such as colour, crystal shape, temperature changes and the appearance of the filtrate rather than relying on mass alone.
| Assessment method | What it indicates | Typical interpretation |
|---|---|---|
| Percentage yield | How much isolated product was recovered | Low yield may reflect incomplete reaction, transfer loss or over-washing |
| Melting point | Purity and identity evidence | A sharp value near the accepted range supports a purer sample |
| Ferric chloride test | Presence of phenolic compounds | A strong colour can suggest unreacted salicylic acid |
| Infrared spectrum | Functional groups in the product | Ester and carboxylic-acid absorptions support aspirin formation |
Calculating yield and checking purity
The limiting reagent is usually salicylic acid when acetic anhydride is added in excess. Students first calculate the moles of salicylic acid, use the 1:1 reaction ratio to determine theoretical moles of aspirin, and convert that amount to theoretical grams. Percentage yield is then calculated as:
percentage yield = actual mass ÷ theoretical mass × 100
A lower-than-expected yield does not automatically mean the reaction failed. Product can remain dissolved in the mother liquor, adhere to glassware or be lost during filtration. Incomplete acetylation, insufficient drying and inaccurate weighing can also affect the result.
Aspirin generally melts at about 135–136 °C, although the accepted range used by a particular laboratory should be checked. A broad or depressed melting range often indicates impurities. The ferric chloride test can provide a contrasting result: unreacted salicylic acid may produce a violet colour because it retains a phenolic group, whereas pure aspirin should respond much less strongly under the same conditions.
Managing hazards and waste
The water bath should be stable, and the reaction flask must not be sealed while heating. Acetic anhydride vapour and ethanoic acid fumes require effective ventilation. If a strong odour, spill or unexpected reaction occurs, students should step back and notify the demonstrator rather than attempting an improvised clean-up.
First-aid arrangements should be known before chemicals are dispensed. Skin or eye exposure requires immediate flushing with water and prompt attention under the local emergency procedure. Broken contaminated glassware belongs in the designated sharps or broken-glass container, while liquid residues should be collected according to the facility’s hazardous-waste instructions.
Australian laboratories may have different arrangements between a Year 12 classroom, a university teaching lab and an industrial facility. A class in Melbourne might use a central waste contractor, while a regional campus may have scheduled collection days. The responsible supervisor must confirm the applicable procedure before the practical begins.
Extending the experiment through analysis
The experiment can be adapted for analytical chemistry by asking students to compare crude crystals with recrystallised aspirin. They can calculate recovery, record melting-point ranges and explain how each operation changes purity and yield. Infrared spectroscopy, if available, provides evidence for the ester carbonyl formed during acetylation.
A useful extension is a quality-control comparison with a commercial Australian aspirin product, without treating the classroom sample as a medicine. Students can examine labelled active ingredients, tablet mass and dissolution behaviour while discussing why pharmaceutical products require validated processes, controlled excipients and regulatory oversight.
The activity also opens a wider discussion about how chemistry connects communities. Australian students can compare laboratory standards used in Perth, Adelaide or Canberra with those in Nepalese universities, while considering the importance of clear records, safe disposal and reproducible measurements across different facilities.
The central lesson is that synthesising aspirin involves more than producing white crystals: a reliable result depends on controlled reaction conditions, safe handling, careful isolation and evidence from purity tests.