Synthesis of aspirin: a step-by-step lab protocol for students

Aspirin is one of the most familiar medicines, yet its laboratory preparation offers valuable lessons in reaction control, purification, and evidence-based reporting. This protocol covers the standard acetylation of salicylic acid with acetic anhydride, using phosphoric acid as a catalyst. Learn more about Pentingnya Kolaborasi Antarpeneliti Untuk Publikasi Bersama 234826.

Australian students should treat this as a supervised teaching experiment, not a route to homemade medicine. Aspirin is familiar in Australian pharmacies and first-aid kits, while universities in Melbourne, Sydney, Brisbane, Perth, and Adelaide commonly conduct practical chemistry with formal risk assessments, fume cupboards, and safety data sheets.

Understand the reaction

The transformation is an acetylation: the phenolic hydroxyl group on salicylic acid becomes an acetate ester, producing acetylsalicylic acid and acetic acid. Phosphoric acid activates the acetic anhydride, helping the reaction proceed at a practical rate. Salicylic acid is the starting material, while aspirin is the isolated product.

For a 5.00 g batch, 5.00 g of salicylic acid represents 0.0362 mol and gives a theoretical maximum of 6.52 g of aspirin. Acetic anhydride is used in excess to reduce the chance that moisture or incomplete mixing will stop the reaction. This theoretical mass later provides the basis for calculating percent yield.

Plan a safe laboratory setup

Acetic anhydride is corrosive, volatile, and capable of damaging the eyes, while salicylic acid and phosphoric acid present their own handling hazards. Wear splash goggles, a laboratory coat, and suitable chemical-resistant gloves. Carry out all transfers and reactions in a functioning fume cupboard, away from ignition sources, and consult the safety data sheet before handling each reagent.

Australian work-health-and-safety rules, including Victoria’s Occupational Health and Safety Act 2004 and New South Wales’s Work Health and Safety Act 2011, require institutions to identify hazards and provide suitable controls. Safe Work Australia guidance supports the use of safety data sheets, GHS-compliant labels, trained supervision, and a written risk assessment. Your instructor should approve the procedure and waste arrangements before the session.

Gather reagents and equipment

For each 5.00 g batch, measure 5.00 g of salicylic acid, 10 mL of acetic anhydride, and approximately 0.25 mL of 85% phosphoric acid. Prepare distilled water and ice for quenching and crystallisation. Required equipment includes a balance, 100 mL reaction flask, stirrer, water bath, thermometer, ice bath, Büchner funnel, filter flask, and vacuum source.

Australian university laboratories may already hold suitable stocks, while chemical suppliers such as ChemSupply and Merck/Sigma-Aldrich list salicylic acid, acetic anhydride, and laboratory-grade phosphoric acid for institutional buyers. Procurement can require an approved account, hazard information, and storage checks. Students should not substitute casually purchased solvents or acquire acetic anhydride through general consumer marketplaces.

Perform the acetylation

Place the salicylic acid in a dry reaction flask and add the acetic anhydride. Stir the mixture in the fume cupboard until the solid is evenly dispersed. Add the phosphoric acid dropwise while stirring; the catalyst should be introduced according to the institutional procedure because its addition can generate heat.

Lower the flask into a water bath maintained near 80–90°C and stir gently for 10–20 minutes. Avoid an open flame, overheating, or tightly closing the flask. A largely clear or homogeneous mixture suggests that much of the salicylic acid has reacted, but appearance alone does not establish completion or product purity.

Remove the flask from the heat and allow it to cool to room temperature. Keep the mixture in the fume cupboard until the next step because residual acetic anhydride remains hazardous.

Recover and wash the product

Slowly add about 50 mL of water to the cooled reaction mixture while stirring. Addition may be exothermic, so use a controlled pour and appropriate heat-resistant equipment. Once the mixture has cooled, place the flask in an ice bath to promote crystallisation.

Collect the crystals by vacuum filtration and wash them with a small quantity of ice-cold water to remove soluble acid and residual reactants. Prolonged contact with water can hydrolyse aspirin, so avoid excessive washing. Transfer the filter paper and damp crystals to a clean, labelled container and dry them gently or in a desiccator. Place contaminated filters and acidic liquid in the designated chemical-waste stream rather than the sink.

Check identity and purity

Pure aspirin normally has a melting point around 134–136°C. Salicylic acid melts at a higher temperature, approximately 158–161°C, so a lower or broader melting range can indicate impurities or incomplete reaction. Where available, a thin-layer chromatogram compared with authentic standards can provide additional evidence. A melting point alone should not be presented as definitive proof of identity.

This synthesis also demonstrates the value of distinguishing visual observations from analytical measurements. A brief explanation of potentiometry and electrode signals shows how electrodes can track chemical changes in solution. Potentiometry is not a direct purity test for this product, but it illustrates why instrumental data must be interpreted within the context of the reaction and its possible interferences.

Calculate the actual yield by dividing the mass of dried aspirin by 6.52 g and multiplying by 100. Record the theoretical and experimental values separately, then explain any substantial loss. Possible causes include incomplete reaction, product remaining in the filtrate, hydrolysis during washing, loss during transfer, and inadequate drying.

Document results and scientific limits

A useful laboratory report includes the reagent quantities, batch identifier, reaction temperature, heating time, filtration details, colour and appearance of the dried solid, melting range, calculated yield, and an account of deviations from the agreed method. Photographs can support those observations, but they cannot replace recorded measurements or analytical evidence.

Transparent records also matter when several people contribute to a study. An Indonesian-language discussion of [research collaboration and co-publication](https://makingthegoldengames.org/articles/pentingnya-k collaborates-antarpeneliti-untuk-publikasi-bersama-234826) provides a useful model for sharing protocols, preserving data, and agreeing on responsibilities before results are written up.

Finally, distinguish teaching material from a commercial medicine. Australian pharmacy aspirin is a therapeutic good regulated by the TGA, whereas a laboratory batch is an unregistered experimental sample that must not be supplied, tested on people, or consumed. The essential lesson is that good chemistry depends on controlled conditions, honest documentation, and evidence rather than the size of the crystals alone.