How to determine the purity of aspirin using titration

Aspirin, chemically known as acetylsalicylic acid, contains a carboxylic acid group that reacts quantitatively with hydroxide ions. This makes a standardised acid–base titration a practical way to estimate the amount of intact aspirin in a weighed sample without relying on the purity printed on a packet.

The method works best when the sample is pure aspirin or its additives are known. It is suitable for teaching chemistry students and community researchers in Australia, provided the endpoint is reached promptly, calculations include all dilution factors, and an appropriate blank is used.

How the titration works

Aspirin behaves as a monoprotic acid during a rapid direct titration: one mole of hydroxide neutralises one mole of acetylsalicylic acid. Its molar mass is 180.16 g/mol, allowing the mass of aspirin to be calculated from the amount of sodium hydroxide consumed.

The titration depends on the original carboxyl group remaining intact. Prolonged exposure to strong alkali can hydrolyse the ester group, causing extra hydroxide to be consumed and making the apparent aspirin content too high.

Reagents and equipment

Use a standardised sodium hydroxide solution, preferably about 0.0500 mol/L, phenolphthalein indicator, and a suitable aspirin solvent such as a one-to-one mixture of ethanol and water. Equipment includes an analytical balance, calibrated pipettes, a 50 mL burette, beakers, a funnel and filter paper if required, plus a white tile or pH meter for endpoint detection.

Australian laboratories commonly report volumes in mL and temperature in °C. Volumetric glassware is usually calibrated at 20 °C, so letting warm equipment reach room temperature is important, particularly during summer in Melbourne, Brisbane or central Australia.

Preparing the sample

Accurately weigh approximately 0.10–0.25 g of finely ground, homogeneous aspirin. Record the actual mass rather than using a nominal amount, and dissolve it in about 25 mL of the solvent with gentle swirling. The chosen quantity should require roughly 10–30 mL of base during titration.

Transfer the solution quantitatively to the titration vessel. If insoluble excipients remain, filter them and rinse the filter with additional solvent, checking that no aspirin is retained. Effervescent products and coated tablets require special care because bicarbonate, coating material and other additives may dissolve slowly or interfere with the result.

Performing the titration

Fill the burette with standardised sodium hydroxide, remove air bubbles from the tip, and record the initial reading. Add several drops of phenolphthalein, then titrate while stirring. As the endpoint approaches, add the base dropwise and stop at the first pale pink colour that persists for about 30 seconds.

Carry out the titration rapidly and avoid excessive agitation after the endpoint appears. A solvent blank containing the same proportions of ethanol, water and indicator can establish the correction volume. Repeat the determination with at least three independently prepared portions and calculate both the mean titre and its spread.

Calculating the purity

The amount of hydroxide delivered equals the amount of aspirin neutralised, so the aspirin mass is calculated as (m=C \times V \times 180.16). Here, (C) is the sodium hydroxide concentration in mol/L, (V) is the corrected titre in litres, and the resulting aspirin mass is in grams.

For example, a 0.1342 g sample requiring 14.86 mL of 0.05000 mol/L sodium hydroxide contains approximately 0.1339 g of aspirin. Its mass purity is therefore (0.1339/0.1342 \times 100 = 99.7%), subject to experimental uncertainty.

If only an aliquot was titrated, multiply the calculated mass by the ratio of total solution volume to aliquot volume. For tablets, divide the total aspirin mass by the number tested, convert to milligrams per tablet, and compare that value separately with the label strength to obtain the percentage of the declared dose.

Recognising interference and error

Effervescent aspirin often contains sodium bicarbonate, which consumes sodium hydroxide and produces a falsely high result. Salicylic acid and other acidic degradation products also consume base, while prolonged alkaline contact can hydrolyse aspirin and require a second hydroxide equivalent. These effects cannot be corrected reliably without knowing the formulation.

Incomplete dissolution, transfer losses, bubbles in the burette and an indistinct endpoint generally lower the measured titre. Carbon dioxide absorbed by the base can increase consumption. Running a blank, using freshly prepared reagents and checking the standardisation factor helps control several of these problems.

A titration result alone also does not prove that the sample is aspirin. A laboratory using this procedure for an unknown material should interpret the percentage as an apparent assay and confirm identity or detect degradation with a suitable method such as HPLC, UV spectroscopy or infrared analysis.

Reporting results in an Australian context

Report the mean result, individual replicate titres, standard deviation and significant figures justified by the balance and volumetric uncertainty. Avoid reporting more decimal places than the method supports, and state any blanks, extraction losses or coating treatment because these can materially affect the comparison.

Australian pharmacies commonly stock acetylsalicylic acid in forms such as 100 mg soluble tablets and 300 mg coated products, with branded and generic manufacturers using different excipients. Packs from Sydney, Melbourne, Brisbane or Adelaide should therefore be treated as separate formulation samples. Heat and moisture during Australian summers can accelerate aspirin degradation, so the product’s storage instructions must also be followed.

Sodium hydroxide is corrosive, and the procedure requires eye protection, suitable gloves and a laboratory coat. Follow the safety data sheet and Safe Work Australia risk controls, and dispose of alkaline aqueous waste through the institution’s approved chemical-waste system rather than the sink. The practical takeaway is to standardise the base, avoid prolonged alkaline exposure, correct any blank and base the reported purity on replicated calculations rather than the label claim.