Acid-Base extraction of an Analgesic Tablet

The purpose of this experiment was to practice technique and acid-base extraction by separating and purifying the components of a common analgesic tablet.

Analysis

The purpose of this experiment was to separate and purify the components of an analgesic tablet. Furthermore, the separations allowed for practice of acid-base extractions. This experiment tested laboratory technique, considering that the pill used, Excedrin, contains only milligrams of the drugs to be separated: acetaminophen, caffeine, and aspirin (see Table A for acid/base comparison by structure).

The separation began with the dissolution of caffeine and aspirin in dichloromethane. The binder, typically made of cellulose or silica gel, as well as acetaminophen, are insoluble in DCM, so a vacuum filtration was performed. The vacuum filtration was unsuccessful in separating the solution from the solids and in that attempt, a large fraction of the components were lost. The solution dried up in the Buchner filter paper, and the solid was scattered throughout the paper and the funnel. In order to recover the components, the filter paper was rigorously scraped and then rinsed with a small amount of dichloromethane.

The procedure was repeated with the recovered components and the vacuum filtration was successful. Now there were two collections, a solid mixture with acetaminophen and the binder, and a solution of DCM with caffeine and aspirin. Acetaminophen was easily separated from the binder by dissolving it in ethanol and using gravity filtration to remove the binder. Unfortunately, most of the solid was washed down the drain in a laboratory accident prior to its dissolution in ethanol. The ethanol was later evaporated to obtain crude acetaminophen.

Table A: Structure & Acidity of Excedrin Components

The next part of the separation, using the dichloromethane solution, employs the acid/base extraction. Since the medicine aspirin is acidic, the addition of aqueous NaOH allows the aspirin to be withdrawn as a water-soluble salt. Caffeine, already in the free-base form remains dissolved in DCM and is recovered by evaporation of DCM. The aspirin dissolved in the aqueous solution is precipitated by acidification (with HCl).

Editor’s note (October 27, 2026): The student’s text and Table A are left as written. Three points need clarifying.

• Aspirin is a “strong” acid only in a relative sense. Table A calls it a “Strong Organic Acid.” In extraction labs, “strong” and “weak” often just compare a carboxylic acid with a weaker acid such as a phenol (LibreTexts). In that sense aspirin (pKa about 3.5) is much stronger than acetaminophen (pKa about 9.4). By general-chemistry standards, though, both are weak acids that ionize only partly in water, unlike HCl. Caffeine is a very weak base (its conjugate acid has a pKa near or below 1; PubChem lists 0.7), so it stays neutral here. PubChem pKa values: aspirin, acetaminophen, caffeine.
• No results are reported. The post gives no masses, percent recoveries or melting points for any component. It describes losses instead: a failed first filtration, and most of the solid acetaminophen–binder mixture washed down the drain in a lab accident.
• Dichloromethane (DCM) safety. DCM is volatile, exposure can occur by inhalation, skin absorption, ingestion, and skin or eye contact, and NIOSH lists it as a potential occupational carcinogen (NIOSH Pocket Guide). Standard handling: work in a fume hood, wear splash-proof goggles and gloves (DCM passes through common glove materials within minutes, so remove gloves promptly if splashed), avoid breathing the vapor, avoid skin contact, and wash promptly if skin is contaminated. IARC classes DCM as Group 2A (probably carcinogenic to humans), and exposure could cause carbon monoxide poisoning (PubChem).

How the extraction works

Acid-base extraction moves a compound between an organic layer and water by changing whether it is charged. NaOH removes the acidic proton from aspirin’s carboxylic acid group, giving a sodium salt that is ionic and dissolves in the water layer, while neutral caffeine stays in the DCM. Adding HCl puts the proton back, and the neutral aspirin, much less soluble in water than its salt, can precipitate (LibreTexts). Aspirin is least stable in water above about pH 8, where its ester group can hydrolyze to salicylate and acetate, so contact with NaOH can convert some of it to salicylic acid (PubChem). Acetaminophen, a phenol (pKa about 9.4), would also be deprotonated by aqueous NaOH, but in the post’s procedure most of it has already been filtered out: it is largely insoluble in DCM, so the vacuum filtration leaves it behind with the binder. For another worked extraction with NaOH and then HCl, see Extraction of Benzoic Acid; for caffeine pulled into DCM, see Isolation of Caffeine from Tea Leaves.