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

Aspirin Acetaminophen Caffeine
Acidity Strong Organic Acid Weak Organic Acid Organic Base

 

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 Since the medicine aspirin is acidic, the addition of aqueous NaOH allows the aspirin to be withdrawn as a water-soluble salt.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).