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Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA
Extraction and filtration can separate compounds based on their solubility properties. In this lab, you'll separate a mixture of cellulose, caffeine, and benzoic acid based on their solubilities in dichloromethane, or DCM, and water.
Caffeine and benzoic acid are both soluble in DCM, but cellulose is not. Thus, in this part, you'll first dissolve as much of the mixture as possible in DCM and filter out insoluble cellulose. You'll continue working with the solution of benzoic acid and caffeine in the next section, and you'll measure the mass of the recovered cellulose once it dries.
|Compound||Empty container (g)||Sample + container (g)||Compound recovered (g)||Recovered mass
of mixture (g)
of mixture (g)
In this part of the experiment, you will separate benzoic acid and caffeine using DCM and water for solvents. This makes a good solvent pair for liquid-liquid extraction because DCM and water are immiscible and have different densities. However, caffeine and benzoic acid are both much less soluble in water than in DCM.
To separate these compounds, you'll mix the benzoic acid and caffeine solution with sodium hydroxide to convert benzoic acid to sodium benzoate, which is highly water-soluble and virtually insoluble in DCM. The caffeine will be mostly unaffected. When the mixture settles into layers, all sodium benzoate will be in the aqueous layer because it's insoluble in DCM.
A small amount of caffeine will be in the aqueous layer, but most of it will stay in DCM. You'll keep the aqueous sodium benzoate layer for the last part of the lab, where you will evaporate the solvent from the organic layer to recover solid caffeine.
Sodium hydroxide is corrosive, so be careful while handling and transporting it. Strong bases can cause ground-glass surfaces like the separatory funnel joint and stopper to fuse together, so always add sodium hydroxide through a long stem glass funnel to keep it from contacting the ground glass.
In the last part of the lab, you'll use hydrochloric acid to reprotonate the benzoate anion. Hydrochloric acid is toxic and corrosive, so be careful with it. Benzoic acid is minimally soluble in water, so most of it will precipitate from solution, allowing you to collect it by filtration.
of mixture (g)
|Error||Source of Error|
|Cellulose mass is very high and the other compound masses are low||Caffeine and benzoic acid might not have dissolved completely at the start of the lab and instead remained on the filter paper labeled for cellulose.|
|Benzoic acid mass is low compared to the recovered caffeine||Benzoic acid was not completely converted to sodium benzoate, leaving a mix of caffeine and benzoic acid in DCM.|
|Caffeine recovered mass is low||Loss due to the aqueous washes, particularly if the aqueous base was warm|
|Low benzoic acid mass recovered||Incomplete protonation of sodium benzoate, solution too warm, or precipitation disturbed|