This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.
Solvent extraction, also called liquid-liquid extraction or partitioning, is a procedure used to separate compounds based on their solubility in two immiscible liquids, usually water and an organic solvent. During solvent extraction, one or more of the solutes in one of the liquid phases migrates to the other liquid phase. The two liquid phases are then physically separated and the desired product is isolated from the phase that contains it.
Solvent extraction is one of the most commonly used laboratory purification methods, particularly in organic chemistry labs. Solvent extractions done in chemistry labs are usually small-scale, batch-mode operations using a separatory funnel. Solvent extraction is also widely used in industrial operations. Some industrial applications use batch-mode extraction, albeit usually on a much larger scale than laboratory solvent extractions. Other industrial applications use continuous-mode solvent extractions, often on a gigantic scale, where the two solvents are continuously added to and removed from a large reaction vessel.
In some solvent extractions, the desired product migrates from the original liquid phase to the second liquid phase. In other solvent extractions, impurities migrate from the original liquid phase to the second liquid phase, leaving the product in the original liquid phase. The phase that contains the desired product is called the product layer. The phase that contains impurities, excess reactants, and other undesirable compounds is called the waste layer. Many first-year organic chemistry students come to grief by discarding what they think is the waste layer, only to learn later that they actually discarded the product layer.
Paul Jones comments
Maybe you’re relying on them learning the hard way, which to be honest really is the only way to learn this lesson, but I like to explicitly tell students not to throw anything away until they have gotten the product they want/expect. But, yes, no one listens. Alas, the life of a teacher. They think you’re an idiot as long as you’re in their presence and only after they’re miles and years away do they realize you weren’t a boob.
In this laboratory, we’ll use solvent extraction to isolate the iodine present in Lugol’s solution, an aqueous solution of iodine and potassium iodide.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- test tube with solid stopper
- test tube rack
- disposable pipette
- watch glass
- Lugol’s solution (1 mL)
- lighter fluid (~2 mL)
- tap water
All of the specialty lab equipment and chemicals needed for this and other
lab sessions are available individually from Maker Shed or other laboratory
supplies vendors. Maker Shed also offers customized laboratory kits at special
prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the
Supplemental Chemicals Kit.
Iodine is toxic and irritating, and stains skins and clothing. Stains can be removed with a dilute solution of sodium thiosulfate or vitamin C (one tablet dissolved in about 10 mL of water). Lighter fluid is flammable. Wear splash goggles, gloves, and protective clothing.
Substitutions and Modifications
- You may substitute a Petri dish, small beaker, saucer, picnic dessert plate, or similar container for the watch glass.
- You may substitute tincture of iodine for the Lugol’s solution.
- You may substitute gasoline, paint thinner, or any other organic solvent that is not miscible with water for the lighter fluid.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Fill a test tube about halfway with water.
- Add about 1 mL (~ 20 drops) of Lugol’s solution to the test tube. Stopper the tube and agitate it until the contents are thoroughly mixed. Record anything unusual about the appearance of the aqueous solution on line A of Table 6-4.
- Use the disposable pipette to carefully add about 1 mL of lighter fluid to the test tube by allowing it to run down the inner surface of the test tube, as shown in Figure 6-5. (If you substitute another solvent for the lighter fluid, make sure it forms a distinct layer on top of the aqueous solution of iodine. If it does not, you’ll have to start again, using a solvent that does not mix with water.) Record the appearance of the organic solvent layer on line B of Table 6-4. (If you want to re-use the pipette, rinse it two or three times with 1 or 2 mL of acetone followed by two or three water rinses; many organic solvents will damage the plastic pipette if they are left in contact with it for any length of time.)
- Stopper the test tube and agitate it vigorously for fifteen seconds. Record the appearance of the mixed layers immediately after agitation on line C of Table 6-4.
- Place the test tube in the rack and allow it to settle for 30 seconds or so. Record the appearance of the aqueous and organic layers on lines D and E of Table 6-4.
- After the two layers have separated completely, use the disposable pipette to draw off the top (organic) layer as completely as possible. Transfer the organic layer to the watch glass and set it aside to allow the solvent to evaporate. (Again, if you want to re-use the pipette later, rinse it thoroughly with acetone and then water. The iodine solution stains anything it comes into contact with, including the plastic of the pipette. This stain is of no consequence, but it does look ugly. If you want to remove the stain, rinse the pipette with a solution of sodium thiosulfate or vitamin C to oxidize the iodine to readily-soluble iodide ions.)
- Add about 1 mL of lighter fluid to the aqueous solution. Stopper the tube, agitate it, and wait for the layers to separate. Record the appearance of the aqueous and organic layers on line F and G of Table 6-4.
Figure 6-5. The aqueous iodine solution with an upper organic layer (right) before extraction.
Table 6-4. Solvent extraction of iodine – observed data
|A. aqueous iodine solution|
|B. organic layer (before agitation)|
|C. mixed layers (after agitation)|
|D. aqueous layer (after settling)|
|E. organic layer (after settling)|
|F. aqueous layer (after second extraction)|
|G. organic layer (after second extraction)|
The aqueous waste can be flushed down the drain with plenty of water. The few milligrams of solid iodine can be flushed down the drain or disposed of with solid household waste.
If you have time and the required materials, consider performing these optional activities:
- Repeat the experiment, beginning with two test tubes, each of which contains 10 mL of water and 1 mL of Lugol’s solution. Add 1 mL of lighter fluid to the first test tube and 5 mL of lighter fluid to the second. Agitate the tubes. After settling compare the appearances of the aqueous layers to determine if using additional lighter fluid causes more of the iodine to be extracted from the aqueous layer.
- Repeat the experiment, substituting three or four different food colorings for the iodine (one color per tube).
Q1: The colors of the two layers after agitating and settling make it clear that most of the iodine was extracted from the aqueous layer into the organic layer, but that does not explain the color change from orange in the aqueous layer to magenta in the organic layer. Use the Internet to research the cause for this color change.
Q2: You are running a solvent extraction to isolate the products of a synthesis, using an aqueous layer and an organic layer. After agitation and settling, the two layers are of very similar appearance. You know that the organic layer contains your product and the aqueous layer is the waste layer, but you are not 100% certain which layer is which. What simple test might you perform to determine which is the organic layer?
August 24, 2009