Liquid-liquid extraction is a separation technique that moves a dissolved substance from one liquid layer into another. It works because some liquids do not mix, such as water and many organic solvents, so they form separate layers in a separating funnel. This method matters because chemists use it to isolate products, remove impurities, and purify reaction mixtures.
It is common in organic chemistry, environmental testing, and pharmaceutical analysis.
The key idea is that a solute distributes itself between two immiscible liquids according to its relative solubility in each layer. This distribution is described by the partition coefficient, K = concentration in organic layer / concentration in aqueous layer, for a specified solute and solvent pair. Shaking increases contact between the layers, while venting releases pressure from volatile solvents or gas-producing mixtures.
After the layers settle, the stopcock lets the lower layer drain out cleanly, leaving the upper layer behind.
Key Facts
- Liquid-liquid extraction separates a solute by using two immiscible liquids, usually an aqueous layer and an organic layer.
- Partition coefficient: K = Corganic / Caqueous for a solute at equilibrium.
- If K is large, more solute dissolves in the organic layer than in the aqueous layer.
- Layer position depends on density: the denser liquid forms the bottom layer.
- Multiple small extractions usually remove more solute than one large extraction using the same total solvent volume.
- Percent extracted = amount extracted / initial amount × 100 percent.
Vocabulary
- Liquid-liquid extraction
- A separation method in which a solute transfers between two immiscible liquid layers based on its solubility in each.
- Immiscible
- Describes two liquids that do not mix evenly and instead form separate layers.
- Separating funnel
- A glass funnel with a stopper and stopcock used to mix, settle, and drain immiscible liquid layers.
- Partition coefficient
- The ratio of a solute concentration in one liquid phase to its concentration in the other liquid phase at equilibrium.
- Washing
- A purification step in which one layer is contacted with a second liquid to remove unwanted impurities.
Common Mistakes to Avoid
- Assuming the organic layer is always on top, which is wrong because layer position depends on density, not whether the liquid is organic or aqueous.
- Forgetting to vent the separating funnel, which is unsafe because pressure can build up during shaking and force liquid or vapor out suddenly.
- Draining the wrong layer into the wrong container, which can lose the product because the desired compound may be in either the top or bottom layer.
- Shaking so hard that an emulsion forms, which is wrong because tiny droplets can prevent clean layer separation and make the extraction slower.
Practice Questions
- 1 A solute has K = Corganic / Caqueous = 4.0. At equilibrium, its concentration in the aqueous layer is 0.20 M. What is its concentration in the organic layer?
- 2 A mixture contains 10.0 g of a compound. After extraction, 8.5 g is found in the organic layer. What percent of the compound was extracted?
- 3 You extract an aqueous solution with dichloromethane, which has a density greater than water. Explain which layer is likely to be on the bottom and why this matters before draining the separating funnel.