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Organic Chemistry
 

Solid-Liquid Extraction

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Extraction is a crucial separation technique in organic chemistry, used to separate components of a mixture based on their solubilities in two different phases that do not mix.

Extractions are performed between two phases. In the case of a liquid-liquid extraction, the dissolved solute is transferred from one liquid phase to another. Extractions are also performed with a liquid and solid phase, called solid-liquid extraction, where the solute is transferred from a solid phase to a liquid phase. A simple example of solid-liquid extraction is coffee brewing, which involves the mixing of solid coffee grounds with water. The coffee flavor compounds are extracted from the grounds into the water to form coffee. This video will illustrate the principles of extraction, and demonstrate solid-liquid extraction in the lab through the removal of organochloride residues from soil.

Extraction uses the property of solubility to transfer a solute from one phase to another. In order to perform an extraction, the solute must have a higher solubility in the second phase than in the original. In general, very nonpolar solutes will partition into an organic phase, while very polar solutes will partition into an aqueous phase. The choice of phases will depend on the solute of interest.

The two phases also must be immiscible. Immiscible solutions never mix and remain as separate phases, like oil and water. Miscible solutions are completely homogeneous after mixing.

In liquid-liquid extraction, a solute is separated between two liquid phases, typically aqueous and organic. This is often performed in a separatory funnel fitted with a stopcock at the bottom and stopper at the top.

In the simplest case, three components are involved: The solute, the carrier liquid, and the solvent. The initial mixture, containing the solute dissolved in the carrier liquid, is mixed with the solvent. Upon mixing, the solute is transferred from the carrier liquid to the solvent, as long as the solute is more soluble in the solvent than in the carrier liquid, and as long as the carrier liquid and solvent are immiscible. The denser solution settles to the bottom.

There are two resulting phases: the raffinate, containing the carrier liquid, and the extract, which contains the solute and the solvent. In reality, there is likely to be residue of each component in both phases. Solid-liquid extraction is similar to liquid-liquid extraction, except that the solute is dispersed in a solid matrix rather than in a carrier liquid. The solid phase, containing the solute, is dispersed in the solvent and mixed. The solute is extracted from the solid phase to the solvent, and the solid phase is then removed by filtration. Now that the principles of extraction have been outlined, the solid-liquid extraction technique will be demonstrated by performing the extraction in the laboratory.

In this experiment, soil samples were collected from a brownfield site, similar to this one in Sewickley, Pennsylvania. Brownfields, as defined by the U.S. EPA, are real property, where the expansion, redevelopment, or reuse may be complicated due to the potential presence of hazardous contaminants. The pollutant of interest in this case is an organochloride: the herbicide atrazine. Once a soil sample has been collected from the site of interest, transfer it into the laboratory.

Weigh out 10 g of the soil in a clean, dry, wide-mouth Pyrex dish. Put the dish into an oven to dry for at least 12 h. Once dry, grind the soil to a uniform powder with a mortar and pestle. Place 5 g of the ground soil into a clean, dry 100-mL round-bottom flask. Add 15 mL of hexane and loosely stopper the flask. Place it into an ultrasonic bath and sonicate for 60 min.

Prepare a Büchner funnel with analytical filter paper. Once sonication is complete, wet the paper with hexane and begin vacuum filtration. Slowly pour the sample over the filter paper. Rinse the residual solids from the flask with hexane and add it to the filter. The stripped soil remains on the filter, while the hexane and extracted organics collect in the flask.

If the hexane solution is cloudy, residual water is present. To dry the solution, add a small spatula of desiccant, such as calcium chloride. Stir the solution until the desiccant is dissolved, and observe the solution.

If the solution is still turbid or if the calcium chloride has aggregated, there is still water in the solution. Repeat the process until the solution is clear and the desiccant is free flowing.

Next, remove the calcium chloride by gravity filtration.

If the concentration of the compound of interest is below the limit of quantification, it must be concentrated. Transfer the filtered extract to a clean, dry 3-necked round-bottom flask. Stopper the center neck, and place a rubber septum over one of the other necks. The third is left open.

Pierce the septum and attach tubing to a nitrogen line. Begin flowing nitrogen through the flask. The gas should be flowing in the headspace above the solution, and not bubbling through it. The flowing gas evaporates the excess solvent. Allow the gas to flow until there is about 50% volume reduction.

Once the organic components of the soil are extracted and concentrated, they can be analyzed by gas chromatography.

The atrazine concentration can be calculated using atrazine standard concentrations. In this case, the approximate atrazine concentration in the brownfield site studied was 2 mg of atrazine per 1 kg of soil.

Solid-liquid extraction is used in a wide range of fields.

This technique can be used to understand the transfer of polychlorinated biphenyls, or PCBs, from fish. PCBs are man-made chlorinated hydrocarbons that have been banned by the EPA. PCBs do not readily decompose in the environment and tend to accumulate in fish.

In this experiment, prey fish containing PCBs were fed to predator fish. The predator fish were then collected and sacrificed. The fish tissue was ground in preparation for extraction.

The PCB in the fish tissue was extracted to an organic phase using a Soxhlet extractor. The Soxhlet extractor setup, composed of a round-bottom flask, condenser, and the Soxhlet apparatus, is frequently used to extract solutes that are poorly soluble in solvents. The Soxhlet extraction enables a small amount of solvent to be used with a large solid sample. The extract was then tested for PCB content using mass spectrometry.

Dry plant matter, called lignocellulose, is the most abundant raw material being researched for bio-derived fuels. However, the carbohydrates used as the fuel are trapped within the rigid plant matrix, called lignin.

When the carbohydrates are removed, the lignin matrix is typically disposed of as waste. However, in this experiment, waste lignin was examined as a fuel source. Solid-liquid extraction was utilized to separate the carbohydrate components from lignocellulose, leaving lignin behind. The lignin was then used for further fermentation experiments.

Solid-liquid extraction can also be used to measure the wax content in fruit skins. In this experiment, the wax content of tomato skins was analyzed.

Exhaustive dewaxing of dried tomato skins was completed using a Sohxlet apparatus, in order to fully remove the wax content in the skins. Tomato skins with wax removed were then further analyzed using nuclear magnetic resonance spectroscopy. This helped elucidate the composition and degradation of native and engineered fruits.

You've just watched JoVE's introduction to solid-liquid extraction. You should now have a better understanding of the extraction of solutes between solid and liquid phases.

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