Liquid–liquid extraction is a method to separate compounds based on their relative solubilities in two different immiscible liquids, often water and an appropriate organic solvent. During extraction process, there is a net transfer of one or more components between the two liquid phases. This partitioning of compounds allows for purification of reaction mixtures or transfer of desirable products to an organic phase that is more easily evaporated than water. Liquid–liquid extraction is a technique used in both chemical laboratories and in industrial separations. Separatory funnels are commonly used for small-scale separations in research or teaching labs. Liquid-liquid extraction is commonly used for organic compounds in the scent/flavor industry, the pharmaceutical industry, and other chemical industries. Metal ions can also be separated using appropriate chelating agents to favor extraction of certain ions into aqueous vs. organic phases like the PUREX process used to separate uranium from plutonium. Liquid-Liquid extraction can be substantially accelerated in microfluidic devices, reducing extraction and separation times from minutes/hours to mere seconds compared to conventional extractors.
Measures of effectiveness
Distribution ratio In solvent extraction, a distribution ratio (D) is often quoted as a measure of how well-extracted a species is. The distribution ratio is a measure of the total concentration of a solute in the organic phase divided by its concentration in the aqueous phase. The partition or distribution coefficient (Kd) is the ration of solute concentration in each layer upon reaching equilibrium. This distinction between D and Kd is important. The partition coefficient is a thermodynamic equilibrium constant and has a fixed value for the solute’s partitioning between the two phases. The distribution ratio’s value, however, changes with solution conditions if the relative amounts of A and B change. If we know the solute’s equilibrium reactions within each phase and between the two phases, we can derive an algebraic relationship between Kd and D. The partition coefficient and the distribution ratio are identical if the solute has only one chemical form in each phase; however, if the solute exists in more than one chemical form in either phase, then Kd and D usually have different values. Depending on the system, the distribution ratio can be a function of temperature, the concentration of chemical species in the system, and a large number of other parameters. Note that D is related to the Gibbs Free Energy (ΔG) of the extraction process. In solvent extraction, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent. In this experiment, the nonpolar halogens preferentially dissolve in the non-polar mineral oil.
Separation factors The separation factor is one distribution ratio divided by another; it is a measure of the ability of the system to separate two solutes. For instance, if the distribution ratio for nickel (DNi) is 10 and the distribution ratio for silver (DAg) is 100, then the silver/nickel separation factor (SFAg/Ni) is equal to DAg/DNi = SFAg/Ni = 10.
Measures of success Success of liquid–liquid extraction is measured through separation factors and decontamination factors. The best way to understand the success of an extraction column is through the liquid–liquid equilibrium (LLE) data set. The data set can then be converted into a curve to determine the steady state partitioning behavior of the solute between the two phases. The y-axis is the concentration of solute in the extract (solvent) phase, and the x-axis is the concentration of the solute in the raffinate phase. From here, one can determine steps for optimization of the process.
Techniques and equipment
Liquid-liquid extraction is often performed on a small scale by synthetic lab chemists using a separatory funnel, Craig apparatus or membrane-based techniques. On an industrial scale, devices like centrifugal contactors, thin layer extraction, spray columns, pulsed columns, and mixer-settlers are used to aid extraction. All of these techniques aim create a high surface area interface between the two liquid phases involved in extraction to aid the transfer of solutes from one phase to the another.
Batch methods
Separatory funnels Separatory funnels, colloquially called a "sep funnel," are commonly used for small-scale extractions in research or teaching labs. To perform an extraction, two immiscible liquids, typically an aqeuous solution and an organic solvent, are added to the sep funnel. The sep funnel is then shaken using appropriate technique to increase the area of contact between the two phases to aid extraction. After the two layers are allowed to settle and separate, each layer is drained from the bottom of the sep funnel into separate containers. Multiple extractions may be performed on the same reaction mixture to increase product recovery. After extraction, the extract phase can be used for further processing Partitioning of organic compounds between the organic and aqueous phases can be controlled by adjusting the pH of the aqeuous phase. For example, increasing the pH of the aqueous solution will deprotanate organic acids, giving them a negative charge that favors partitioning into polar solvents like water. Adding a strong acid to decrease pH can have the opposite effect, neutralizing conjugate bases to form neutral compounds that can partition more easily into the organic phase. This technique can be used in teaching labs to extract compounds like caffeine from coffee or tea using ethyl acetate as the organic extractant.
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