v Liquid-liquid extraction (LLE):
Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid phase. Liquid-liquid extraction is a basic technique in chemical laboratories, where it is performed using a separatory funnel. This type of process is commonly performed after a chemical reaction as part of the work-up.
In other words, this is the separation of a substance from a mixture by preferentially dissolving that substance in a suitable solvent. By this process, a soluble compound is usually separated from an insoluble compound. Solvent extraction is used in nuclear reprocessing, ore processing, the production of fine organic compounds, the processing of perfumes, and other industries.
Liquid-liquid extraction is possible in non-aqueous systems: In a system consisting of a molten metal in contact with molten salt, metals can be extracted from one phase to the other. This is related to a mercury electrode where a metal can be reduced, the metal will often then dissolve in the mercury to form an amalgam that modifies its electrochemistry greatly. For example, it is possible for sodium cations to be reduced at a mercury cathode to form sodium amalgam, while at an inert electrode (such as platinum) the sodium cations are not reduced. Instead, water is reduced to hydrogen. A detergent or fine solid can be used to stabilize an emulsion, or third phase.
is an important unit operation that allows one to separate fluids based on different solutes being soluble to different degrees in different solvents. In the this experiment, acetone is extracted by contact with water from a feed stream containing an unknown concentration of acetone in butyl acetate (BA).
v Purpose:
The goal of this laboratory exercise is to study the performance of a liquid-liquid extraction column.
v Learning Objectives
The main learning objective of this laboratory exercise is to gain experience in characterizing the behavior of a liquid-liquid extraction column. To do this, one must examine such parameters as the composition of the top and bottom products at various feed rates and compositions. These parameters should be calculated theoretically using available computer programs and determined experimentally from the column.
v Distribution ratio:
In solvent extraction, a distribution ratio is often quoted as a measure of how well-extracted a species is. The distribution ratio (D) is equal to the concentration of a solute in the organic phase divided by its concentration in the aqueous phase. 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 ΔG of the extraction process.
Sometimes, the distribution ratio is referred to as the partition coefficient, which is often expressed as the logarithm. See partition coefficient for more details. Note that a distribution ratio for uranium and neptunium between two inorganic solids (zirconolite and perovskite) has been reported.[1] 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 nonpolar mineral oil.
v 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.
v Extraction in the organic chemistry teaching labs:
Liquid-liquid extractions using a reparatory funnel are essentially the only kind of extraction performed in the organic teaching labs. The "liquid-liquid" phrase means that two liquids are mixed in the extraction procedure. The liquids must be immiscible: this means that they will form two layers when mixed together, like oil and vinegar do in dressing. Some compounds are more soluble in the organic layer (the "oil") and some compounds are more soluble in the aqueous layer (the "vinegar").
Extractions use two immiscible phases to separate a solute from one phase into the other. The distribution of a solute between two phases is an equilibrium condition described by partition theory. Boiling tea leaves in water extracts the tannins, the bromine, and caffeine (the good stuff) out of the leaves and into the water. More typical lab extractions are of organic compounds out of an aqueous phase and into an organic phase.
v Illustration of an extraction in a reparatory funnel:Analytical Extractions
Elemental analysis generally requires fairly simple (not necessarily easy) sample preparation. Solids are usually dissolved or digested in caustic solution and liquids are sometimes extracted to separate the analyte from interferences.
Organic analysis is often much more complicated. Real-world samples can be very complicated matrices that require careful extraction procedures to obtain the analyte(s) in a form that can be analyzed.
v Techniques:
§ Salvation mechanism:
Using solvent extraction it is possible to extract uranium, plutonium, or thorium from acid solutions. One solvent used for this purpose is the organophosphate tri-n-butyl phosphate. The PUREX process that is commonly used in nuclear reprocessing uses a mixture of tri-n-butyl phosphate and an inert hydrocarbon (kerosene), the uranium(VI) are extracted from strong nitric acid and are back-extracted (stripped) using weak nitric acid. An organic soluble uranium complex [UO2(TBP)2(NO3)2] is formed, then the organic layer bearing the uranium is brought into contact with a dilute nitric acid solution; the equilibrium is shifted away from the organic soluble uranium complex and towards the free TBP and uranyl nitrate in dilute nitric acid. The plutonium(IV) forms a similar complex to the uranium(VI), but it is possible to strip the plutonium in more than one way; a reducing agent that converts the plutonium to the trivalent oxidation state can be added. This oxidation state does not form a stable complex with TBP and nitrate unless the nitrate concentration is very high (circa 10mol/L nitrate is required in the aqueous phase). Another method is to simply use dilute nitric acid as a stripping agent for the plutonium. This PUREX chemistry is a classic example of a salvation extraction.
Here in this case DU = k TBP2[[NO3]]2.
§ Ion exchange mechanism:
Another extraction mechanism is known as the ion exchange mechanism. Here, when an ion is transferred from the aqueous phase to the organic phase, another ion is transferred in the other direction to maintain the charge balance. This additional ion is often a hydrogen ion; for ion exchange mechanisms, the distribution ratio is often a function of pH. An example of an ion exchange extraction would be the extraction of americium by a combination of terpyridine and a carboxylic acid in tert-butyl benzene. In this case
DAm = k terpyridine1carboxylic acid3H+−3
Another example is the extraction of zinc, cadmium, or lead by a dialkyl phosphoric acid (R2PO2H) into a no polar diluents such as an alkane. A non-polar diluents favors the formation of uncharged non-polar metal complexes.
Some extraction systems are able to extract metals by both the salvation and ion exchange mechanisms; an example of such a system is the americium (and lanthanide) extraction from nitric acid by a combination of 6,6'-bis-(5,6-dipentyl-1,2,4-triazin-3-yl)-2,2'-bipyridine and 2-bromohexanoic acid in tert-butyl benzene. At both high- and low-nitric acid concentrations, the metal distribution ratio is higher than it is for an intermidate nitric acid concentration.
v The Physical Basis of Extractions:
Let's go back to our example of the chemical dissolved in water that makes its way into methylene chloride through a liquid-liquid extraction. While the procedure is successful in our hypothetical case, this laboratory trick does not always work. The following conditions must be satisfied for our liquid-liquid extraction example to be successful:
· The compound (solute) that is initially dissolved in water must also be soluble in methylene chloride.
· The compound must be more soluble in methylene chloride than it is in water (which is the case for many organic compounds).
· Water and methylene chloride must be immiscible (incapable of forming a homogeneous mixture).
We can extend our example to illustrate another useful property of extraction. Assume that the compound being transferred into methylene chloride from water is initially mixed with another solute in water. If the other solute in water is not soluble in methylene chloride, then the extraction separates the two solutes: one moves into methylene chloride, and the other stays behind in water. Here we see extraction being used for its most common purpose: purification.
Liquid-liquid extractions work because solutes preferentially migrate to or remain in the solvent in which they are most soluble. By requiring that the solvents used be immiscible (e.g. water and methylene chloride), physical separation of the solutions becomes easy: the less dense solution floats on top of the more dense one without mixing. The separatory funnels used in extractions are specially designed to facilitate separations of liquid layers.
v References:
1. ^ Scholz, F.; S. Komorsky-Lovric, M. Lovric (February 2000). "A new access to Gibbs energies of transfer of ions across liquid". Electrochemistry Communications (Elsevier) 2 (2): 112–118. doi:10.1016/S1388-2481(99)00156-3.
2. ^ Danil de Namor, A.F.; T. Hill (1983). Journal of the Chemical Society, Faraday Transactions: 2713.
3. ^ Mackenzie, Murdoch. "The Solvent Extraction of Some Major Metals". Cognis GmbH. http://www.cognis.com/NR/rdonlyres/62A4BDA0-2B5F-4579-9761-968114B57A2A/0/thesolve.pdf. Retrieved 2008-11-18.
4. ^ M. Filiz, N.A. Sayar and A.A. Sayar, Hydrometallurgy, 2006, 81, 167-173.
5. ^ Yoshinari Baba, Minako Iwakuma and Hideto Nagami, Ind. Eng. Chem. Res, 2002, 41, 5835-5841.
6. ^ J. M. Sánchez, M. Hidalgo, M. Valiente and V. Salvadó, Solvent Extraction and Ion Exchange, 1999, 17, 455-474.
7. ^ Lee W. John. "A Potential Nickel / Cobalt Recovery Process". BioMetallurgical Pty Ltd. http://www.biomet.com.au/Extract/NiCoFS.htm.
8. ^ "Precious Metals Refining By Solvent Extraction". Halwachs Edelmetallchemie und Verfahrenstechnik. http://www.halwachs.de/solvent-extraction.htm. Retrieved 2008-11-18.
9. ^ P. Giridhar, K.A. Venkatesan, T.G. Srinivasan and P.R. Vasudeva Rao, Hydrometallurgy, 2006, 81, 30-39.
^ K. Takeshita, K. Watanabe, Y. Nakano, M. Watanabe (2003). "Solvent extraction separation of Cd(II) and Zn(II) with the organophosphorus extractant D2EHPA and the aqueous nitrogen-donor ligand TPEN". Hydrometallurgy 70: 63–71.