02 Solvent Extraction

02 Solvent Extraction - II. Solvent Extraction CHM 614/714...

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Unformatted text preview: II. Solvent Extraction CHM 614/714 Fall 2011 What is extraction? Extraction is the transfer of a solute from one liquid phase to another. A significant method that is based on relative solubility of an analyte in two immiscible liquids. It is used to (a) remove interference , (b) isolate or concentrate the desired species prior to analysis, and (c) produce measurable form of a species. The basic theory of extraction is applicable to chromatography. http://www.youtube.com/watch?v=vcwfhDhLiQU Organic Solvents The most common case is the extraction of an aqueous solution with an organic solvent . Diethyl ether , toluene , and hexane are common organic solvents that are immiscible with and less dense than water. They form a separate phase that floats on top of the aqueous phase. Chloroform , dichloromethane , and carbon tetrachloride are common solvents that are denser than water. Miscible or Immiscible To be miscible means that the two liquids form a single phase when they are mixed in any ratio . To be immiscible means that liquids remain in separate phases. Organic solvents with low polarity are generally immiscible with water which is highly polar. Requirements for Organic Solvent Must be a good solvent for the solute to be separated. Must coalesce and separate from aqueous phase quickly. The density of the solvent should be significantly different from water. Popular Organic Solvents Chloroform (ρ = 1.49) Benzene (ρ = 0.88) Diethyl ether (ρ = 0.71) Methyl isobutyl ketone (ρ = 0.80) Whenever a choice exists between CHCl 3 and CCl 4 , the less toxic CHCl 3 should be tried. Also, toluene is greatly preferred over benzene (a carcinogen), if that choice must be made. Facts and assumption In a two-phase mixture, some of each solvent is found in both phases, but one is predominantly water and the other phase is predominantly organic. The volumes of each phase after mixing are not exactly equal to the volumes that were mixed. For simplicity, however, we will assume that the volumes of each phase are not changed by mixing. Solvent extraction theory For a solute S exists in equilibrium between phase 2 (aqueous) and 1 (organic). The equilibrium constant, K p (or K), for the system is the partition coefficient of the solute. where A S1 and [S] 1 refer to the activity and concentration of solute in phase 1. In dilute solution, the ratio of activities can be replaced with a ratio of concentrations. (1) [S] [S] [S] [S] t coefficien Partition organic) 1, (phase S aqueous) 2, (phase S aq org 2 1 2 1 = ≈ = = ⇔ S S p A A K Figure 1. Partitioning of a solute between two liquid phases Solvent extraction theory Suppose that solute S in V 2 mL of phase 2 (water) is extracted with V 1 mL of phase 1 (toluene) . Let n be the mmol of solute S in the system and let q be the fraction of S remaining in phase 2 at equilibrium. The molarity in phase 2 is therefore qn/V 2 . The fraction...
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This note was uploaded on 10/20/2011 for the course CHEM 653 taught by Professor Wei, robert during the Spring '11 term at Cleveland State.

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02 Solvent Extraction - II. Solvent Extraction CHM 614/714...

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