Analytical Chemistry Solvent Extraction Quiz

Solvent Extraction

• Solvent extraction is a technique used to transfer desired analytes from one matrix to another.
• This process is often used when the initial matrix (e.g., soil) isn't suitable for standard analytical methods.
• Water is frequently unsuitable for direct use in chemical instruments, thus requiring extraction.
• Extraction involves transferring a solute from one liquid phase to another or from a solid phase to a liquid phase.
• Typically, an aqueous solution is extracted with an organic solvent.
• Common organic solvents include diethyl ether, benzene, chloroform, dichloromethane, and carbon tetrachloride.
• Organic solvents are less dense than water, forming a separate phase.
• Chloroform, dichloromethane, and carbon tetrachloride are denser than water.
• The choice of solvent should prioritize less toxic options.
• A two-phase mixture will have some of each solvent present in both phases, but one phase will be predominantly water and the other organic.

Analytical Separations

• Chemical analyses of samples often require separation of the analyte from interfering substances in the sample.
• Analytical chemistry significantly focuses on achieving these separations.
• Organic analytical chemistry routinely uses chromatography for separations.
• Gas and liquid chromatography are instrumental methods used in separations.
• Solvent extraction is a manual separation technique still used in modern analytical laboratories.

Extraction (Solid-Liquid)

• A weighed solid is placed in a container with a solvent.
• Mixing allows some portion of the solid to dissolve.
• The dissolving material has stronger interactions with the solvent than with the remaining solid, promoting transfer into solution.
• This process isolates the analyte of interest from the remaining part of the solid.

Extraction (Liquid-Liquid)

• A common method for separation.
• Involves two immiscible liquids that don't dissolve into each other.
• Two layers form after mixing, and these are separated by draining one layer.
• Organic product isolation from inorganic substances uses liquid-liquid extraction.

Solvent Extraction - Criteria

• The organic solvent used in extraction needs to dissolve the substance readily.
• The solvent should not chemically react with the substance extracted.
• The solvent should not mix with water (this is usually the second solvent used).
• It must have a low boiling point.

Solvent Extraction - Examples

• A solvent extraction of butyric acid involves 4 grams of butyric acid from 500 mL of water. Solvent used in extraction is ether with an extraction coefficient (K) of 3.
• Single batch - 3 grams extracted to the ether layer, leaving 1 gram in water layer.
• Two succesive extractions, each with 250 mL portion - 2.4 gram is extracted to the ether phase, leaving 1.6 gram in the water phase.

pH Effects

• Solvent extraction of an amine depends on pH as well.
• The equilibrium constant for the extraction process that uses a base needs to be taken into account.

Chelating Ligands

• Metal ions are considered Lewis acids, which can accommodate electron pairs donated by the ligand.
• Cyanide (CN) is considered a "monodentate" ligand that binds to a metal ion through a single atom.
• Chelating ligands bind through more than one atom to form a complex with the metal ion. This allows for separation by reacting with different metal ions and creating selective extraction.
• Dithizone, 8-Hydroxyquinoline (oxine), and Cupferron are common chelating ligands used for metal ion separation.

Extraction with a Metal Chelator

• A scheme used for separating metal ions involves forming complexes with an organic ligand.
• The metal-ligand complex is then extracted into an organic solvent.

Metal Chelators (further details)

• The ligands work as weak acids and lose a proton when they bind to a metal ion.
• Most complexes that can be extracted into organic solvents must be neutral.
• Many of the formed charged complexes are not soluble in organic solvents.

Metal Chelators (Distribution Coefficient)

• An equation for the distribution coefficient of a metal between the two phases can be derived.
• It is assumed that essentially all of the metal in the aqueous phase is in the form Mn+ and all of the metal in the organic phase is in the form MLn.
• The partition coefficients for the ligand and the complex are defined.
• The distribution coefficient (D) is defined as: D = [total metal]org/ [total metal]aq

Fractional Distillation

• Separation of components from a liquid mixture via fractional distillation.
• The more volatile component leaves the mixture first at higher temperatures.
• The process repeats until a pure component is extracted. A column with packing material is used to create many sequential vaporization and condensation steps that improves purity of the distillate (more volatile phase).
• The remaining component is called residue and is less volatile than the more volatile component. Components are extracted from a mixture based on boiling point differences.
• Involves many repetitions of vaporization and condensation.
• Useful in separating mixtures of similar boiling points (i.e., benzene-toluene mixture).

Distillation of Petroleum

• Petroleum (crude oil) is a mixture of alkanes, other elements, and organic compounds.
• It is separated into different fractions through distillation in a distillation column.
• Gasoline, kerosene, diesel fuel, and other petroleum fractions obtained are extracted from petroleum in a refining process based on differing boiling points.

Liquid-Vapour Equilibrium

• A graph depicting the relation between vapor and liquid compositions in a binary mixture (e.g. benzene-toluene mixture).
• The graph is useful in explaining how fractional distillation works.