Separation and Chromatographic Techniques - Chapter 2: Simple Separation Methods

Vocabulary practice flashcards covering chromatographic mechanisms, selectivity and detectability, simple separation methods (evaporation, precipitation, crystallization, filtration, membrane separations, distillation), and liquid-liquid extraction theory.

Affinity Chromatography

A highly selective form of chromatography based on a specific interaction between a target analyte and an immobilized ligand on the stationary phase, such as antigen-antibody or enzyme-substrate binding.

Ion-Exchange Chromatography

A separation method based on ion-exchange affinity between sample ions and fixed charged functional groups covalently attached to the stationary phase, such as fixed negative groups ($-\text{SO}_3^-$) in cation exchangers or fixed positive groups ($-\text{N(CH}_3)_3^+$) in anion exchangers.

Molecular Exclusion Chromatography

Also called gel filtration or gel permeation chromatography; a technique that separates molecules based on size and pore penetration, where larger molecules are excluded from the pores and elute faster, while smaller molecules enter the pores and elute slower.

Adsorption Chromatography

A separation technique based on differences in the affinities of solute molecules for the active surface of a solid stationary phase via interactions such as polar interactions, hydrogen bonding, van der Waals interactions, or $\pi\text{-}\pi$ interactions.

Partition Chromatography

A separation technique based on differences in the distribution (partitioning or solubility) of solute molecules between a liquid stationary phase (thin film) and a mobile phase.

Selectivity

The intrinsic capability of a separation method to distinguish and separate components with slight variations in physical properties or molecular characteristics, such as molecular weight differences (isotopic replacement of $\text{H}$ with $\text{D}$) or stereochemical structures (geometric and optical isomers).

Detectability

The minimum quantity of material that can be detected by a system, typically determined at two to three times the baseline noise level and controlled by detector sensitivity, separation efficiency, and minimizing band broadening.

Evaporation

The process of removing solvent from a sample by heating or by utilizing air currents to concentrate the sample or convert it into a solid state, such as evaporating seawater to yield sea salt ($\text{NaCl}$).

Precipitation

The formation of an insoluble solid ($\text{ppt}$) that occurs when the concentration of a solute in a solution increases above its solubility limit.

Solvent Precipitation

A precipitation method achieved by adding another miscible solvent to a solution to lower the solubility of the target solute.

Precipitation via Chemical Reaction

The addition of a chemical species that reacts with the target analyte to form a lower solubility salt, such as reacting $\text{BaCl}_2$ with $\text{H}_2\text{SO}_4$ to precipitate $\text{BaSO}_4$.

Common Ion Effect

The reduction in solubility of an ionic precipitate upon the addition of a soluble compound providing an ion common to the equilibrium, shifting equilibrium toward precipitate formation.

Precipitation by Adjustment of pH

A precipitation technique that alters solubility equilibria by adding strong acid or strong base; used for weak acids (converting them to their less-ionized, insoluble form) and sparingly soluble metal hydroxides (precipitating when $Q > K_{sp}$).

Crystallization

A purification technique used to obtain pure material in the crystalline state by dissolving an impure solid, heating to evaporate solvent, cooling to induce crystal formation, and isolating the crystals by filtration and drying.

Simple Filtration

A liquid-solid separation technique that uses gravitational force and cellulose filter papers capable of retaining particles down to $2.5\,\mu\text{m}$.

Vacuum Filtration

A filtration technique that applies suction to a collection flask (often using a Buchner funnel or membrane filter) to increase filtration speed, yield a drier solid residue, and improve separation efficiency for fine particles.

Membrane Filters

Thin porous membranes with precisely controlled pore sizes down to $0.02\,\mu\text{m}$, constructed from materials such as cellulose esters/nitrate, polycarbonate ($0.2\text{ to }10\,\mu\text{m}$), or PTFE with polyester support depending on chemical compatibility.

Membrane Separations

Separation processes utilizing semipermeable membranes to permit the passage of specific chemical species completely while restricting or stopping the permeation of others without requiring phase changes.

Ultrafiltration

A membrane-based separation technique that uses applied pressure to separate larger molecules and particles from smaller ones across a semipermeable membrane.

Reverse Osmosis (RO)

A membrane separation process that utilizes high applied pressure to force water through a semipermeable membrane against osmotic pressure, removing dissolved salts and small molecules.

Dialysis

A membrane-based separation process in which molecules are separated across a semipermeable membrane driven by concentration gradients based on molecular size.

Electrodialysis

A membrane separation process that uses an applied electric field to drive and separate ions through charged ion-exchange membranes.

Distillation

A separation method for liquid mixtures based on differences in volatility, depending on the equilibrium distribution of constituents between the liquid phase and the vapor phase.

Sublimation

A specialized distillation technique used for solids, in which a solid component volatilizes directly into vapor upon heating without passing through a liquid phase.

Fractional Distillation

A distillation method used to separate liquid mixtures containing components with close boiling points by utilizing a fractionating column.

Flash Distillation

A distillation process in which a portion of a liquid feed is rapidly vaporized (flashed) by a sudden drop in pressure, generating vapor and liquid phases in equilibrium.

Vacuum Distillation

Distillation carried out under reduced pressure to lower the boiling point of liquids, preventing thermal degradation of heat-sensitive compounds or separating high-boiling liquids.

Steam Distillation

A distillation method where steam is bubbled through a liquid mixture to co-distill volatile, water-immiscible compounds at temperatures below their normal boiling points.

Azeotropic Distillation

A distillation process used to separate constant-boiling azeotropic mixtures by altering vapor-liquid equilibrium, frequently through the addition of a third component known as an entrainer.

Distribution Coefficient ($K_c$)

An equilibrium constant representing the ratio of solute concentration in solvent 1 (typically the organic phase) to its concentration in solvent 2 (the aqueous phase), defined as $K_c = \frac{C_1}{C_2}$ and calculated for the neutral, un-ionized species only.

Distribution Ratio ($D$)

The ratio of the total analytical concentration of a solute across all chemical forms in the organic phase to its total concentration across all chemical forms in the aqueous phase ($D = \frac{\text{total concentration in organic}}{\text{total concentration in aqueous}}$).

Successive Extractions

A liquid-liquid extraction practice where the extraction solvent is divided into multiple smaller portions used sequentially rather than in a single large volume, thereby removing a significantly greater total amount of solute.