Chapter 5 - Separation and identification of the components of mixtures

Solutes, Solvents, and Solubility

• A solution consists of a solvent (major component) and a solute (minor component dissolved in the solvent).

• Aqueous solutions (aq) are formed when a solute is dissolved in water.

• The solubility of substances varies in different solvents.

• Definitions:

  • Solute: A substance that dissolves in a solvent.

  • Solvent: A substance that dissolves a solute.

  • Solution: The product of dissolving a solute in a solvent.

  • Solubility: The extent to which a solute dissolves in a solvent.

• "Like dissolves like": Polar solutes dissolve in polar solvents; non-polar solutes dissolve in non-polar solvents.

• Non-polar substances include non-polar molecules with dispersion forces as the strongest intermolecular forces.

• Polar substances include spectator ions and polar molecules.

• Polar molecules with H bonded to F, O, or N can form hydrogen bonds with F, O, or N on adjacent molecules.

• Other polar molecules can undergo dipole-dipole attractions.

• Examples of polar substances: water (H2O), ammonia (NH3), methanol (CH3OH), vitamin C, methanoic acid (HCOOH), lactose.

• Examples of non-polar substances: cyclohexane (C6H12), oxygen (O2), ethene (CH2CH2), vitamin D, benzene (C6H6), triglyceride.

• Worked Example: Water is polar, so polar molecules like hydrogen chloride (HCl) and ammonia (NH3) dissolve in it. HCl dissolves via dipole-dipole bonding, and NH3 dissolves via hydrogen bonding. Non-polar molecules such as hydrogen, oxygen, chlorine, nitrogen, carbon dioxide, methane, ethane, and ethene do not dissolve in water and may form an insoluble layer depending on density.

Chromatography

• Chromatography Definition: A technique for separating mixtures based on the relative solubility of each substance in a solvent.

• Components of Chromatography: Stationary phase (does not move, e.g., paper) and mobile phase (moves over the stationary phase, e.g., a solvent).

• The technique was originally used to separate colored substances, but can now separate colorless substances as well.

• Thin Layer Chromatography (TLC) uses coated galss or plastic as the stationary phase, and a solvent as the mobile phase.

• Process: A mixture is placed on the origin, and the solvent moves from below the origin towards the solvent front.

• Substances more attracted to the stationary phase adsorb to it, with a series of adsorptions/desorptions taking place.

• The substance most soluble in the mobile phase moves the fastest.

• Definitions:

  • Mixture: Two or more substances not chemically combined.

  • Origin: The starting point on the stationary phase where the mixture is applied.

  • Solvent Front: The endpoint of the chromatogram where the solvent reaches.

Calculating $R_f$$$R_f$$ Values

• Rf value is the distance the component traveled from the origin divided by the distance traveled by the solvent front.

• Formula:

  

High-Performance Liquid Chromatography (HPLC)

• The stationary phase is a solid packed into a column.

• The mobile phase is a solvent fluid that is pumped through the column under pressure.

• The sample is injected into the machine above the column.

• The eluent is the solvent that leaves the chromatography machine.

• Definition: HPLC is a technique where the mobile phase is pumped through a tightly packed stationary phase under pressure.

• The number of peaks on a chromatogram indicates the number of components in the mixture.

• Retention Time: The time taken for a component to pass through the HPLC column, equivalent to the $R_f$$$R_f$$ value in paper and TLC.

• Under the same conditions, retention time can be used to qualitatively identify a substance.

• Peak Area: The area under the curve corresponds to the concentration of the substance quantitatively compared to a calibration curve.

Factors Affecting Retention Time in HPLC

• For sugars (polar molecules) in HPLC with a polar stationary phase and non-polar mobile phase:

  • Decreasing the viscosity of the mobile phase can reduce retention time.

  • Using a more polar mobile phase also reduces retention time.

  • Using a more tightly packed stationary phase on it's own would not reduce retention time, as it doesn't effect how quickly the sugar would move through the column.

Key Terms

• Mixture

• Chromatography

• Stationary Phase

• Mobile phase

• Origin

• Solvent Front

• $R_f$$$R_f$$ Value

• HPLC

• Eluent

• Retention Time

• Peak Area

Summary

• Chromatography separates mixtures based on their relative solubility in the mobile phase versus adsorption to the stationary phase.

• $$Rf \text{ (formula) value} = \frac{h1}{h_2}$$

• High performance liquid chromatography (HPLC) – qualitative and quantitative

• Paper and thin layer chromatography (TLC) – qualitative only