Chapter 8

• Separation Methods


  • Separation by Precipitation
  • Separation of Species by Distillation
  • Separation by Extraction
  • Separating Ion by Ion Exchange
  • Chromatographic Separation

  
• Separation by Precipitation


  Generally based on the large solubility differences between the analyte and potential interferents
• Precipitation of Analytes/Interferent


  1. pH control

  2. Debye-Huckel Theory (Salting out & Salting in)

  3. Electric potential

  4. Solubility with inorganic and organic precipitant such as sulfides, chlorides, or sulfates

  
• Separation of Species by Distillation


  Mainly based on differences in the boiling points of the materials in a mixture
• Types of Distillation


  • Vacuum Distillation
  • Molecular Distillation
  • Pervaporation
  • Flash evaporation
• Separation by Extraction


  • Based on the solubility of two immiscible compounds
  • The partition of a solute between two immiscible phases is an equilibrium phenomenon that is governed by the distribution law
• Separation by Extraction Example 1

  • Answer: (a) 0.062 or ≈ 6% (b) 0.000 98 or ≈ 0.1%
• Separation by Extraction Example 2

  • Answer: (a) 1.16 x 10^-5 M (b) 5.28 x 10^-7 M (c) 5.29 x 10^-10
• Separation of Ions by Ion Exchange


  Ion exchange is a process by which ions held on a porous, essentially insoluble solid are exchanged for ions in a solution that is brought in contact with the solid
• Ion Exchange Resin


  Synthetic ion-exchange resins are high-molecular-weight polymers that contain large numbers of an ionic functional group per molecule
• Ion Exchange Equilibria


  Ion-exchange separations are ordinarily performed under conditions in which one ion predominates in both phases
• Order of K values for univalent ions


  • Decreasing order
• Order of K values for divalent cations


  • Decreasing order
• Chromatographic Separation


  Based on differences in migration rates among the mobile-phase components
• Types of Chromatography


  • Column Chromatography
  • Planar Chromatography
• Chromatogram


  Concentration profiles of solute bands A and B at two different times in their migration down the column
• Methods for Improving Column Performance


  • Increase the rate of band separation
  • Decrease the rate of band spreading
• Separation Methods


  • Separation by Precipitation
  • Separation of Species by Distillation
  • Separation by Extraction
  • Separating Ion by Ion Exchange
  • Chromatographic Separation

  
• Separation by Precipitation


  Generally based on the large solubility differences between the analyte and potential interferents
• Precipitation of Analytes/Interferent


  1. pH control

  2. Debye-Huckel Theory (Salting out & Salting in)

  3. Electric potential

  4. Solubility with inorganic and organic precipitant such as sulfides, chlorides, or sulfates

  
• Selectivity factor (α)


  Defined as the ratio of the retention factors of two solutes A and B
• Separation of Species by Distillation


  Mainly based on differences in the boiling points of the materials in a mixture

  
• Types of Distillation


  • Vacuum Distillation
  • Molecular Distillation
  • Pervaporation
  • Flash evaporation
• Chromatographic Separation Example


  • Rf (Benzene) = 5.0, Rf (Toluene) = 6.9, Relative Retention = 1.39, Unadjusted Relative Retention = 1.33

  
• Separation by Extraction


  • Based on the solubility of two immiscible compounds
  • The partition of a solute between two immiscible phases is an equilibrium phenomenon that is governed by the distribution law
• Fronting and tailing


  Common causes of fronting and tailing in chromatography
• Separation by Extraction Example 1

  • Answer: (a) 0.062 or ≈ 6% (b) 0.000 98 or ≈ 0.1%
• Separation by Extraction Example 2

  • Answer: (a) 1.16 x 10^-5 M (b) 5.28 x 10^-7 M (c) 5.29 x 10^-10
• Quantitative Measures of Column Efficiency


  N = Plate Count, H = Plate Height, L = Column Packing Length
• Separation of Ions by Ion Exchange


  Ion exchange is a process by which ions held on a porous, essentially insoluble solid are exchanged for ions in a solution that is brought in contact with the solid
• Ion Exchange Resin


  Synthetic ion-exchange resins are high-molecular-weight polymers that contain large numbers of an ionic functional group per molecule
• Variables that lead to Band Broadening


  • Large particle diameters for stationary phases
  • Large column diameters
  • High temperatures (important only in gas chromatography)
  • For liquid stationary phases, thick layers of the immobilized liquid
  • Very rapid or very slow flow rates
• Ion Exchange Equilibria


  Ion-exchange separations are ordinarily performed under conditions in which one ion predominates in both phases
• The Effect of Mobile-Phase Flow Rate


  Plate heights can be decreased, and column efficiency increased, by decreasing the particle size of column packings, by lowering the viscosity of the mobile phase, and by increasing temperature
• Order of decreasing K for univalent ions

  • Cations
  • Anions
• Order of decreasing K for divalent cations

  • Cations
• Chromatographic Separation


  Based on differences in migration rates among the mobile-phase components
• Column Resolution


  The goal in chromatography is the highest possible resolution in the shortest possible elapsed time. Unfortunately, these goals tend to be incompatible.
• Types of Chromatography


  • Column Chromatography
  • Planar Chromatography
• Sample Preparation


  • Sampling is an important step in analytical chemistry. It determines the quality of an analysis.
  • The lengthy process of sample preparation accumulates great errors.
  • Selective sample handling technique can go a step further to restore, or even enrich, the concentration from dilution.
  • The analyst must care about the sample cleanup to remove interference by fractionation, derivatization or extraction.
  • The traditional liquid-liquid extraction (LLE) serves a need for sample preparation.
  • If dilution from sample preparation calls for enrichment, solid phase microextraction (SPME) is a choice.