Chromatography + qualitative analysis

• What is chromatography?

  A family of separation techniques based on the principle of separating mixtures.
• What is the mobile phase in chromatography?

  The phase that carries the soluble components of the mixture.
• What relationship between a sample and the mobile phase makes the sample move faster?

  More soluble components or components with more affinity to the solvent move faster.
• What does the stationary phase do in chromatography?

  It holds back components of the mixture that are attracted to it.
• What relationship between a sample and the stationary phase makes the sample move slower?

  More affinity for the stationary phase means that a component moves slower, often involving hydrogen bonding.
• How are substances separated by chromatography?

  By the balance between affinity for the mobile phase and affinity for the stationary phase, which differs for each component.
• Why will different substances show different Rf values?

  They are bonded differently and have different polarities, affecting their retention time.
• What does TLC stand for?

  Thin Layer Chromatography.
• What is the stationary phase in TLC?

  A plastic/glass/metal sheet coated in silica or alumina.
• What are the advantages of TLC over paper chromatography?

  • Runs faster
  • Smaller amounts of a mixture can be separated
  • TLC plates are more robust than paper
• How can you observe colourless spots in chromatography?

  By shining UV light on them or spraying with a developing agent like ninhydrin.
• How do you calculate the Rf value in chromatography?

  Rf = distance moved by spot / distance moved by solvent front.
• What does Rf value stand for?

  Retention factor; a measure of the rate of movement of a component.
• How could you confirm the identity of a substance from its Rf value?

  By comparing your Rf value to accepted values for that substance in the same solvent and setup.
• What is the mobile phase in gas-liquid chromatography?

  A carrier gas, typically inert like N2 or He.
• What is the stationary phase in gas-liquid chromatography?

  Powder coated with oil, packed into a long, thin capillary tube.
• What do you measure in gas-liquid chromatography?

  Retention time; different components take different amounts of time to move through.
• What are the advantages of GLC?

  It is very sensitive and can detect minute traces of substances.
• What are GLC’s uses?

  Testing athletes’ and horses’ blood and urine for drugs.
• How can you use GC or GCMS to identify substances?

  By matching retention time to that of a known substance under the same conditions.
• How does GCMS work?

  Gas Chromatography is run, retention time is recorded, then the mixture is analyzed by Mass Spectrometry.

  
• How do you test for alkenes and what is the result?

  Shake with bromine water; the result is decolourisation from orange to colourless.
• How do you test for haloalkanes and what is the result?

  Add NaOH, warm, acidify with HNO3, then add AgNO3; result is a precipitate of AgX.
• How do you test for alcohols and what is the result?

  Add acidified K2Cr2O7 and heat; result is a color change from orange to green for primary and secondary alcohols.
• How do you test for aldehydes and what are the results?

  Warm with Fehling’s solution for a brick red precipitate or with Tollens’ reagent for a silver mirror.
• How do you test for carboxylic acids and what is the result?

  Add Na2CO3; the result is CO2 gas given off, indicating effervescence.
• How do you test for phenols?

  By weak acidity; there is a neutralization reaction with NaOH but no reaction with CO3^2-.
• How do you test for carbonyl compounds?

  React with 2,4-DNP to form an orange precipitate.