Chromatography and qualitative analysis

Stationary phase: Attraction

The more attracted a component is to the stationary phase, the slower it moves

Mobile phase: Attraction

The more soluble a component is in the mobile phase, the faster it moves

TLC

• Stationary phase: TLC Plate

• Mobile phase: Solvent

• The efficient separation depends in the relative adsorption of the components on to the stationary phase

Which property affects how strongly a component is adsorbed onto the stationary phase?

Polarity

Rf Value

Rf = distance moved by component/ distance moved by solvent front

Why can some components have different Rf values

• The more polar soluble adsorbs more strongly to the stationary phase

Limitations of TLC

Similar compounds have similar Rf values, so the spot may contain more than one component

Gas Chromatograhpy

• Stationary phase: Alkane with a high boiling point coated onto a solid

• Mobile phase: Helium or Nitrogen

• The efficient separation depends on the relative solubilities of the components in the stationary phase

• If a component is more soluble in the stationary phase it will have a longer retention time

3 things to consider when interpreting a chromatogram

• Number of peaks - gives the number of components present in the mixture

• Retention time - used to identify components by comparing them to known values

• Area below each peak - shows the relative quantity of each component

Limitations of Gas chromatography

• Similar compounds can have similar retention times so it can be hard to identify

• Retention times are dependent on temperature, pressure and choice of carrier choice

• The chromatogram will contain many peaks

How is a calibration curve obtained

• Make up 4 solutions of known concentrations

• Run a chromatogram for each solution

• Plot the peak area against known concentration ‘

• Draw a line of best fit

Alkene

• Reagent: Bromine Water

• Observation: Decolourisation orange → colourless

Haloalkane

• Regents: Aqueous NaOH then acidified silver nitrate

• Observation: White ppt, Cream ppt, Yellow ppt

First and Secondary alcohols

• Reagents: acidified potassium dichromate

• Observation: Orange → Green

Aldehydes

• Reagents: Tollens reagent, 2,4-DNP

• Observations: Silver Mirror, Orange precipitate

Ketones

• Reagents: 2,4-DNP

• Observation: Orange Precipitate

Carboxylic acids

• Reagents: Na₂CO₃

• Observation: Effervescence

• Equation: CO₃^2- + 2H⁺ → CO₂ + H₂O

Phenol

• Reagents: Bromine water, Universal indicator followed by Na₂CO₃

• Observations: White ppt, Red / Orange to show acidity then NVC with carbonate