6.3.1 Chromatography and Qualitative Analysis

What is chromatography used for?

Separate individual components from a mixture of substances

What do all forms of chromatography have?

A stationary phase and a mobile phase

What is a phase?

Physically distinctive form of matter, such as the solid, liquid and gas phases of ordinary matter

What is the mobile phase?

Phase that moves in chromatography - normally a liquid or gas

What is the stationary phase?

Phase that does not move and is normally a solid or a liquid supported on a solid

What is adsorption?

The process by which a solid holds molecules of a gas or liquid or solute as a thin film on the surface of a solid or liquid (rarely)

What does the TLC technique use?

A TLC plate which is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance - usually silica

What is the stationary phase in TLC?

The adsorbent

What is the mobile phase in TLC?

Suitable organic solvent

How does TLC work?

•Solvent rises up the TLC plate

•Each component dissolves in the solvent

•Separation is achieved by the relative adsorptions of substances with the mobile phase

How would you carry out TLC?

•Place a small amount of solvent in beaker

•Add filter paper to chamber

•In pencil, draw straight line across TLC plate about 1cm from one end of the plate

•Place a drop of solution on line with capillary tube

•Add silica TLC plate with samples spotted on to developing chamber - make sure solvent does not go over line

•Seal container

•Once solvent has moved 5mm from end, take out plate and draw line where solvent reached

•Circle visible spots or visualise

What is a chromatogram?

Visible record showing the result of separation of components from a mixture by chromatography

What factors will affect how far a substance travels up a TLC plate?

•Solubility in mobile phase

•How polar the substance is

•Size

Why do non-polar substances travel further up a TLC plate with silica gel?

Silica gel is polar and can form hydrogen bonds with polar substances

How are TLC chromatograms analysed?

By calculating the retention factor for each component

How do you calculate Rf value?

Distance travelled by substance / distance travelled by solvent

How might you be able to identify a compound from its Rf value?

Compare Rf value with known values recorded using the same solvent system and adsorbent

Why might you not be able to identify a compound from its Rf value?

•Components have very similar Rf values

•A new chemical has been formed

•Conditions such as temperature may be different to conditions used when measuring Rf values of compounds

What factors affect Rf value?

•Temperature

•Solvent

•Thickness and amount of spot

•Other compounds

What is gas chromatography useful for?

Separating and identifying volatile organic compounds present in a mixture

What is the stationary phase in gas chromatography?

High boiling point liquid adsorbed onto an inert solid support

What is the mobile phase in gas chromatography?

Inert carrier gas

What happens during gas chromatography?

•Very small amount of sample injected into machine

•Injector is contained in an oven

•Sample boils and is carried along a thin, long column by an inert carrier gas

•Time taken to travel through tube depends on how much time is spent moving in the gas instead of the liquid

•Detected by a detector at a certain point in column

What two pieces of information can be gauged from a gas chromatogram?

Retention time and peak integrations

What is retention time?

Time taken for the compound to travel through the column to the detector, from the time the compound is injected to the time its peak shows its maximum height

What does retention time depend on?

•Boiling point - higher boiling point, longer it takes to boil, higher retention time

•Solublility in stationary phase - greater solubility, longer it takes to be detected, higher retention time

How can retention times be used to identify a compound?

Compare retention times for known components - different retention times, different compounds

What does peak integration tell you?

Area under each peak - proportional to the amount of compound present, can be used to determine concentration of components in sample

How can you find the concentration of a component in a sample in GC?

Compare its peak integration with values obtained from standard solutions of the component

How do you form a calibration curve for peak integration?

•Prepare standard solutions of known concentrations of the compound being investigated

•Obtain gas chromatograms for each standard solution

•Plot a calibration curve of peak area against concentration - external calibration, can be used to convert peak area to concentration

What reagent is used to test for alkenes?

Bromine water

What is the positive result for alkenes?

Bromine water decolourised from orange to colourless

What reagent is used to test for haloalkanes?

Add silver nitrate and ethanol and warm to 50°C in a water bath

What is the positive result for haloalkanes?

Chloroalkane - white precipitate

Bromoalkane - cream precipitate

Iodoalkane - yellow precipitate

What reagent is used to test for carbonyl compounds?

Add 2,4-dinitrophenylhydrazine

What is the positive result for carbonyl compounds?

Orange/yellow precipitate forms

What is the reagent used to test for aldehydes?

Add Tollens' reagent and warm

What is the positive result for aldehydes?

Silver mirror

What is the reagent used to test for primary and secondary alcohols and aldehydes?

Add acidified potassium dichromate (VI) and warm in a water bath

What is the positive result for primary and secondary alcohols and aldehydes?

Colour change from orange to green

What is the reagent used to test for carboxylic acids?

Add aqueous sodium carbonate

What is the positive result for carboxylic acids?

Effervescence