C7.4 chemistry full

Qualitative analysis is used to find out which chemicals are in a sample.

Quantitive analysis is used to find out the amounts of chemicals in a sample.

chemists use standard methods to collect, store and prepare samples. 

- Collection: the sample must represent the bulk of the material.

- Storage: The sample must not 'go off' or be tampered with or contamiated.

- Preparation: The sample is often dissolved in a solution.

Chromatography can:

- separate and identify the chemicals in a mixture.

- Determine the purity of the chemical.

In chromatography a mobile phase moves through a stationary phase.

A sample is added to the stationary phase. The mobile phase flows along. This makes the chemicals in the sample move through the stationary phase. Each chemical moves at a different speed. So the chemicals separate.

The position of an equilibrium determines how quickly a chemical moves:

- If the equilibrium lies towards the mobile phase, the chemical moves quickly.

- If the equilibrium lies towards the stationary phase, it moves slowly.

In Paper chromatography the stationary phase is chromatography paper.

In Thin layer chromatography the stationary phase is an absorbent solid supported on glass or plastic.

In both, the mobile phase is a solvent. It may be an aqueous solvent ( water ) or a non- aqueous solvent ( no water ).

TLC can be used to analyse dyes from clothing fibres and to test for drugs, by forensic scientists. PC is rarely used.

1  prepare the paper or slide.

- dissolve the sample in a solvent                                                                           - Draw a pencil line on the paper or the plate                                                         - put a drop of sample solution on the line                                                            - Put drops of reference material solutions on the line. A reference material is a    material you suspect might be pesent in the unknown mixture.

2  Run the chromatogram.

- When the solvent gets near the top take out the TLC plate/ paper.                         - Mark the solvent front.   

3  Locate the separated chemicals

- Use a pencil to draw round coloured spots.                                                              - Use a locating agent or ultraviolet lamp to locate colourless spots.

4  Interpret the chromatogram.

- Identify spots by comparing them with those from the reference materials.          - Calculate the chemicals Retardation factor.

RF = distance moved by chemical / distance moved by solvent.

Gas ghromatography seaprates compex mixtures and detects tiny amounts of chemicals. Chemists use GC qualitatively and quantatitvely. 

The stationary phase is a thin film of liquid on the surface of a powdered solid. The powder is packed into a log, thin tube. The mobile phase is called the carrier gas and is helium.

To analyse a sample by GC: 

1  Turn on the oven and wait for the column to reach the correct temperature.

2  Turn on the carrier gas and adjust its pressure to get the correct flow rate.

3  Inject the sample at the start of the column, where the column enters         the oven. The chemicals in the sample become gases and mix with the carrier       gas. The mixture passes through the column to the detector. 

4  interpret the chromatogram. 

- each peak on the graph represents one compound.                                          - The position for a peack represents its Retention time ( time taken to pass           through the column)                                                                                          - The height of a peak represents the relative amount of that compound in the      sample.

Chemists use titrations to measure concentration, to check purity and to find out the amounts of chemicals that react together.