Chromotography Flashcards
What is paper chromatography used for
To separate mixtures of soluble substances and to provide information on the possible identity of the substances present in the mixtures
What are the substances often like in chromatography?
Coloured such as food colourings, inks, dyes or plant pigments
Phases of paper chromatography
Spots of ink or plant dye are placed on a pencil line, as the paper is lowered into the solvent some of the dye spreads further up the paper and the paper has absorbed rhe solvent and the dye has spread further up the paper
What two phases does chromatography rely on
The mobile phase and the stationary phase
What is the mobile phase
The solvent that moves through the paper carrying different substances with it
What is the stationary phase
It is contained on the paper the solvent does not move through it
What causes the dissolved substances to move at different rates through the paper during chromatography
The dissolved substances are attracted to the two phases in different proportions
What does separation by chromatography produce
A chromatogram
What can a paper chromatogram distinguish
Pure and impure substances
How do we know if a substance is pure using a chromatogram
It produces one spot on the chromatogram
How do we know if the substance is impure using the chromatogram
The substance produces two or more spots
How else can a paper chromatogram be used to identify substances
By comparing them with known substances
How are two substances likely to be the same
If they produce the same number of spots and match in colour and if the spots travel the same distance up the paper (have the same Rf value)
Why can Rf values be used to identify unknown chemicals
The Rf value is always the same for a particular substance if they run in the same solvent
What is the equation for the Rf value of a spot?
Distance travelled by substance divided by distance travelled by solvent
If the Rf value is 0 what does it mean
The substance is not attracted to the mobile phase
If the Rf value is 1 what does this mean
The substance is not attracted to the stationary phase
What do Rf value vary from
0-1
Chromatography method
Draw a pencil line across the chromatography paper one to 2 cm from the bottom, use a pipette or capillary tube to add small spots to the line on the paper, place the paper into a container with a suitable solvent in the bottom, allow the solvent to move through the paper but remove the chromatogram before it reaches the top and allow the chromatogram to dry and measure the distance travelled by each spot and by the solvent
What is the harm and precaution taken from a harmful solvent
Skin irritation or breathing difficulties and avoid skin contact e.g. by wearing gloves and ensure adequate ventilation or use a fume cupboard
