REQUIRED PRACTICAL 6: Chromatography Flashcards
describe fully the practical
1 sample food colouring (mixture of chemicals) UNKNOWN (U)
4 KNOWN food colourings that sample 1 could contain. LABELLED (A-D)
first use a ruler to draw a horizontal line on the chromatography paper near the bottom of the paper just above the solvent (approx 2cm from the bottom)
mark 5 pencil spots at equal spaces across the line leaving at lease 1cm clear at each side
use a capillary tube to put a small spot of each of the unknown food colours and the unknown colour onto the pencil spots
pour water(solvent) into a beaker to a depth of 1 cm
attach the paper to a glass rod using tape and lower the paper into the beaker
the bottom of the paper should dip into the water
when the colours are carried up the chromatography paper, remove the paper when the water has travelled around 3 quarters up
use a pencil to mark the point where the water has reached
hang the paper up to dry
what is a capillary tube
a very thin glass tube
why is it important to keep the spots of food colouring relatively small
to prevent the colours from spreading into each other later
3 points that affect the experiment
the pencil line with the spots of ink must be above the surface of the water otherwise the water will wash the ink off the line
sides of the paper must not touch the side walls of the beaker otherwise it will interfere with the way the water moves
place a lid on the beaker to reduce evaporation of the solvent, the water will move up the paper and the colours will be carried up
becareful to NOT MOVE the beaker
explain the results
the unknown colour has separated into spots, spots aligned vertically will tell us that this is a mixture of three colours, spots aligned horizontally tells us that the unknown colour is a mixture of colours (e.g A, U, D)
if we wanted to identify the chemicals in these colours we calculate the Rf values
if the unknown colour has separated into 3 spots, what does this mean?
it’s a mixture of 3 colours
what does it mean for spots to line up with the unknown colour horizontally?
the unknown colour is a mixture of these spots
if a spot does NOT line up with the unknown colouring spots, what does this mean?
the unknown colour does NOT contain that colour
how do u calculate Rf values?
Measure distance from pencil line to the centre of each spot
measure distance moved by the water from the pencil line
Rf= distance moved by chemical/ distance moved by solvent
look this Rf value up in a database and that will tell us the identity of the chemical
how do we identify chemicals in the colours?
by calculating their Rf value
what is the unit for an Rf value
Rf values have no units
many different chemicals could have the same Rf value, how can we further narrow the result down
we might need to repeat the experiment using a different solvent to narrow it down further to a more accurate result
why might there not be an Rf value on a database for a chemical?
because the chemical has never been analysed before
