8. Chemical analysis (required practical 6 - chromatography) Flashcards
What is the practical investigating?
how paper chromatography can be used to identify substances
What are the steps for investigating how paper chromatography can be used to identify substances?
- Use a ruler to draw a horizontal pencil line onto the chromatography paper, around 2cm from the bottom of the paper
- Mark 5 pencil spots at equal spaces across the line, leaving 1cm clear at each side
- Use a capillary tube to place four known food colouring spots (A-D) onto the pencil spots and a fifth unknown (U)
- Place water into a beaker to a depth of 1cm
- 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
- Place a lid over the beaker to reduce evaporation of the solvent
- Wait for the water to take the dyes up the paper
- Remove the paper, allow to dry and draw the solvent front
Why must the sides of the paper not touch the side walls of the beaker?
as it will interfere with the way the water moves
What is the equation for calculating the Rf value?
Rf = distance moved by chemical / distance move by solvent
Do Rf values have a unit?
no
How do we identify the chemicals in these colours?
- calculate the Rf values
- look the Rf value up in a database, as it will tell us the identity of the chemical
How do you calculate the Rf value?
- measure the distance from the pencil line to the centre of each spot
- measure the distance moved by the water from the pencil line
- Use the equation for Rf
True or false. Rf values are always lower than 1?
true
If several different chemicals have the same Rf value, what could be done to narrow it down further?
repeat the experiment using a different solvent
If there is not the correct Rf value for the chemical on the database, what does this mean about the chemical?
that the chemical has never been analysed before
