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Unit 2 Assignment C
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- PEARSON (PEARSON)
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Separate to identifyIntroduction
I am a newly appointed technical assistant at a large chemical plant, Chemcalequip. As part of my
induction period and to progress in my role, I have to demonstrate skills in a range of practical
procedures and techniques. The company is often required to identify substances.
I need to demonstrate that I can carry out different chromatographic techniques to separate and
identify components in mixtures. In a report of the techniques I have used, I will need to explain the
techniques, analyse and evaluate my results and suggest improvements.
What is chromatography?
Chromatography is a process that is used for separating components of a mixture. For the entire
process to begin the mixture has to be dissolved in a mobile phase substance, which then gets
carried out into the stationary phase substance.
Chromatography is used to separate mixtures, identify substances and it also purifies liquids.
Chromatography works by separating substances in a mixture. The mixture is dissolved in a fluid, the
mobile phase, which carries it through a structure holding another material, the stationary phase.
The various components of the mixture travel at different speeds, causing them to separate. The
separation happens because different substances stick to the stationary phase at different levels.
Rf Values
The Rf value in chromatography is calculated by dividing the distance travelled by the substance by
the total distance travelled by the solvent. Rf values are useful because they help identify substances.
Each substance has a unique Rf value under set conditions, making it easier to identify each
substance.
Polarity
Polarity in molecules means there's an uneven distribution of electron density. This happens when
atoms in a molecule share electrons unequally, creating a dipole, or two poles: a positive pole where
there are fewer electrons and a negative pole where there are more. It's this polarity that determines
how molecules interact with each other. The size of a molecule can affect Rf values because larger
molecules often move slower through the stationary phase due to increased interactions, resulting in
lower Rf values.
Equipment:
- Pencil
- Strips of chromatography paper
- Strips of TLC plates
- Beakers
- Wooden stick
- Paper clip
- Propanone solution
- TLC solution
- Ruler
, Methods
Part 1a: Using chromatography to separate the
pigments in a leaf (Propanone)
Aim
The aim is to separate the pigments that are present in leaves by using paper chromatography
Risk Assessment
Risk Hazard Precaution If it goes wrong
Glass Sharp if broken Wear gloves Get a med kit
Propanone Irritant & highly Wear safety goggles & Tell a teacher and
flammable lab coat wash skin or eye.
1- Finely cut up spinach leaves and fill a mortar to about 2 cm depth.
2- Add a pinch of sand and about six drops of propanone from the teat pipette.
3- Grind the mixture with a pestle for at least three minutes.
4- On a strip of chromatography paper, draw a pencil line 2 cm from the bottom.
5- Use a fine glass tube to put liquid from the leaf extract onto the centre of the line. Keep the
spot as small as possible.
6- Allow the spot to dry, then add another spot on top. Add five more drops of solution, letting
each one dry before putting on the next. The idea is to build up a very concentrated small
spot on the paper.
7- Attach the paper to the pencil or splint using Sellotape so that when placed in the beaker,
the paper is just clear of its base.
8- Place no more than about 10 cm3 of running solvent in the beaker and hang the paper so it
dips in the solvent. Ensure the solvent level is below the spot.
9- Avoid moving the beaker in any way once the chromatography has started.
10- Leave the experiment until the solvent has soaked near to the top, and then remove the
paper from the beaker.
11- Mark how high the solvent gets on the paper with a pencil and let the chromatogram dry.
[This is the method provided by Exeter College]
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