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Unit 2c - chromatography
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Unit 2c - chromatography This paper got graded as a distinction All the points in the pearsons btec level 3 applied science specification are included
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Btec Applied ScienceUnit 2c - Chromatography
Unit 2c - Chromatography
Chromatography is an analytical method that separates components from a mixture.
To begin with, there is the mobile phase, which is where the mixture is dissolved in a
solvent, which carries it through a second substance called the stationary phase. [1]
Industry: [2]
In the industry, chromatography is used for many different reasons, such as:
- Separating compounds
- Testing products for quality control
- Testing for trace amounts of substances
- Purifying chemicals
- Analysing components in the food industry
There are many different ways to carry out chromatography, including paper
chromatography, thin layer chromatography, high performance liquid
chromatography and gas chromatography.
Stationary phase:
The strip or piece of paper placed in the solvent. It does not move.
Mobile phase:
This is the phase where the solvent moves through the paper, carrying different
substances with it.
Rf value:
The Rf value is used to compare and identify compounds. It is equivalent to the
distance travelled by the compound divided by the distance travelled by the solvent
front.
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,Btec Applied Science
Unit 2c - Chromatography
Adsorption:
Adsorption describes the bonds which are formed between one substance and a
surface - however, these bonds are not permanent. It is known as “sticking to the
surface”. In adsorption chromatography, the mobile phase is usually a liquid or a gas,
and the stationary phase is a solid. This means that the gas or liquid sticks to the
surface of the solid. The compounds are constantly moving between this stationary
and mobile phase. A material (the adsorbate) travels from a liquid or gas phase, to
form a superficial monomolecular layer on the surface of a solid. [3]
When the compounds are dissolved in the solvent (mobile phase) they are able to
move. When this compound is adsorbed by the surface, it is temporarily stopped but
the solvent keeps on moving. Therefore, the more strongly a compound is adsorbed,
the less distance it can travel up the chromatogram.
Due to the stationary phase being a solid, the pigments are adsorbed onto its
surface via the solid particles or fibres. Separation occurs due to the relative forces
of attraction between the stationary phase, mobile phase and the pigment.
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2mZAOcHkgpOZ0RR6XxpHAJrVXT5y5IDlQH9NBdxfalU6fagT
Choice of developing solvent and vessel:
Solvent -
First of all, the compound you are testing must be soluble in the solvent, otherwise it
would be permanently adsorbed. As well as this, the solvent should be less polar
than the compound. Different solvents will dissolve different substances. For
example, water (a polar solvent) will dissolve water soluble ink (polar substances).
This is the same for non polar solvents and substances. Common liquid solvents are
water, isopropanol, and methanol.
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, Btec Applied Science
Unit 2c - Chromatography
We chose our solvents because they allowed the mixture we were experimenting on
to be absorbed, leading to separation. We also considered the above reasons when
choosing solvents. [4]
Vessel -
The vessel is what will hold the solvent and the chromatogram plates. It must be big
enough to hold the chromatogram plates, as well as enough solvent and a lid or
cover. It is advisable to have a clear, see-through vessel so that you can see the
separation of pigments take place. The most common vessels used are glass
beakers or vases, along with a lid or plastic covering. For our experiments, we used
a large glass beaker to hold our solvent and chromatogram, and used an empty
glass petri dish for our lid. [5]
Preparative methods for samples: [6]
Solvent extraction -
Solvent extraction is the ability to separate two compounds based on their solubilities
in two different immiscible liquids, such as water and an organic solvent. A liquid,
known as the solvent, is used to dissolve a target molecule so that they can then be
separated. An example of how solvent extraction works is with petrol and water:
When petrol is added to water they do not mix because they are both immiscible.
However, if a chemical compound is already dissolved in the water but it can
dissolve more readily in the petrol, when the mixture is shaken the compound moves
to the petrol. After this mixture has been left to stand, the water and petrol will
separate and the chemical compound will be dissolved in the petrol.
Filtration -
A mixture of solids and liquids must be filtered. This is done by pouring the mixture
through fluted filter paper that has been placed in a funnel. The liquid will flow
through the filter paper but the solids are insoluble so residue will be left behind in
the filter paper. This left over residue is then rinsed in the liquid again and passed
through the filtration paper. To improve filtration results, the correct porosity (volume
of holes in a material / total volume of the material) has to be chosen for the mixture
passing through it and the length of time this process takes. To speed up this
process, vacuum filtration is sometimes used.
Concentration by evaporation -
Evapouration cycles can be used to increase the concentration of the sample that
needs to be separated. This means that the identification of the substance can be
determined with more clarity. Evaporation is the process of separating a soluble
chemical compound by removing excess water from the chemical solution. This is
achieved via heating the sample. The heating process must be done carefully and
slowly, otherwise over-evapouration could occur.
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