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Applied Science - Unit 2: Aim C
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- Unit 2 - Learning aim C
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- PEARSON (PEARSON)
This is My Coursework for Applied Science - Unit 2: Aim C. I got a Distinction on this Coursework. Hope this helps!
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Unit 2: Practical Scientific Procedures and TechniquesSeparate to identify
Learning Aim C: Undertake chromatographic techniques to identify components in mixtures
Introduction
In this report I am going to be looking at all the various ways on how to separate mixtures
and identify substances. I will be looking at the different types of chromatography: mainly
Paper chromatography and Thin Layer chromatography. I will give a detailed method for my
practicals and make an equipment list and risk assessment. I will also write the problems I
encountered and the improvements I should make to my practical so that it becomes more
precise. My practicals will be looking on how to separate amino acids and extracted plant
pigments. I will also be explaining the theory behind chromatography.
Chromatography
Chromatography is a process used to separate mixtures and identify substances, and from
finding those you can find the purity of a substance. There are many types of chromatography
- like paper chromatography, thin layer chromatography, gas chromatography and ion
exchange chromatography. Even though there are different types of chromatography they all
have a mobile phase and a stationary phase; a stationary phase is usually a solid that absorbs
the mixture flowing through and the mobile phase is usually a gas or a liquid that transports
the substance mixture through the stationary phase. There are 2 main types of
chromatography: liquid chromatography and gas chromatography, liquid chromatography
consists of the mobile phase being a liquid likewise gas chromatography consists of the
mobile phase being a gas. The way chromatography works is that as the mobile phase carries
the mixture through the stationary phase the mixture gets separated into different substances.
This is because the different (separate) substances are attracted to either the mobile phase or
the stationary phase, this causes them to travel at different rates which means that some
substances move further than others in the set period of time. Learning Aim C, C.P5 and C.P6
Principles of paper chromatography
Paper chromatography is a type of liquid chromatography as the mobile phase is a liquid.
Paper chromatography is used to separate the different substances in a mixture. The principle
in paper chromatography is almost always partition chromatography. Partition
chromatography is the separation (partition) of the substances in the mixture between 2 liquid
phases. The 2 liquid phases used in paper chromatography is the mobile phase, as this is the
liquid which carries the mixture up the paper, and the other phase is the water held in the
pores of the chromatography paper – as the chromatography paper has a thin coating of water
molecules. However, sometimes the principle can also be adsorption chromatography – this is
the separation (adsorption) of the substances in the mixture between a solid and a liquid
phase. The solid phase is the stationary phase which is the solid surface of the paper, and the
liquid phase is the mobile phase. Adsorption is the process in which the substance (mainly a
gas but it can be a liquid as well) forms a thin layer on the surface of a solid. In paper
chromatography the stationary phase is the chromatography paper, and the mobile phase can
be an aqueous (water-based) liquid like for example water, or it can be a non-aqueous organic
(carbon-based) solvent (a substance, usually a liquid, that can dissolve other substances) like,
,Unit 2: Practical Scientific Procedures and Techniques
for example, acetone. How the substances get separated from the mixture depends on their
differences in their affinity (strength of attraction between 2 substances) towards water, if a
substance has a high affinity for the water, then the substance will travel at a fast rate – this
substance can be said to have a low affinity for the chromatography paper, and likewise if a
substance has a low affinity for the water, then it will travel at a slow pace – this substance
can be said to have a high affinity for the chromatography paper. Also, some substances can
have some affinity for both water and the chromatography paper this substance will travel at
an intermediate pace. Learning Aim C, C.P5 and C.P6
Principles of thin layer chromatography (TLC)
Thin layer chromatography (TLC) is also a type of liquid chromatography as the mobile
phase is a liquid. TLC is also used to separate the different substances in a mixture usually
non-volatile (substance which does not vaporise easily) mixtures, but it is also used to
identify the purity of the substances. The principle in TLC is adsorption chromatography -
this is the separation (adsorption) of the substances in the mixture between a solid and a
liquid phase. The solid phase is the stationary phase which is the glass or plastic plate coated
with silica or alumina gel. And the liquid phase is the mobile phase which is the solvent used
in the practical (I used acetone). Adsorption is the process in which the substance (mainly a
gas but it can be a liquid as well) forms a thin layer on the surface of a solid. Alumina and
silica are adsorbent materials – this means that they are capable to adsorb (gather the liquid or
dissolved substances to the surface of the solid in a thin layer) another substance. How it
works is similar to paper chromatography, the substances get separated from the mixture
depending on their differences in their affinity towards water, if a substance has a high
affinity for the water, then the substance will travel at a fast rate – this substance can be said
to have a low affinity for the TLC plate , and likewise if a substance has a low affinity for the
water, then it will travel at a slow pace – this substance can be said to have a high affinity for
the TLC plate. Also, some substances can have some affinity for both water and the TLC
plate this substance will travel at an intermediate pace. Finally, once separation has happened
the individual substances are seen as spots at different position on the TLC plate depending
on their rate of travel. Learning Aim C, C.P5 and C.P6
Chromatograms comparison between paper chromatography and TLC
Paper chromatography and TLC both have advantages and disadvantages. But TLC has some
advantages over paper chromatography: The first advantage is that the mobile phase in TLC
moves more quickly through the stationary phase than in paper chromatography. This is
because there is less diffusion occurring in TLC than in paper chromatography, so clear spots
are formed, and TLC has faster development. The reason TLC has less diffusion is because
TLC has a more rigid coating (glass or plastic plate), and this contributes to the rate of
development. The second advantage of TLC is that the mobile phase moves more evenly
through the stationary phase in TLC than in paper chromatography. This is also because of
less diffusion occurring in TLC. Another advantage is that there are a range of absorbencies
in TLC for the stationary phase compared to paper chromatography.
A chromatogram is the result, it shows the substance separated at the end on the paper or
plate. Useful chromatograms are the ones where it shows greater separation of the substances
in the mixture, and this makes it easier to analyse. TLC tends to produce more useful
, Unit 2: Practical Scientific Procedures and Techniques
chromatograms than paper chromatography as TLC does show greater separation of the
substances than paper chromatography and this allows TLC to be analysed more easily.
To have more dependable results (be able to repeat those results), it will be linked to how
constant you keep: The solvent used (in my case acetone), the amount of stationary phase
(adsorbent) used on the plate, the amount of substance spotted onto the plate and how
constant you keep the temperature controls. Learning Aim C, C.P5 and C.P6
Preparative methods for samples
There are many methods that you can use to prepare samples:
Solvent extraction – this is a method that separates compounds based on their solubilities in 2
different immiscible (liquids that do not mix) liquids.
Filtration – this is a method used to separate insoluble solids from a solution. In this method
filter paper is used and this is a semi-permeable paper barrier. The filter paper is placed
perpendicular to the flow of the liquid, the liquid goes through the filter paper and there
should be solid residue still on the filter paper. Then rinse the residue off the filter paper and
pass the liquid through the filter paper again. For this to be as accurate as possible you will
need to choose the filter paper with the correct porosity (volume of tiny holes in a
material/total volume of the material). This will affect the amount of time needed for it to
filter and it will affect the purity of the solution. Vacuum filtration is used sometimes to
speed up the process.
Evaporation – this is a method used to separate a mixture (solution) of a solute (solid which
dissolves into a solvent to make a solution) and a solvent. And this will be done by heating
the solution or mixture, the heating must be carried out carefully so that over-evaporation
does not occur. If you do this method repeatedly it can lead to an increased concentration of
the sample to be separated which then allows you to identify the substance more clearly.
Locating agents
A locating agent is used to help analyse colourless substances, such as amino acids, by
making them visible. Locating agents are chemical substances which are added to the sample
by spray, then the locating agent should react with the colourless substances and change its
colour as a result.
Theory behind chromatography
Substances can be separated between polar and non-polar. Polar molecules release positive
charge on one end of the molecule, and they release a negative charge on the other. Non-polar
molecules do not have opposite positive and negative poles, instead they are equally charged,
and they consist of the molecules not having a dipole in contrast to polar molecules all form
dipole forces. Water is a polar molecule as the hydrogen atoms are covalently bonded to an
oxygen atom and that produces a net positive and negative charge. More polar solvents allow
the substances of the mixture to move faster than less polar solvents.
Molecule size is a factor in its ability to be soluble in water. The molecule size can affect the
speed at which a substance can dissolve. Smaller molecules dissolve more quickly in a
solvent. Temperature is another factor of solubility in water, when the temperature increases
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