Chromatography
The experiment's objective is to provide evidence that I can accurately carry out chromatography to
calculate Rf values and use them to draw conclusions.
The theory of chromatography:
Chromatography is used to separate mixtures into their components based on their interactions with
a stationary and mobile phase: The stationary phase is a material that a sample of the mixture is
placed on. The mobile phase is typically a solvent which flows through the stationary phase and
carries the mixture as it travels.
For the mobile phase to separate the components of the mixture, it must be of low viscosity to
solubilize the sample. Once the sample is solubilized and starts travelling along the stationary phase,
the components of the mixture travel at different rates. The stationary phase should remain in a fixed
place as it acts as a constraint on the mixture components, ensuring they move slower than the
solvent. This allows the mixture components to fully interact with the stationary phase because the
polarity of the component molecules being separated, and the polarity of the stationary phase are
essential for the components to separate.
Electronegativity is the ability of an atom to attract a pair of electrons in a covalent bond. If the
atoms of a molecule have a similar electronegativity (below 0.5), they will attract each other with
equal force; this is called a non-polar molecule. If the atoms of a molecule have a significant
difference of electronegativity (0.5 to 2.1), the electrons will shift towards the more electronegative
atom meaning that one atom will be slightly negative (delta minus) while the other is slightly positive
(delta plus); this is called a polar molecule. Intermolecular forces are the attraction between
molecules. Non-polar molecules form weak intermolecular forces (Van Der Waal forces) while polar
molecules form strong intermolecular forces (induced dipole-dipole forces).
The chromatography results are called a chromatogram which allows you to perceive the
components of the mixture and measure how far each component has travelled. These
measurements allow you to calculate the Rf which is a value that varies from 0-1 and determines
how far the component has travelled in comparison to how far the solvent has travelled – distance
travelled by component/distance travelled by solvent. Components that are more soluble travel
further than those that are less soluble. So, the Rf value and solubility are directly proportional.
There are several factors that affect the solubility/Rf value of a component:
The polarity of the component molecules:
If the molecules of a mixture component are polar, the intermolecular forces between the
component and stationary phase will be stronger meaning more energy is required to overcome
these intermolecular forces. So, the component would spend more time attached to the stationary
phase hence less time in the mobile phase and has a low Rf value. Non-polar molecules have weaker
, attraction to the stationary phase hence not much energy is required to overcome the intermolecular
forces, and the component spends less time attached to it. So, non-polar components spend more
time within the mobile phase and have a higher Rf value.
Size of molecules:
A molecule's overall size physically influences its solubility. Components consisting of larger
molecules have a low Rf value as they require more energy to move and do not travel as far.
Components consisting of smaller molecules have a higher Rf value as they do not need as much
energy to move, hence travel further.
Experiment technique:
Procedures such as calibration must be carried out to obtain accurate results. Calibration is the
process of checking a measuring instrument to ensure there are no faults that could affect the
accuracy of the results. You must calibrate the equipment being used throughout the experiment; if
not, the results will have a higher uncertainty. Each piece of equipment has a calibration method
individual to its function:
Piece of equipment Calibration method
Beaker/conical flask/measuring cylinder Rinse out with distilled water to remove any
residue. Also, ensure there are no cracks/chips
because if there is product could be lost.
Dropping pipette Repeatedly pipette distilled water to remove
any residue. Ensure there are no water droplets
within the pipette before using it.
Experiment no.1: Paper chromatography of ink
In this experiment, the mixture being separated into its components is ink. The stationary
phase is the paper which absorbs the mixture while the mobile phase that carries the
soluble components down the paper is ethanol. A disadvantage of paper chromatography
is that it is prone to smearing.
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