In the previous section, we saw some methods for the purification of organic compounds. In this section, we will see chromatography.
• Chromatography can be used for different purposes:
♦ To separate a mixture into it’s components.
♦ To purify compounds.
♦ To test the purity of compounds.
• The word chromatography is derived from the Greek word ‘chroma’ which means ‘color’. This method was first used to separate colored compounds in plants.
• First we will see the different types of chromatography. After that, we will see the definition of chromatography.
Paper Chromatography
This can be explained in 12 steps:
1. Fig.12.114(a) below shows a strip of paper.
• A horizontal line is marked near the bottom of the strip. This line is known as the base line.
 |
| Fig.12.114 |
2.
We place a small quantity of a mixture on this base line. The quantity
is so small that, it looks like a spot. It is shown in the fig.b.
• Suppose that, the mixture contains three components A, B and C. We want to separate those three components.
3. We take a small quantity of a suitable organic solvent in a beaker.
♦ The solvent is shown in magenta color in fig.c
• Then we suspend the strip in the beaker. It can be suspended from a card board.
• The quantity of organic solvent in the beaker must be such that, the top surface of the solvent must be below the base line.
4. The solvent will rise through the paper strip due to capillary action.
• The components A, B and C in the sample will dissolve in the rising solvent.
• After some time we will see three spots as shown in fig.d.
♦ Spot A contains component A. It is nearer to the base line.
♦ Spot B contains component B. It is far away from the base line.
♦ Spot C contains component C. It is at an intermediate distance from the base line.
5.
So we have successfully separated the mixture into it’s components. The
theory behind such a separation can be written in 3 steps:
(i) The component B readily dissolves in the solution.
♦ Molecules of B has greater attraction towards the solvent.
♦ Molecules of B has lesser attraction towards the paper.
• So the molecules of B prefer to move upwards along with the solution.
(ii) The component A does not readily dissolve in the solution.
♦ Molecules of A has lesser attraction towards the solvent.
♦ Molecules of A has greater attraction towards the paper.
• So the molecules of A do not travel upwards much with the solution.
(iii) The component C is an intermediate case.
• Attraction between molecules of C and the solvent is
♦ greater than that in A
♦ lesser than that in B
• Attraction between molecules of C and the paper is
♦ lesser than that in A
♦ greater than that in B
• So distance traveled by C will be
♦ greater than the distance traveled by A
♦ lesser than the distance traveled by B.
6. We see that, for each component, there is a competition between two forces:
(i) The force of attraction between component and the solvent.
(ii) The force of attraction between component and the paper.
• If the first force is larger, the component will travel greater height
• If the second force is larger, the component will travel lesser height
◼ We can write:
The competition between the two forces causes the separation of various components.
7.
The above steps help us to understand paper chromatography. The strip of
paper used is known as chromatography paper. This paper contains
water trapped in it.
8. In this type of chromatography,
• The paper is the stationary phase.
♦ Because, the paper remains stationary.
• The solution taken in the beaker is the mobile phase.
♦ Because, the solution moves through the paper.
9. The final paper strip in fig.d is known as chromatogram.
10. In our present case, the initial mixture contained three components A, B and C.
• In some cases, there may be more than three components. In such cases, the chromatogram will contain more colored spots.
11. In some cases, some of the separated components may be color less. So they will not be visible.
•
In such cases, we place the chromatogram under ultraviolet light. The
color less components will absorb energy from the ultraviolet light.
After absorbing energy, they begin to emit visible light. This
phenomenon is called fluorescence. Thus the color less components can be
detected.
• Another detection technique is to place the
chromatogram in a jar containing a few crystals of iodine. The color
less components will absorb iodine and will become brown in color. Thus
they can be detected.
• Yet another detection technique is to spray
an appropriate reagent on the chromatogram. For example, amino acids can
be detected by spraying ninhydrin solution.
12. Consider the chromatogram in fig.e.
• We see that:
♦ The solvent has moved a distance of y from the base line.
♦ Component A has moved a distance of xA from the base line.
♦ Component B has moved a distance of xB from the base line.
♦ Component C has moved a distance of xC from the base line.
• From the above distances, we can calculate the retardation factor (Rf) of each component.
♦ Rf of A = $\frac{x_A}{y}$
♦ Rf of B = $\frac{x_B}{y}$
♦ Rf of C = $\frac{x_C}{y}$
• We see that, distance moved by the component is in the numerator. So if the distance is large, Rf will be large.
• If a component has a large distance, it indicates that, attraction of that component towards the chromatography paper is low.
• So we can write:
If a component has a large Rf value, it indicates that, attraction of that component towards the chromatography paper is low.
• We can define retardation factor as:
$R_f\;=\; \frac{\text{Distance moved by the substance from base line (x)} }{\text{Distance moved by the solvent from the same base line (y)}}$
Thin layer chromatography (TLC) is similar to paper chromatography. Instead of chromatography paper, a thin layer of silica gel or alumina is used. We can write the details in 11 steps:
1. Fig.12.115(a) below shows a glass plate coated with a thin layer (0.2 mm thick) of silica gel or alumina.
• The glass plate is known as thin layer chromatography plate or chromaplate.
• A horizontal line is marked near the bottom of the strip. This line is known as the base line.
 |
| Fig.12.115 |
2.
We place a small quantity of a mixture on this base line. The quantity
is so small that, it looks like a spot. It is shown in the fig.b.
• Suppose that, the mixture contains three components A, B and C. We want to separate those three components.
3. We take a small quantity of a suitable organic solvent in a jar.
♦ The solvent is shown in magenta color in fig.c
• Then we place the plate in the jar and close the lid.
• The quantity of organic solvent in the beaker must be such that, the top surface of the solvent must be below the base line.
4. The solvent will rise through the silica layer due to capillary action.
• The components A, B and C in the sample will dissolve in the rising solvent.
• After some time we will see three spots as shown in fig.d.
♦Spot A contains component A. It is nearer to the base line.
♦Spot B contains component B. It is far away from the base line.
♦Spot C contains component C. It is at an intermediate distance from the base line.
5.
So we have successfully separated the mixture into it’s components. The
theory behind such a separation can be written in 3 steps:
(i) The component B readily dissolves in the solution.
♦ Molecules of B has greater attraction towards the solvent.
♦ Molecules of B has lesser attraction towards the silica gel.
• So the molecules of B prefer to move upwards along with the solution.
(ii) The component A does not readily dissolve in the solution.
♦ Molecules of A has lesser attraction towards the solvent.
♦ Molecules of A has greater attraction towards the silica gel.
• So the molecules of A do not travel upwards much with the solution.
(iii) The component C is an intermediate case.
• Attraction between molecules of C and the solvent is
♦ greater than that in A
♦ lesser than that in B
• Attraction between molecules of C and the silica gel is
♦ lesser than that in A
♦ greater than that in B
• So distance traveled by C will be
♦ greater than the distance traveled by A
♦ lesser than the distance traveled by B.
6. We see that, for each component, there is a competition between two forces:
(i) The force of attraction between component and the solvent.
(ii) The force of attraction between component and the silica gel.
• If the first force is larger, the component will travel greater height
• If the second force is larger, the component will travel lesser height
◼ We can write:
The competition between the two forces causes the separation of various components.
7.
The above steps help us to understand TLC.
8. In this type of chromatography,
• The silica gel is the stationary phase.
♦ Because, the silica gel remains stationary.
• The solution taken in the jar is the mobile phase.
♦ Because, the solution moves through the silica gel.
9. In our present case, the initial mixture contained three components A, B and C.
• In some cases, there may be more than three components. In such cases, the plate will contain more colored spots in the final stage.
10. In some cases, some of the separated components may be color less. So they will not be visible.
•
In such cases, we can follow the same procedure that we saw in step 11 for paper chromatography.
11. For TLC also, we can measure x and y and calculate Rf values just as we saw in the case of paper chromatography.
Column Chromatography
Details about column chromatography can be written in 11 steps:
1. Fig.12.116 (a) below shows a glass tube fitted with a stopcock at the bottom.
 |
| Fig.12.116 |
2. First we put some cotton or glass wool at the lower end of the glass tube just above the tap. This is shown in fig.b
3. Next we pack the tube with alumina or silica gel. This is shown in fig.b
• The alumina or silica gel is packed in the form of a slurry. This slurry is the stationary phase.
• The cotton mentioned in the previous step (2) helps to prevent the flow of the stationary phase even when the stopcock is open.
4. Next step is to fill the tube with a suitable organic solution. This is shown in fig.d
5. Next we place the mixture to be analyzed, at the top of the packing. This can be done using a pipette. It is shown in fig.e.
Suppose that, the mixture contains three components A, B and C. We want to separate those three components.
6. After some time we will see three bands as shown in fig.f.
♦ Band A contains component A. It is nearer to the top.
♦ Band B contains component B. It is nearer to the bottom.
♦ Band C contains component C. It is at an intermediate distance from the top.
• Note that, we do not have to open the stopcock to get these bands.
7. So we have successfully separated the mixture into it’s components. The theory behind such a separation can be written in 3 steps:
(i) The component B readily dissolves in the solution.
♦ Molecules of B has greater attraction towards the solvent.
♦ Molecules of B has lesser attraction towards the silica gel.
• So the molecules of B prefer to move downwards through the solution.
(ii) The component A does not readily dissolve in the solution.
♦ Molecules of A has lesser attraction towards the solvent.
♦ Molecules of A has greater attraction towards the silica gel.
• So the molecules of A do not travel downwards much through the solution.
(iii) The component C is an intermediate case.
• Attraction between molecules of C and the solvent is
♦ greater than that in A
♦ lesser than that in B
• Attraction between molecules of C and the silica gel is
♦ lesser than that in A
♦ greater than that in B
• So distance traveled by C will be
♦ greater than the distance traveled by A
♦ lesser than the distance traveled by B.
• As mentioned earlier, we do not have to open the stopcock to get these bands. The bands move down through the solvent, due to gravity.
8. We see that, for each component, there is a competition between two forces:
(i) The force of attraction between component and the solvent.
(ii) The force of attraction between component and the silica gel.
• If the first force is larger, the component will travel greater depth.
• If the second force is larger, the component will travel lesser depth
◼ We can write:
The competition between the two forces causes the separation of various components.
9. Once the components are separated into different bands, we can open the stopcock. After opening, we pour the solvent gently from the top. The bands will move downwards. First we collect component B. Immediately after collecting B, we close the stopcock and put up a new container to collect C. In this way, by regulating the stopcock, we can collect all the components in separate containers. The components can be separated from the solution by distillation.
10. In analytic chemistry and organic chemistry, elution is the process of extracting one material from a mixture by washing the mixture with a solvent.
• This is exactly what we saw in the above steps. We washed the mixture with the solvent and extracted components A, B and C.
• The solvent is called eluent.
11. We often come across the term adsorption in chromatography.
• Adsorption is the adhesion of atoms or molecules on to a surface. As a result, a thin coating will be formed on the surface. The atoms or molecules do not go deeper below that surface.
• In our present case of column chromatography, the molecules of component A gets adsorbed onto the surface of the silica gel. The force of this adsorption is larger.
• The molecules of component B also gets adsorbed onto the surface of the silica gel. The force of this adsorption is weaker.
• The molecules of component C also gets adsorbed onto the surface of the silica gel. The force of this adsorption is intermediate.
Now we will see the definition of chromatography. It can be written in 4 steps:
1. Chromatography is an important technique used to separate mixtures into their components. It can also be used to purify compounds and to test the purity of compounds.
2. In this technique, the mixture is applied onto a stationary phase. This stationary phase can be a solid or a liquid.
3. A mobile phase is allowed to move slowly over the stationary phase. This mobile phase can be a pure solvent, a mixture of solvents or a gas.
4. The components of the mixture gets gradually separated from one another.
• Based on the principle involved, there are different types of chromatography.
• The most important types are: Adsorption chromatography and Partition chromatography.
• Column chromatography and Thin layer chromatography comes under the category of Adsorption chromatography.
• Paper chromatography comes under the category of Partition chromatography.
In the next section we will see qualitative analysis of organic compounds.
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