Chromatography of leaves- Chemistry Solved Assignment Solution Sample

QUESTION

 

Chromatography of leaves

Introduction

Most leaves are green due to chlorophyll. This substance is important in photosynthesis (the process by which plants make their food). In this experiment, the different pigments present in a leaf are separated using paper chromatography.

Protocol

1. Finely cut up some leaves and put into a mortar.
2. Add a pinch of sand and six drops of propanone from the teat pipette.
3. Grind the mixture for at least three minutes.
4. On a strip of chromatography paper, draw a pencil line 1 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. Put a small amount of propanone in a beaker and hang the paper so it dips in the propanone. Ensure the propanone level is below the spot.
8. Leave until the propanone has soaked near to the top.
9. Mark how high the propanone gets on the paper with a pencil and let the chromatogram dry.
10. Draw circles around any spots which appear on the paper.

Apparatus

Mortar and pestle.

Chromatography paper.

Teat pipette.

100cm³ beaker.

Capillary tube.

Safety

Wear eye protection and prior to starting the experiment obtain the appropriate COSHH data. Propanone is highly flammable.

Record

The chromatogram produced in this experiment can be dried and kept.

Report

Write-ups should be set out in the usual way. Each person in your team must submit an individual report. In your report of this laboratory exercise you need to include:

Record

1. The number substances are on the chromatogram
2. Their colours
3. The distance the spots moved
4. The Retention Value (Rf) of the spots

Questions

1. Why did the separation of pigments in the leaves extract occur as it did? (i.e. How does paper chromatography work?)
2. What would happen if we used hexane as a solvent?
3. Why are chlorophyll a & b green and why do the other species have different colours?
4. Why do plants have more than one pigment?
5. Based on what you have learned in this lab, explain why leaves tend to change colour in autumn.

In your write-ups please state clearly who you worked with.

Please supply relevant MSDS and COSHH sheets.

Before your team leaves the laboratory, hand in any sample and then the demonstrator will check that your bench is clean and tidy.

ANSWER

Title: Paper Chromatography of leave

Lab partner: XXX and YYY

Date: xyz

 

Purpose

The purpose of this experiment is to determine retention value (factor) of solute particle (leaves) which has spotted on chromatography paper.

INTRODUCTION

Paper chromatography, in analytical chemistry, technique for separating dissolved chemical substances by taking advantage of their different rates of migration across sheets of paper. It is an inexpensive but powerful analytical tool that requires very small quantities of material.

The method consists of applying the test solution or sample as a spot near one corner of a sheet of filter paper. The paper is initially saturated with some suitable solvent to create a stationary liquid phase. An edge of the paper close to the spot is then immersed in another solvent in which the components of the mixture are soluble in different degrees. The solvent penetrates the paper by capillary action and, in passing over the sample spot, carries along with it the various components of the sample. The components move with the flowing solvent with different velocities that are dependent on their solubilities in the stationary and flowing solvents. Before the flowing solvent reaches the farther edge of the paper, both solvents are evaporated, and the location of the separated components is identified, usually by application of reagents that form coloured compounds with the separated substances. At the end of the experiment one can determine retention factor which can be calculated by distance spot moved divided by distance solvent moved.

Retention value =

Retention factors are useful in comparing the results of one chromatogram to the results of another. If the conditions in which the chromatogram are run are unchanged (same mobile and stationary phases), the retention factor for a given material should remain constant. This allows unknowns to be compared to known materials. If the retention factor of an unknown does not match that of a known material, they are not the same compound. The retention factors will vary slightly from sample to sample. Interactions of the individual components with each other and the concentration of the component in the sample will both affect the Rf value.

Materials used

• Chromatography papers

• Teat paper

• Capillary tube

• 100cm³ beaker.

• Mortar and pestle

• Pencil

• Pinch of sand

• Propanone

 

Method

Collect leaves and Finely cut up some of them and put into a mortar. Now add a pinch of sand and six drops of propanone solution from the teat pipette. Then grind the mixture for at least three minutes. Now, take chromatography paper and on a strip of chromatography paper, draw a pencil line 1 cm from the bottom. The next step is to put liquid from the leaf extract onto the centre of the line by using a fine glass tube to Keep the spot as small as possible. Then, 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. Now, put a small amount of propanone in a beaker and hang the paper so it dips in the propanone by ensuring the propanone level is below the spot. leave the setup until the propanone has soaked near to the top. Mark how high the propanone gets on the paper with a pencil and let the chromatogram dry. Draw circles around any spots which appear on the paper. Now calculate the retention value by below equation.

Retention value =

{left (distance spot moved right )} over {(distsnce solvent moved)}

Results & discussion

Different solvent used to have different results.

No. of substances (leaves solution) with color and distance of spot moved which can seen in image below.

Substances with color	Distance of spot moved
Dark Green	4.5cm
Light Green	4.6cm
Yellow	4.4cm

 

 

RETENTION TIME OF EACH SUBSTANCES:

1. Retention time of dark green colour substance = 4.5/4.8 = 0.94

2. Retention time of light green colour substance= 4.6/4.8 = 0.96

3. Retention time of Yellow colour substance = 4.4/4.8 = 0.92

Where the distance of solvent (propanone ) travelled = 4.8cm

 

According to the retention time of three substance light green colour has more retention time of 0.96 compare to other two substances which is dark Green and Yellow which indicates that light green substance has more affinity towards stationary phase (chromatography paper). Also, one can say that yellow has low retention time which indicate that yellow colour substance is more polar substance compare to other two substances because it has more affinity towards stationary phase which is solvent (propanone).

 

Questions

1. Why did the separation of pigments in the leaves extract occur as it did? (i.e. How does paper chromatography work?)

Answer. Because different substances has different affinity towards stationary phase and mobile phase according to their polarity and concentration. That is why different substances has different velocity with mobile phase and this is how they separated from each other by chromatography paper. Here, Light green substance is less polar than other two substances so, it has more retention time because of less affinity towards polar mobile phase.

2. What would happen if we used hexane as a solvent?

Answer. Hexane is non-polar solvent. If hexane used as mobile phase then light green color would have less retention time as it is more non-polar among three substances and yellow color would have more retention time compare to other two substances.

3. Why are chlorophyll a & b has green and why do the other species have different colours?

Answer. Different compounds or components absorb different range of wavelength from visible range spectrum (400-800nm). Chlorophyll a & b pigments allows a broader range of wavelengths to be absorbed, and thus, more energy to be captured from sunlight which absorb between 55-650nm wavelength and appear as green where other compound has different color.

4. Why do plants have more than one pigment?

Answer. Plants depend strictly upon light energy to make food. Since, light is a combination of various wavelengths, so leaves are exposed to almost all of them. Now, chlorophyll absorbs only a particular wavelength of light to make energy and rest is wasted. To account for such a loss, other accessory pigments are present, like: carotenoids, fucoxanthin etc.

5. Based on what you have learned in this lab, explain why leaves tend to change colour in autumn.

Answer. Leaves are colored by molecules called pigments. The pigment that causes leaves to be green is chlorophyll. Chlorophyll is important for plants to make food using sunlight. During spring and summer when there is plenty of sunlight, plants make a lot of chlorophyll. In autumn when it starts to get cold, some plants stop making chlorophyll. Instead, those plants break down chlorophyll into smaller molecules. As chlorophyll goes away, other pigments start to show their colors. This is why leaves turn yellow or red in fall.

Conclusion

In conclusion the experiment showed that three substances which has different concentration and polarity are separated by Paper chromatography method. Where different color substances have different retention time according to their affinity toward mobile phase. It indicate that separation of component depends on chosen solvent or mobile phase because different solvent has different polarity and according to that polarity we can separate variety of substances.

 

References

1. BRITANNICA. (2018).Introduction of paper chromatography. (Dipti Mhajan, 2018) Retrieved from: https://www.britannica.com/science/paper-chromatography

2. Journal of Chromatographic Science. (1973). “Paper Chromatography- Past, Present and Future” (Gunter Zweig,1973, volume-16, page- 279-283).

3. Journal of Chemical Education. (1953). “Paper Chromatography for student” (R. Quentin Blackwell, 1956, 30(12), page-614).

4. Journal of Chemical Education. (2012). “Normal and Reverse phase Chromatography of leaf extracts” (Maria. H. Du. Toit, (2012), 89 (10), page 195-196).