Question

Introduction Your instructor has prepared an extract of chloroplast pigments from fresh green grass or fresh spinach. A blender was used to rupture the cells, and the pigments were then extracted with acetone, an organic solvent. Work- ing with one other student, begin this exercise by separating the pigments extracted using paper chromatography. To do this, you will apply the pig- ment extract to a cylinder of chromatographic paper. You will then place the ylinder in a jar with the organic solvents petroleum ether and acetone. The solvents will move up the paper and carry the pigments along: the pigments will move at different rates, depending on their different solubilities in the solvents used and the degree of attraction to the paper. The leading edge of the solvent is called the front. Discrete pigment bands will be formed from the front back to the point where pigments were added to the paper. The following information will be helpful to you as you make predictions and interpret results: 1. Polar molecules or substances dissolve (or are attracted to) polar molecules 2. Nonpolar molecules are attracted to nonpolar molecules to varying degrees 3. Chromatography paper (cellulose) is a polar (charged) substance. 4. The solvent, made of petroleum ether and acetone, is relatively nonpolar. 5. The most nonpolar substance will dissolve in the nonpolar solvent first. 6. The most polar substance will be attracted to the polar chromatography paper; therefore, it will move last.
Pre lab

Answer 1

INTRODUCTION

Plant system contain various pigments which help in various processes such as photosynthesis. The primary pigments in the plants are Chlorophyll, xanthophyll and carotenoids, these pigmets are known to capture solar energy and convert it into the chemical energy.

Common characteristics of plant pigments

1)Chlorophyll a

it is most abundant pigment found in plants. it absorbs light with wavelength of 430 nm(blue) and 662 nm(red). it reflects green light, and hence this pigment appear green in colour.

2)Chlorophyll b

It is olive green coloured pigment,it absorbs light at wavelength 453 nm to 642 nm

3)beta- carotene

This is a class of accessory pigments that occur in all photosynthetic organisms. they absorb light between 460 to 550 nm and appear red orange in colour

4) xanthophylls

They are essentially oxidised carotenoids, they are usually red or yellow in colour and do not absorb light.

HYPOTHESIS

Above question is based upon the principle of Chromatography, that is differential migration of individual components of mixture(plant pigments) under stationary phase (which is water molecules trapped in cellulose paper in this case) and mobile phase (petroleum ether and acetone in this case ).

once the chromatogram is ready Rf value is calculated (Rf means retention factor and it shows the degree of affinity)

Rf =distance moved by solute from origin divided by distance moved by solvent from origin.

RESULTS

1) It is seen that chlorophyll b, which has polar structures move very less distance because it has high affinity for the chromatographic paper which is also polar and charged in nature. due to this very high affinity it is not affected much by the organic solvents. the similar explaination exist for chlorophyll a as it has structural similarities with that of chlorophyll b.

2) carotene move highest distance because of its good affinity for the non polar solvents. hence instead of getting stuck with the paper molecules it is carried away with the organic solvent phase.

3) xanthophyll has less affinity towards paper as compared to chlorophyll and hence is the third molecule to get separated out.

CONCLUSION

paper chromatography is a very good technique for separation of plant pigments. The pigments are moved and separated based on their affinities for mobile or stationary phase (which is decided by the chemical nature of the pigment, the polarity etc.). The Rf values suggest that carotenes are pigments which are most soluble in the solvent (petroleum ether and acetone) whereas chlorophyll b is least soluble.