[Chemistry Class Notes] on Separation of Pigments of Leaves and Flowers using Chromatography Technique Pdf for Exam

In this article the chromatography technique can be widely used to separate, purify, and also identify the compounds. The substance balances in the chromatography between a stationary phase and a mobile phase. Following are the different types of chromatography techniques:

• In paper chromatography, paper soaked in liquid and a liquid are used as filters in the mobile and stationary phase to separate the components of the mixture. After the separation, the other components leave a spot on the paper.

• In the thin layer chromatography technique, glass or metal is coated with a thin layer of aluminium or silica based on the availability and this sheet is used as a filter to separate the components of the mixture.

• The column chromatography technique is similar to the layer chromatography technique but the only difference is the time taken for the process. 

Aim

The aim is to separate the pigments that are present in flowers and leaves by the paper chromatography technique and determine their R_f values.

Theory

Paper chromatography is mainly partition chromatography. In this paper chromatography, the stationary phase is defined as paper. The paper contains 22 per cent of water molecules absorbed on about 78 per cent of cellulose.

The components mixture separation takes place by partitioning the components between the mobile phase and stationary phase. The mobile phase travels via paper by the capillary action. Depending on the ways the solvent travels on the paper, there exist three types of chromatography, given as follows.

• Ascending Paper Chromatography

• Descending Paper Chromatography

• Circular Paper Chromatography

The distribution takes place in a definite ratio that represents the solution’s characteristic distribution coefficient.

The Rf coefficient ration is given as follows:

R_f=distance travelled by solute, distance travelled by the solvent

Various substances possess various R_f values. R_f depends on several factors which are listed below.:

If any compound is coloured, it can be easily located on chromatographic paper. If the same substance is colourless, however, a reagent can be used to treat it, which produces a characteristic colour. The term developer is given to this reagent. And iodine is the most frequently used paper chromatography developer.

Materials Required

Apparatus Setup

()

Procedure

• Take the Whatman filter paper and then draw a line using a pencil above 4cm from one end.

• Then, grind the flowers and leaves in a motor and transfer the paste into the test tube.

• In the crushed material, add methanol or acetone, shake well and filter the respective mixture.

• The filtrate is collected in a test tube to perform experiments.

• With the help of a capillary tube, add one drop of the filtrate on the filter paper and allow it to dry.

• After that, hang the filter paper in a jar containing 20ml of chloroform and petroleum ether.

• Keep the same jar till the mobile phase rises to 2/3rd of the length of the paper.

• Then, remove the filter paper from the jar and mark the solvent front.

• Outline the spots using a pencil and allow the filter paper to dry.

• Now, measure the distance between the centre of different spots and the solvent front about the reference line as indicated.

• Then, determine the pigment number in the flowers and leaves extract.

• Finally, calculate the R_f value of different spots using the expression.

Observations and Inference

The noticed Observations and Inferences can be recorded in the respective fields of the table, which is given below.

Results and Discussions

Precautions

• Always choose a fine capillary tube.

• Do not allow the spots to spread while spotting the test solution available on the paper.

• Do not disturb the jar once the experiment is arranged as long as the development of the chromatogram has been completed.

• Use the capillary finely drawn to place the spot on the same paper.

• Prior to developing the spots, allow the paper strip to become dry perfectly.

• Handle the organic solvents carefully.

Finding the Number of Pigments in Leaves with the Help of Chromatography

If there exist more samples, it’s spots are bigger on the TLC plate.

Like when we spill coffee on a white shirt , the more the coffee, the larger the spot! In general, on a shirt, the water carries the solute, which is considered a poor example.

Remember that a sample is applied in a tiny spot that goes to thin-layer chromatography (TLC) glass or near the bottom of the paper or the plastic plate. Then, the plate is placed carefully in the appropriate solvent and moves in an upward direction uniformly. Solutes, which are otherwise called pigment molecules, dissolve. After that, move upward at various rates with the solvent while the paper or the TLC plate adsorbent wants to “hold on” to the molecules. This is the main reason why the molecules having different polarities are separated and hence generate an R_f value.