ORGANIC cHEMISTRY - Iv

Experiment 3

Aim of the Experiment

Aim: To separate the colors from the given sample using paper chromatography (gel inks)

To identify components of inks by calculating Rf values

Principle

The principle involved can be partition chromatography or adsorption chromatography. Partition chromatography because the substances are partitioned or distributed between liquid phases. The two phases are water held in pores of the filter paper and the other phase is a mobile phase which passes through the paper. When the mobile phase moves, the separation of mixtures takes place. The compounds in the mixture separate themselves based on the differences in their affinity towards stationary and mobile phase solvents under the capillary action of pores in the paper. Adsorption chromatography between solid and liquid phases, wherein the solid surface of the paper is the stationary phase and the liquid phase is the mobile phase

Materials required

Large glass jam jar with cover (eg flat piece of plastic or glass), hair dryer or warm oven, ruler, scissors, pencil, sheet of chromatography paper, or large filter paper, pack of assorted food colours (red, yellow, green and blue), 0.1%(w/v) aqueous sodium chloride solution (mobile phase)

Procedure

Part A: Preparation of Chromatography Paper

1. Wash your hands thoroughly to remove excess oils from your skin. Obtain a ruler and a piece of chromatography paper from your instructor. Handle the paper only on the edges to avoid leaving fingerprints, as these may hinder the elution process.

2. Place the chromatography paper on a sheet of clean notebook paper or paper towel to avoid picking up dirt or contaminants from the bench top. Orient the paper into a “landscape” position and write your name on the top edge of the paper in one corner. Using a pencil and ruler to measure accurately, draw a straight line across the paper, about 1.5 cm above the bottom edge. This is the starting line. Draw another line about 10 cm above the bottom edge. This is the finish line.

3. On the starting line, measure in from one side about 2.5 cm and lightly draw a small “X” centered on the starting line. Draw seven more, 1.5 cm apart.

4. In the center of each X, make a small spot of ink color in this order: black, burgundy, red, pink, violet, turquoise, green, blue.

5. Go back over each ink spot a second time to ensure there is enough ink in the spot.

6. Obtain a small piece of tape and gently curl the paper into a cylinder, with the spots on the outside. Tape the ends together near the top and bottom, taking care that the two edges of the paper do not touch. If they do touch, the eluent will creep on a diagonal, and the spots will run together, or not in straight lines.

Part B: Acquisition of Chromatogram

1. Take a 500-mL beaker and pour about 25-mL of eluting solution into the beaker. Obtain a piece of plastic wrap to cover the top.

2. Gently place the paper cylinder into the beaker and cover the top with the plastic wrap. Remember that the spots must be above the liquid level for the experiment to work. Watch the eluent creep up the paper until it begins to move some of the ink. It will take about 45-90 minutes for the solvent front to reach the finish line.

3. When the solvent front reaches the finish line, remove the paper from the beaker, being careful to touch only the top. Let excess eluent drip into the beaker. Gently remove the tape and lay the chromatogram on a piece of paper towel in the hood. Leave the paper in the fume hood, where it will dry completely. If needed, use a heat lamp (in the fume hood) to dry the chromatogram; if using the heat lamp, allow 5-10 minutes to dry.

Part C: Interpretation of Chromatogram

1. Write the names of the original ink colors beneath each “X” mark (from the order you added the colors). Each ink sample should no longer be on the “X” mark, and have travelled up the paper, becoming one or more separate color spots between the starting and finish lines. Circle around each color spot.

2. Use a ruler and draw a plus sign in the center of each spot. Measure the distance from the starting line to each plus sign. Record this distance for each spot on your lab report. These are the values, in cm.

3. Measure the distance between the starting line and the finish line or, the farthest up that the solvent front reached. Record this distance. This is the value, in cm.

4. Calculate the retention factor (Rf ) for each spot and record the values in your lab report.

5. You and your lab partner will hand in your lab reports at the same time, with the paper chromatogram stapled to the lab reports.

Calculation

Some compounds in a mixture travel almost as far as the solvent does; some stay much closer to the base line. The distance travelled relative to the solvent is a constant for a particular compound as long as you keep everything else constant - the type of paper and the exact composition of the solvent, for example.

The distance travelled relative to the solvent is called the R_f value. For each compound it can be worked out using the formula:

Result

Reference Material

1. Chem LibreTexts

2. Demo videos

https://www.youtube.com/watch?v=iPpy4khqtks

https://www.youtube.com/watch?v=FQqVqVZgKjA&t=124s

Questions

1. Principle of paper chromatography?

2. What is the definition of Rf value? and factors affecting it?

3. Similarity and difference between TLC and paper chromatography?