Question

Please I need all answers about questions in TLC LAB.

(Q1) In our lab, what is the mobile phase and what is stationary phase? Discuss the polarity of each.

(Q2) Do your results suggest that the chemical characteristics of these pigments might differ according to their color? Explain.

(Q3) Which of your pigment molecules was the most non-polar? Polar?

(Q4) What is the relationship between Rf values and polarity for this experiment? Is this always true?

(Q5) Why should you not use ink on the coating to mark your pigment placement?

Here is procedure of lab.

Procedure

(1) On a balance weigh ~0.5 grams of algae powder and ~0.5 grams of anhydrous magnesium sulfate.

(2) Transfer (1) to a large test tube and combine with 2.0 ml of acetone. Stopper the test tube and shake vigorously for approximately one minute. You need to make sure that the solvent and solid are well mixed. Allow the mixture to stand for few minutes.

(3) Use a pipette to carefully transfer the solvent above the solid (should be green) into a small test tube. Cover the tube to avoid evaporation.

(4). Obtain a TLC chamber (a glass jar with a cover) and add developing solvent (a mixture of pet ether, acetone, cyclohexane, ethyl acetate and methanol). The solvent should completely cover the bottom of the chamber to a depth of approximately 0.5 cm.

(5). For each person, obtain a TLC plate (a silica gel coated plastic sheet) which has been precut and make a dot with a pencil on the coated side (the matt side) approximately 1.0 cm from the bottom of the strip.

(6). Fill a capillary tube by placing it in the leaf extract. Keep your finger on the end of the tube. Apply the extract to the center of the dot on the TLC plate by quickly touching the end of the TLC applicator to the plate. Allow to dry. Repeat several times to make a concentrated dot of extract. Be sure to let dry between applications.   The best ‘dot’ would be a concentrated but small in size. PRACTICE DOTTING ON A PAPER TOWEL BEFORE USING THE TLC PLATE !

(7) Carefully place the TLC plate in the TLC chamber. The TLC plate should sit on the bottom of the chamber and be in contact with the solvent (solvent surface must be below the extract dot). Screw the lid on the TLC chamber.

(8). Allow the TLC plate to develop (separation of pigments). As the solvent moves up the TLC plate you should see the different colored pigments separating.

(9). Remove the TLC plate from the chamber when the solvent from is approximately 1.0 cm from the top of the TLC plate. With a pencil, mark the level of the solvent front (highest level the solvent moves up the TLC plate) as soon as you remove the strip from the chamber. IT WILL FADE QUICKLY.

(10) Using a pencil, gently trace all the pigments. These will fade also. Tape the TLC plate onto your lab notebook. Sketch the TLC plate next to it including the solvent line, trace of pigments. Now add on the descriptions such as color to the sketch. Be specific.

Answer 1

(Q1) Stationary phase: silica gel, it has polar -OH on the surface

Mobile phase: mixture of solvents (a mixture of pet ether, acetone, cyclohexane, ethyl acetate and methanol), as the composition of pet ether is high the mobile phase is non-polar

(Q2) Yes, they posese different functional groups which is indicated by different degree of polarity.

(Q3) The spot at the top of the correspondes to non-polar compound -carotene. The spot at the bottom of the plate correspondes to polar compound xanthane (it contains polar groups, -COOH, -OH)

(Q4) The realtion between R_f values and polarity depends on nature of mobile and stationary phase. For the present system where statiionary phase is polar and mobile phase is non-polar, higher R_f values indicate that compound is non-polar and lower R_f values indicate that compound is polar

(Q5) Ink spread on the TLC staionary phase due to the polar nature of ink.