Treatment of data and report format: nknown numbers, identities of your unknowns

Question

Treatment of data and report format: nknown numbers, identities of your unknowns, your TLC plate in a Be sure to baggie, the include the u R, values of each substance (showing calculations), and a discussion. Postlab Questions 1. Why do you mark your TLC plate with pencil, rather than pen? 2. Jo College misread the instructions and put 2 cm of ethyl acetate in the 400 mL beaker The liquid level was above the level of the spots on the TLC plate. Will Jo get valid data? Explain 3. Explain why the solvent moves upward in thin layer chromatography 4. After a thin layer chromatography experiment, spots of J, K and L appeared 8.4, 3.4, and 12. 5 cm, respectively, above the starting line. The edge of the solvent was 16.0 cm above the starting line. a. Calculate the Ri of each compound. b. Which compound has the greatest tendency to go into the solvent? Explain how you go your answer. esic

Explanation / Answer

Ans. #1. Pen ink consists of organic dyes in aqueous or organic solvents. So, they are soluble in the organic as well as the aqueous phase depending on the nature of ink. So, the ink used to mark the TLC plate would dissolve in the aqueous/organic phase and contaminate the whole samples loaded on the plate simultaneously with deteriorating the quality of analysis.

The pencil mark is constituted of graphite (carbon). Being almost insoluble, it remains at the mark without interfering with the TLC analysis. Therefore, TLC pencil is marked with pencil, but not with pen.

#2. No. When the loaded samples are directly dipped in the mobile phase, the samples dissolve in it. Once dissolved in the mobile phase, they non-spread non-specifically through the plate during the time of elution. Therefore, no specific spots can’t be identified in the specified lane.

#3. The stationary phase is polar, porous and dry (oven dried) at the beginning. When it comes in contact with the solvent, the solvent molecules are adsorbed onto the dry stationary phase. The porous nature of plate facilitates the capillary paths through which the solvent moves upward through the plate in attempt to saturate the dry plate.

#4.a. Rf = Distance travelled by a spot / The distance travelled by solvent front   

            Rf (J) = 8.4 cm / 16.0 cm = 0.525

            Rf (K) = 3.4 cm / 16.0 cm = 0.213

Rf (L) = 12.5 cm / 16.0 cm = 0.781

#b. In TL chromatography, the molecule with greater polarity moves slowly and the one with low polarity (i.e. non-polar) moves faster along with the solvent.  

The stationary phase (silica) is a polar molecule. It has higher affinity for polar molecules and retard their movement. However, the non-polar solutes exhibit very low affinity for the stationary phase and higher affinity for non-polar mobile phase. So, they move faster dissolved in the solvent without being retarded by stationary phase.

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