Question

Two compounds I and II eluted by column chromatography (adsorption of I>II). Which one of the following is a correct statement?
A. II moves slower and has higher ${{\text{R}}_{\text{f}}}$ value than I.
B. II moves faster and has higher ${{\text{R}}_{\text{f}}}$ value than I.
C. I move faster and have a higher ${{\text{R}}_{\text{f}}}$ value than II.
D. I moves slower and has a higher ${{\text{R}}_{\text{f}}}$ value than II.

Answer 1

Hint: Column chromatography is based on adsorption. The more the adsorption, the slow the movement of the solvent. The less the adsorption, fast will be the movement of the solvent.

Complete step by step solution:
Chromatography is a term used to describe a separation technique in which a mobile phase carrying a mixture is caused to move in contact with a selectively adsorbent stationary phase.
This is a solid-liquid technique in which the stationary phase is a solid and the mobile phase is liquid.
The solvent used as the mobile phase is called eluent.
Compounds attracted more strongly by the mobile phase will move fast through the column and elute from the column dissolved in the eluent.
The compound which is more strongly attracted to the stationary phase will move slowly through the column.
Here, adsorption of I is greater than II. It indicates that I is more strongly attached to the stationary phase than II. Therefore it moves for a very little distance and very slowly. II is not so strongly attached to the stationary phase. Thus it can move very fast and at long distances.
${{\text{R}}_{\text{f}}}$ value is also called retention factor. It is the ratio of the distance moved by the compound in the TLC plate to the distance moved by the solvent.

Therefore the option B is correct.

Additional information:
There are different types of chromatographic techniques based on the difference in the mobile and stationary phases used. Column chromatography is a useful method for the separation and purification of both solids and liquids. It is also used in the isolation of active constituents.

Note: There are a lot of factors affecting the column efficiency. They are dimensions of a column, particle size, temperature, and quality of solvents. Solvents having low viscosities will give better results.