Question

You are given a sample of several compounds to separate by paper chromatography. You draw a pencil line exactly 1.0 cm from the bottom of the paper, and place a spot of sample on it. You dry the sample, then develop it in a solvent.

When the chromatogram is taken out of the solvent, the paper is wet up to 9.0 cm from the bottom of the sheet. The compound you are interested in shows up as a spot 7.3 cm from the bottom of the paper.

Calculate the following:

How far did the compound move?

In the same time, how far did the solvent move?

What is the Rf factor for the compound?

Answer 1

Solution: Assuming that the line drawn at 1.0 cm mark as the origin of spot, we can calculate the above values:

             (a) In the experminent, the compound of our interest has moved above upto 7.3 cm from the bottom of the paper. Since the spot has orginated from a point marked 1.0 cm above the bottom of the paper, therefore:

                The distance the compound has traversed = 7.3 - 1.0 cm = 6.3 cm

             (b) Similarly, we have to calculate the distance of the solvent front from the base line as well. Therefore,

                 Distance traversed by the solvent = 9.0 - 1.0 cm = 8.0 cm

An example to show, the base line, and how to measure the distance for solvent and compound

         (c) The Rf = Migration distance of Compound/ Migration distance of solvent front

                       = 6.3/8.0 = 0.7875