Question

Explain why the center of the band is used to calculate the Rf value for a cation rather than the leading edge of the band in chromotagraphy.

Answer 1

In paper Chromatography,Rf value is the distance moved by the solute (generally colored pigments like dyes) to the distance moved by the solvent (in which different solutes participating in the chromatogram are differentially soluble).These distances are liable to be measured only from the centre of the point of initial spotting of the mixture of solutes on the baseline of the paper.The solutes forms different bands on moving along the paper with the solvent.Ideally, the band of the colored solute moved along the chromatography paper is small,compact spherical zone/disc like structure,however ,the spots of solute get elongated to form streak like structures.This shows an uneven distribution of the solute ,so generally, the centre of the zone is considered to measure the distance of the solute moved.

The center of the band is used to calculate thhe Rf value for the cation rather han the leading edge of the band due to the bands being elongated or irregularly shaped, a standard for determining the D ion value is by measuring from the center of the band. Ideally this practice will produce more accurate results.

So, in simple words we can say that , the center of the band is used to calculate the Rf value rather than the leading edge of the band to minimize error and increase accuracy.