Question

why can the Rf value be used to distinguish the substances from one another? and Why is anRf value rather than the distance the spot moved used to help identify a substance by TLC?

Answer 1

• Every compound has a polar or non-polar nature in it. The extent of it, however, does not exactly match with other substance.In TLC, we seperate the compounds based on difference in their polarity. In TLC, stationary phase is polar silica gel. We use a low polarity solvent or solvent mixture as mobile phase. When we run the TLC, then, compounds with high polarity wants to stick with the TLC plate. Compounds with low polarity tries to go up with solvent. Even if there is small polarity difference, the adsorption-desorption phenomenon during running TLC play an important role to seperate out the spots from a mixture of components.

Now, R_f value is the ratio of distance covered by the component to the distance covered by solvent from the starting line. Now, distance covered by solvent is same for all components. So, R_f changes based on the distance covered by individual components. So, definitely R_f value will differ based on the components and each component can be distinguished based on their R_f value.

• Up to now, what I said, would lead you to think that, then what is the need of measuring distance covered by solvent. Of course, there is several reasons. Our experimental conditions are not always same. Same environment, temperature, pressure etc can not be reproduced every time. The distance covered by component is affected by all of these. Even the length of TLC plate, how TLC plate is placed in solvent each and every smallest parameter affects the distance covered. So, may be, in different situation you'll get different distance covered for same component in same solvent system. Then, you're lost. What would be the identification of your compound then? Here comes the significance of the relative scale i.e, our R_f value. The each and every small parameters that affect your component to run over TLC also affect your solvent. So, if you divide distance covered by component by distance covered by solvent, then errors occuring due to this experimental conditions will be minimized. That is why, we consider R_f value, not the distance.