Question

Would the Rf of a given compound increase, decrease, or stay the same if you doubled the length ofyour TLC plate and eluted the plate a correspondingly longer distance? Explain your answer.

Answer 1

Rf = distance moved by the compound or spot / distance moved by the solvent.

The more polar a compound has the lower Rf value. Because Polar compound want to bond with polar silica, while non-polar compound will move fast with eleuent hence this non-polar has highest Rf value.

The factors that influence the Rf of a compound on silica get TLC are following:

1. the TLC plate should be coated uniformity
2. sample should be dry on load time.
3. The sample should be same at every time (wt/ volume)
4. The chamber should be saturated by the mobile phase
   

Hence Rf is the ratio it would not change with the length of the plate.

For example we are running a 15 cm plate with a single compound.

And here

distance moved by the solvent = 15 cm

distance moved by the compound or spot = 7.5 cm

then Rf

Rf = distance moved by the compound or spot / distance moved by the solvent

= 7.5 cm/ 15 cm

= 0.5 cm

Now double the length of plate then

These distance also doubled,

distance moved by the solvent = 30 cm

distance moved by the compound or spot = 15 cm

then Rf

Rf = distance moved by the compound or spot / distance moved by the solvent

= 15 cm/ 30 cm

= 0.5 cm

Hence, The numerator changes in proportion to the denominator. Longer plates help when separating compounds with similar RFs, but RF is designed to be independent of plate size.