Rf values TLC - Organic Chemistry LAB

Question

 

 

For my developing solvent, I used 6:4 hexane:ethyl acetate. For my reference spot, I mixed some salicylic acid and ethyl acetate. And for my test spot, I used some impure aspirin and ethyl acetate.

After letting my TLC plate develop in the developing solvent, marking the solvent line, and letting it dry, I put the TLC plate under a UV to marked the spots.

I got 3 spots total, 1 spot for my reference and 2 spots from my impure sample of aspirin.
Reference spot traveled 2.47 cm
Spot 1 from impure aspirin samples traveled 1.51 cm
Spot 2 from impure aspirin sample traveled 2.45 cm

And for my Rf values I got…
Rf (reference - salicylic acid and ethyl acetate) = 2.47 ÷ 5.30 = 0.466
Rf (1.51 spot) = 1.51 ÷ 5.30 = 0.2849056604 = 0.285
Rf (2.45 spot) = 2.45 ÷ 5.30 = 0.4622641509 = 0.462

Reading the lab handout again, I’m suppose to get a Rf value of 0.20 to 0.35…

So, did I do the lab wrong? Was the 6:4 hexane:ethyl acetate too polar? I’m so confused! Can someone explain to me what my values mean???

Answer 1

The Rf (retention factor) depends on many variables, and should not be taken as an absolute value.  It will vary from experiment to experiment, from location to location, etc.  So, not getting the exact value reported shouldn't be of such concern. Probably the most likely possibility is how your tank was equilibrated, if at all. Here are some other concepts to contemplate.

 

In TLC, the stationary phase (silica) is fairly polar, so more polar molecules will be retained longer and have smaller Rf values.  Vice versa for non polar molecules.  Since your experimental Rf value is somewhat greater than the one reported in the handout, this suggests that your spot ran further up the plate than expected, thus suggesting greater polarity of the mobile phase.  Is it possible that you prepared the mobile phase as 6:4 ethyl acetate : hexane instead of the other way around? Asking if the 6:4 hexane:ethyl acetate is too polar is an appropriate questions, and does agree with the science of TLC and Rf values, as explained above.  Again, if you look in the literature for TLC results of aspirin and salicylic acid, you will find varying Rf values, so one shouldn't be overly concerned about it. These values will vary with temperature, humidity, and most importantly, how the tank is equilibrated with mobile phase.