posted by H .
How can I tell which of two mobile phases is more polar? Does the fact that one solvent (e.g. isopropanol/ammonia) moves up the TLC plate slower than another solvent (e.g. hexane/ethyl acetate/ethanol)?
Or should I look at how far up the dyes have moved?
I think you can use both methods. The first one if you compare the move up the plate IF you know that the TLC plate is coated with a polar/non-polar material. The second IF you know the polarity of the dyes.