Chromatography

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?

Answered above.

To determine which of two mobile phases is more polar in chromatography, you can look at how far up the dyes have moved on the TLC (thin-layer chromatography) plate.

In chromatography, different compounds have different affinities for the stationary phase (the solid or stationary layer on the TLC plate) and the mobile phase (the liquid or mobile solvent that moves through the stationary phase). Compounds that are more polar have a stronger attraction to the stationary phase, while less polar compounds have a stronger affinity for the mobile phase, allowing them to move more quickly up the TLC plate.

If one solvent mixture (e.g., isopropanol/ammonia) moves up the plate slower than another solvent mixture (e.g., hexane/ethyl acetate/ethanol), it suggests that the former is likely more polar than the latter. Slower movement indicates stronger interaction with the stationary phase, which typically occurs with more polar compounds.

Alternatively, you can also compare how far up the dyes have moved in each mobile phase. If a dye in one mobile phase moves higher up the plate compared to the other mobile phase, it indicates that the former is less polar and has a weaker interaction with the stationary phase.

In summary, both the relative speed of movement up the TLC plate and the distance traveled by the dyes can provide information about the polarity of the mobile phase. Slower movement and shorter distance traveled generally indicate a more polar mobile phase.