|MadSci Network: Chemistry|
Hi, Selecting the appropriate mobile phase in thin layer chromatography (TCL) requires a bit of luck and experience. In most cases it is done by trial and error. The main thing to keep in mind is that you want to be able to separate your mixture before any of the spots reaches the end of the plate. The criterion for this is that the Rf values between 0.2 and 0.5. If you remember, the Rf value is the ratio of the distance traveled by the compound (spot) to the distance traveled by the solvent front. If the Rf value is 1, it means your spot (compound) traveled the same distance as your solvent front. Most TLC plates are made of silica or alumina. These are polar phases. The mobile phases used in TCL range in polarities from that of hexane (low polarity) to that of water (the most polar one). So a good starting point is to use solvent of medium polarity (See Table below) for starters, then adjust your polarity up or down by adding a miscible solvent of higher or lower polarity as needed. If your are separating the pigments in ink, you will have to adjust the polarity of the mobile phase until you get the maximum number of spots. Remember that some solvents are not miscible with each other. For examples hexane does not mix with water, but it does with toluene and chloroform. Good luck Polarity Table Water HIGHEST POLARITY Methanol Ether Acetone Ethyl acetate Diethyl Ether Dichloromethane Chloroform Toluene Hexanes LOWEST POLARITY
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