Question

 

What issue would you run into if you used 100% 18M acetic acid as your TLC (Thin Layer Chrom.) solvent in a normal phase TLC run?

 

Answer 1

 In the case of Thin Layer Chromatography (TLC), selecting the proper TLC solvent system (also called a mobile phase or eluent) is perhaps the most critical aspect of achieving efficient chromatographic separation between mixtures of compounds. Once a sample of interest has been applied on a TLC plate, it is drawn up with a solvent or mixture of solvents through capillary action. Based on the compound's solubility with the solvent and the difference in their affinity with mobile phase and stationary phase adsorbent, different analytes ascend the TLC plate at different rates achieving the desired separation.

 In a normal phase of TLC runs, volatile organic solvents (nonpolar, polar, or mixture) are usually used depending upon the nature of the compounds. After running up the TLC plate with solvent (mobile phase), the solvent should be thoroughly dried to get the best result; otherwise, the solvent may interfere in detecting desired compounds.

 100% 18M acetic acid (glacial acetic acid) is a viscous, dense (1.05 g/cc), corrosive, high boiling (118 °C), and polar (1.74 D) solvent. So, after running up the TLC plate in the glacial acetic acid solvent, it will not be dried up quickly. Moreover, glacial acetic acid can dissolve both polar and nonpolar compounds simultaneously. Further, as glacial acetic acid is a much weaker base than water, the amide compound interacts with acetic acid and behaves as a strong base.

 Thus, due to these physical and chemical properties, using 100% 18M acetic acid (glacial acetic acid) is not feasible as a TLC solvent. To separate acidic components, dilute acetic acid could be used in the solvent mixture (0.1 to 2.0 %).