Common Polar Adsorbent
General requirements for adsorbents: large surface area and certain adsorption capacity. Does not chemically change with the developing agent, does not react with or catalyze, decompose or associate with the substance to be separated, and the particles are uniform.
Silica gel and alumina are polar adsorbents, which are characterized by:
a) It has a strong affinity for polar substances, and solute with strong polarity will be preferentially adsorbed.
b) If the solvent polarity is weak, the adsorbent will exhibit a strong adsorption capacity for the solute. When the polarity of the solvent is increased, the adsorption capacity of the adsorbent to the solute is also weakened.
c) Even if the solute is adsorbed by silica gel or alumina, once it is added with a more polar solvent, it can be replaced by the latter.
The judgment of the polarity (depending on the type, number and arrangement of the functional groups): the hydrophilic group is proportional to the polarity, and the lipophilic group is inversely proportional to the polarity; the free compound is weak in polarity, Lipophilic, dissociative compounds are highly polar and hydrophilic; the polarity of the solvent is determined by the dielectric constant.