Question

A chemist started to carry out column chromatography on a Friday afternoon, got to the point at which the two compounds being separated were about three-fourths of the way down the column. The stopcock was closed and the column capped so that no solvent was lost. On Monday, when she returns to continue the elution, do you think she can separate the two compounds? Explain your answer

Answer 1

What is an important factor in the column chromatography? solvent system! correct. The rate at which a compound traverses through the column is highly dependent on the mobile phase being utilized. Typically, the more polar the solvent the faster the compounds will pass through the column. Polar solvents have a greater affinity for the solid phase, limiting the interactions between the compound(s) and the solid phase, allowing the compounds to elute more rapidly. Caution must be taken to ensure that the solvent system chosen for the column chromatography has the appropriate polarity to create separation between the compounds in the mixture. The solvent choice is crucial to successful separation using column chromatography.

Sice she stopped the column on friday afternoon and re-starts at monday, during these 3 days the coulmn experieces different temperature environments due to sunset and sunrise. The pressure development inside the column due to evaporation of one of the low boiling point mobile phase (no one will use higher boiling point solvents - we use generally mixture of hexane & ethyl acetate or methanol solvent system) to top of column forces mobile phase go down (remember we use external pressure for alumina columns). In addition due to evaporation of one of the mobile phase solvent the concentration of mobile phase varies inside the column and probably polarity increases, this leads to two compounds non-seperable as she already got to the point at which the two compounds being separated were about three-fourths of the way down the column.

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