Question

In column chromotography why is it important that thelevel of th solvent is close to the level of the alumina or sand when the sample is added? Why not fill the column and funnel with solvent, and then add the sample?

Answer 1

It is because, when there is large excess of the solvent in the column, there are two possibilities:

1. If the solvent used has the ability to solubulize the sample, whole of the sample gets dissolved in the solvent and there would be no efficient passage of the sample through the stationary phase (i.e., alumina or sand).

2. Even if the solvent has no ability to solubulize the sample, it may get dispersed in the large quantity of solvent and unless the solvent passes through the column, the sample does not pass through the column. Therefore causing the loss of resolution to separate the mixture and remove impurities and the sample may be eluted even without any purification.

When the solvent is close to the level of stationary phase and when the sample is added, it has very less probability of dispersion and a cotton plug is placed after the sample is settled in the solvent before a large excess of the eluent (solvent) is poured in the column. This enhances proper elution of analytes as seen in TLC.