Column Chromatography: Sample Run

Pour the solvent carefully into the top of the column.

In this example we will be using non-polar petroleum ether as the solvent. Be sure the valve at the bottom of the column is open. By itself, gravity will be sufficient to pull the liquid through the column, however, the vacuum aspirator may be turned on to speed up the process.

Be very careful not to allow the solvent level to drop into the sand layer. Periodically it will be necessary to add additional solvent to the column.

Turn the column off when:

1. The alumina and sand are completely wet with the solvent (they must also be free of air spaces)

2. The solvent level has dropped as far as the filter paper (..and no further!)

This is very important since the next liquid to be added to the column will be the sample plug. We want NO air spaces between in between additions to the column.

Pour the sample mixture into the column and open the column valve.

The sample plug should be prepared quite concentrated. Gravity will slowly pull the sample mixture into the column packing. Us the vacuum aspiration system or a squeeze bulb to speed things up if necessary. You will observe the separation of the sample plug into colored bands even at this early point (Note: the sample plug used in this example is much larger than you would normally use. Be sure to make your sample plug as small and concentrated as possible.).

Turn the column valve off when the upper sample plug level has dropped to the level of the filter paper.

The sample plug will be immediately followed by additional solvent that is used to propel one component through the column (petroleum ether in this case).

Pour additional solvent into the column and turn the column valve on.

The additional solvent propels only one (hopefully... if a separation is to obtained) through the column while leaving other components adhering to the stationary phase (alumina). In this case, non-polar petroleum ether propels non-polar ferrocene through the column observable as the bright yellow band midway. The orange band at the top of the column is identified as the relatively polar acetylferrocene.

Pour additional solvent into the top of the column as necessary.

Don't let the level of the solvent drop below the filter paper!

Just before the ferrocene is flushed from the column, empty the lower filtering tube.

The point is that we want to collect the sample components in the most concentrated form. Letting the ferrocene/solvent drip into an already partially filled container would dilute the sample unnecessarily.

Replace the filter tube collect the ferrocene sample component.

Stop collection and turn off the column valve when the ferrocene has entirely moved from the column into the filter tube.

Pour the ferrocene/pet. ether solution into a clean beaker

Turn off the column first. If necessary, rinse the filter tube with a small amount of pet ether and add this to the bulk solution. Put the beaker containing the sample away in a safe place. Reposition the filter tube beneath the column.

When the level of the first solvent level reaches the filter paper, add the second solvent.

In this example, the second solvent will be a 1:1 mixture of petroleum ether and ethylacetate. This solvent is effectively polar and capable of dissolving the polar acetylferrocene that is presently stuck on the column. Consequently, the acetylferrocene will now move down the column (photo at right)

Collect the acetylferrocene sample using the same method as for ferrocene.

Keep adding the 1:1 pet. ether/ethyl acetate solvent mixture to the top of the column. When the last remaining acetylferrocene has passed out of the column, turn off the column valve to avoid unnecessary dilution of the sample with the pure solvent.

Pour the acetylferrocene/mixed solvent solution into a clean beaker

Turn off the column first. If necessary, rinse the filter tube with a small amount of pet ether and add this to the bulk solution. Put the beaker containing the sample away in a safe place.

Now....can you identify which of the two samples in the picture above is ferrocene and which is acetylferrocene????????