ION EXCHANGE CHROMATOGRAPHY

Ion exchange chromatography can be used to measure the concentration of ions usually in aqueous solutions.
In a case of cations, a cationic resin is used as the column
The resin is a polymer with sulphanate end-groups represented here as a lipsis which rapidly exchange cations.

For instance sodium ions might replace hydrogen ions.
To prepare the resin for use, suspend it in deionised water and stir well to remove air bubbles.

New dry resin needs to be soaked for about 5mins and any fine powder decanted from the surface of the water.
The resin that has been used already will be damped and does not need soaking.
The glass columns that are used for chromatography often have a wider section at the top to act as a reservoir.
There is a tap at the base of the column and in this case, there is a glass fret above it.

The fret will prevent ion exchange resin blocking the tap and such column needs no preparation.
If the column does not have a fret, first add about 5 cubic cm of deionised water to the column.
Then take a piece of glass wool and using a glass rod, push it to the bottom of the column.

Clog the glass wool to release any air bubbles and pack it down the formal plug which will prevent resin entering the tap.
Put the resin slurry carefully into the column.
Allow the resin to settle and then drain off excess water from the bottom of the column but don’t let the top of the column become dried out.
Leave enough room at the top of the column to add the sample to be analyzed.
Close the tap when the water level at the top of the column is just above the resin.

If any air bubble becomes trapped, it must be released by stirring the column with a glass rod
This is to ensure that the solution uniform and maximum contact with the resin.
When old resin is reused, it will contain a variety of cations M+ bound to the sulphanate groups.
Such resin must be regenerated by treating it with concentrated acid so to drive the equilibrium to the right and leave the resin in the acid form.

To do this, carefully add about 25cubic cm of 6M Hydrochloric acid to the column, disturbing the top of the column as little as possible.
Partially open the tap and let the acid slowly run through the column.
This will result in all the cation being replaced by hydrogen ions.
When the acid level as come down to the top of the resin, turn off the tap and add deionised water to the top of the column.
Open the tap and run out the water. This will remove excess acid from the column.

It requires several refills to wash all the excess acid from the column.
From time to time, sample the washing and test them with an indicator such as screened Methyl Orange.
The reddish color indicates that the test portion is acid and that further washing is needed.
Continue washing the column until the test portion shows that the effluent in neutral.

With the water level just above the top the resin, withdraw 25cubic cm of the sample using a pipette.
And add this to the top of the column taking care to disturb the column as little as possible.
With a large conical flask in place below the column, partially open the tap so the liquid slowly percolates down the column.
When the liquid level reaches the top of the resin, add deionised water.
Cations from the sample solution shown as (M) displace hydrogen ion on the resin.
The washing carries the resulting acid solution into the conical flask
Periodically test the washings until the indicator shows they are neutral.
The red color indicates that washing must continue.
Any acid test solution must be added to the conical flask.
When the test indicates that the washing leaving the column are neutral, titrate the acid effluent collected in the large conical flask against standard alkali using screened Methyl Orange as the indicator.
The titre will give the acid equivalent of all the cations contained in the 25cubic cm of sample solution.