Ion Exchange Chromatography
I spend a few minutes introducing ion exchange chromatography, showing how it is possible to attach fixed cations or anions to polymeric resins and how these then have an exchangeable counterion.
1. Describe a scheme using ion exchange chromatography that would enable you to deionize water. Say something about the capacity of the ion exchange resins you would use for this purpose.
Allow students about few minutes to work on this problem. Students should have no trouble recognizing that you would need to run the water through a pair of columns in order to remove the anions and the cations.
What ions should be used as the counter-ions in the column?
Within a few minutes the groups can usually figure out that hydronium and hydroxide ions are needed if the goal is to deionize the water. They also realize that high capacity resins will be the best for deionizing water. I then spend time summarizing what would happen to each ion in the two-column system. I make sure that the students understand that the capacity is limited by the number of derivatized aromatic rings. I discuss the water purification systems that we use in the department and how the measurement of conductivity is used as a way to determine how well the water has been deionized.
2. Would ion exchange resins that are useful for deionizing water be useful for analytical separations?
The students will need to know what is meant by analytical separations (trace levels of ions). I also ask them to consider what it would take to be able to actually have the ions elute from the column. The groups can usually reason out that the high capacity resins used for deionizing water would lead to exceptionally long analysis times if used for trace analyses. They also propose including eluent ions in the mobile phase. I ask them to specify what ions they might use as their mobile phase counterions and they can usually come up with hydronium (hydrochloric acid) for cations or hydroxide (sodium hydroxide) for anions. I then summarize these concepts at the board.
3. What would be the order of retention for the ions Li(I), Na(I), and K(I) on a cation exchange resin? Justify your answer.
I allow students about ten minutes to consider this problem on their own. They always address this by considering how strongly the ions might associate with the resin. They may find compelling arguments for both sides including the charge density of lithium or the steric hindrance of potassium. I ask them to think about the equation that describes the electrostatic attraction of two ions, which has the two radii in it, and most eventually conclude that the lithium will associate more strongly with the resin and elute last.
After summarizing this as a reasonable prediction, I then ask them to consider mobile phase effects. In particular, I ask them to think about what they know about the structure of water and what would happen to this structure when ions dissolve in water. I also ask them to think about drawing a picture for the environment around a lithium, sodium or potassium ion in water. Students may realize that lithium is more stable in the eluent water because it causes the least disruption to the physical network of hydrogen bonds in the liquid water and has the strongest electrostatic attraction with the negative ends of the water molecules. They eventually conclude that a consideration of mobile phase effects would lead one to predict the opposite retention order.
With two opposite predictions, I suggest that we try the experiment. We don’t actually try the experiment, but I indicate that experimental data shows that for ions of the same charge, the mobile phase effects are more important and smaller ions elute first. I also introduce the concept of the solvophobic effect.
What if you had ions of varying charge but the same size (a +1 and +2 ion of similar size)? What is the retention order based on mobile phase effects? What about the retention order based on stationary phase effects?
Students can reason out that, again, the two different considerations lead to different retention orders. Experimental data shows that the +2 ion elutes last, and I discuss how ions of higher charge have much stronger association with the resin because they can bind simultaneously to two of the fixed ion sites and require two mobile phase counterions to be pushed out of the resin and migrate down the column.
4. Consider the case of separating the alkali ions in (3) on a polystyrene resin using a fairly dilute solution of hydrochloric acid as the mobile phase.
a) What is the bound ion and the mobile counter ion?
Groups can readily answer this.
b) One problem is how to detect these ions. They do not absorb ultraviolet or visible light in the accessible portion of the spectrum. They do not absorb infrared light. Conductivity might work except that the hydrochloric acid in the mobile phase produces too high of a background signal. Devise a way to remove the conductivity of the eluent ions (HCl) but retain the conductivity of the alkali ions you wish to detect.
Students understand the problem but usually struggle to come up with a way to solve it. I usually draw a representation of the column on the board and show the NaCl and HCl eluting simultaneously and indicate that the goal is to remove the conductivity of the HCl while retaining the conductivity of the NaCl. Many of the groups start to think that there should be some way of neutralizing the acid but when asked what they would use (and the typical response is NaOH), they realize that this would eliminate the conductivity of the H+ (by reacting it with OH- and converting it to water) but replace it with Na+ leaving NaCl, which has not solved the problem. Through prompting they come to the realization that they need a way to selectively replace the Cl- with OH-, which converts the HCl to water and the NaCl being analyzed to NaOH, which is still conducting. Groups then realize that running the sample eluting from the analytical column through an anion exchange column in the hydroxide form will solve the problem. I then summarize the use of suppressor columns in ion chromatography at the board. I also explain the possibility of using an indirect spectrophotometric detection method as well.