Ion Chromatography Determination of Anions in Water

READING (FROM HARRIS TEXT):

• • 642-644 (What is Chromatography?)

  • 752-753 (Ion Exchangers)

  • 757-759 (Ion Chromatography)

OBJECTIVES:

• • To determine the concentrations of various inorganic anions in water samples

  • To become familiar with ion chromatography.

This experiment will be an introduction to ion chromatography (IC). Ion chromatography is a very powerful technique used for the quantitative determination of individual ion concentrations in an aqueous sample, and it’s very simple to use and understand. The diagram below shows the basic design of the IC instrument.

As in all of our chromatography techniques, the sample is injected into a flowing stream of mobile phase. In our case the mobile phase is a carbonate/bicarbonate buffer. The sample is carried by the mobile phase through an ion-exchange column where the ions in the sample interact with the stationary phase and separate from each other, with each of the different ions ideally emerging from the column in a pure band. Now we just have to detect the ions. Most IC systems use a conductivity detector that simply detects the conductivity of the solution as it passes through. But since solution conductivity is largely determined by the concentration of ions in solution, even our buffered mobile phase will produce a large signal. So the signal from our analyte ions could very well be swallowed up by the background signal from the mobile phase itself. We need some way of reducing this background signal, and this is the job of the suppressor. The suppressor effectively reduces the conductivity of the eluent (mobile phase), but not the conductivity of the analyte. The mechanism of suppression will be explained below, but first we need to focus on how the separation occurs in the ion-exchange column.

SEPARATION BY ION EXCHANGE:

As stated in your Harris text, most IC columns are packed with small porous beads of polystyrene resin that contain varying amounts of divinylbenzene for cross-linking of the polymer chains (PS-DVB). The benzene rings in the polymer are modified to contain either cation-exchange [e.g. -SO3-] or anion-exchange [e.g. -N(CH3)3+] sites. Usually these sites are occupied by ions from the mobile phase buffer (like HCO3- or CO32-), but these ions can be displaced by analyte ions (such as Cl-) in the following illustration:

This overall process can be described by the following equilibrium reaction where RN(CH3)3+ is a cation-exchange site and Ax- is an analyte ion:

xRN(CH3)3+HCO3- + Ax-(aq)  →  [RN(CH3)3+]xAx-+ xHCO3-(aq)

Since the mobile phase has a much higher concentration of HCO3- than Ax-, this equilibrium is shifted left according to Le Chatelier’s principle. This shift causes a bound analyte ion to reenter the mobile phase and move down the column to another exchange site where the process can be repeated. Ions in a sample separate from each other based on their charge and size. Highly charged ions tend to be retained longer, as do larger ions. (This is somewhat counter-intuitive, but larger ions tend to have smaller hydrated radii than smaller ions. This means that more water molecules bind to the small ions and actually make them too large to access the ion-exchange sites.)

CONDUCTIVITY SUPPRESSION:

As stated above, we must decrease the overall conductivity of the mobile phase so that our conductivity detector will be able to distinguish our analyte ions from the background. This is the purpose of the suppressor. A suppressor system is also based on ion-exchange principles, but it uses the opposite form from that of the separation column. In other words, a cation-exchange separation needs an anion-exchange suppressor, and an anion-exchange separation needs a cation-exchange suppressor.  

To illustrate how this works, let’s use an example of a sodium bicarbonate buffer with chloride ions as the analyte.  The separation column is anion exchange, so the suppressor is cation exchange. The suppressor must first be charged with acid solution so that the cation exchange sites are occupied by H+ ions. Let’s first consider what happens when only the sodium bicarbonate mobile phase is moving through the system. Sodium and bicarbonate ions emerge from the column and enter the suppressor. Here the Na+ ions stick to the cation exchange sites and release H+ ions to the mobile phase. These H+ ions now combine with HCO3– ions to form neutral carbonic acid (H+ + HCO3– → H2CO3). So the ions have been removed and the resulting solution is non-conductive!  The following cartoon illustrates the entire process:

But now what happens when our chloride analyte comes through?  The chloride ions will take the place of some of the bicarbonate ions in our picture above. Sodium ions will once again release H+ ions in the suppressor, but these H+ ions will not combine with the Cl– ions because this would form HCl which is a strong acid that can’t exist in solution. So the solution remains conductive and a peak is generated by our detector!

You’ll notice that the schematic diagram of the instrument above shows a peristaltic pump along with reservoirs for water and sulfuric acid. This is for regeneration and rinsing of the suppressor. As shown in the figure below, our suppressor consists of three identical channels, each containing cation-exchange sites. The eluent flows through one channel and this is where the suppression of the eluent occurs before going to the detector. Sulfuric acid is flowing through another channel, regenerating all the cation exchange sites with H+. The third channel is being washed with water to remove excess acid. After each sample run, the connections to the suppressor are all rotated. So in the next run, the channel that had water now has eluent, the channel that had eluent now has sulfuric acid, and the channel that had sulfuric acid now has water. This system ensures that there are always freshly charged H+ sites in the eluent channel for suppression of the eluent conductivity.

Diagram of suppressor showing three channels

METROHM IC SYSTEM:

Below is a picture of our IC system. You’ll notice that the single sample injector from the schematic diagram above is replaced with an autosampler that can be programmed to run up to 148 separate samples automatically.   

The picture below shows the inside of the separation center. Notice that in addition to the conductivity suppressor, there is also a CO2 suppressor whose purpose is to eliminate dissolved CO2 from the eluate after H2CO3 has been formed in the conductivity suppressor. The tall columns in front of the CO2 suppressor are for removing CO2 and water vapor from the air that mixes with the eluate in the CO2 suppressor.

DETERMINATION OF ANIONS IN WATER:

In this experiment, you will use ion chromatography to measure the concentrations of common anions in water samples taken from various sources.