Ion Chromatograph (IC)

Determination of Sulfur in Coal by Ion Chromatography

Reading:

pp. 634-646 and 707-708 in your Harris text

Objectives:

Introduction:

This experiment will be an introduction to ion chromatography (IC).  Ion chromatography is a very powerful technique used for the quantitative determination of individual ions 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 then passes through an ion-exchange column where the various types of ions interact with the stationary phase and separate from each other.  Ideally, each ion emerges from the column in a pure band.  At this point the separation is complete and all that is left is detection of 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 bicarbonate buffer mobile phase will produce a large signal.  So the signal from our analyte ions could very well be swallowed up by the signal from the background.  Some way is needed to reduce the background signal.  This is the job of the suppressor system.  The suppressor effectively reduces the conductivity of the eluent, 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, 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 counterintuitive, 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 shows a peristaltic pump along with reservoirs for water and sulfuric acid.  This is for regeneration and rinsing of the suppressor.  When a sample has been injected, the eluate from the separation column runs through the suppressor so the mechanisms just described can occur.  At the end of the run, sulfuric acid is run through the suppressor to regenerate the cation-exchange sites with H+ ions.  Then the excess acid is rinsed with water.

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 Sulfur in Coal:

The presence of sulfur in coal leads to well-known environmental problems.  Combustion of sulfur produces gaseous sulfur dioxide, which in turn reacts with water in the atmosphere to produce sulfuric acid.  This sulfuric acid can then precipiate as acid rain:

S  +  O2 → SO2 2 SO2  +  O2  →  2 SO3 SO3  +  H2O  →  H2SO4

In some coals, sulfur can constitute up to 10% of the total mass. We will be using IC to determine the amount of sulfur in a sample of coal.  The sulfur is converted to sulfate ion in a fusion reaction using a flux of Eschka mixture (2 parts MgO to 1 part Na2CO3) at 800°C according to the following reaction sequence:

S(as organosulfur and metal sulfides)  +  O2 → SO2 Na2CO3·MgO  +  SO2  +  ½O2  →  Na2SO4  +  MgO  +  CO2

The soluble Na2SO4 is then dissolved in water and the sulfate ion is determined by IC.