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| Table of Contents | |
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Introduction
Coulometer analysis determines carbonate concentration in a variety of samples, including pure carbonates, soils, rocks, and liquids. Coulometry quantifies the carbon dioxide evolved from acidified samples and uses this to determine the carbonate content in the original sample. The inorganic carbon value obtained from this method is used in conjunction with TC (total carbon) measurements from the CHNS to arrive at an organic carbon value.
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- Coulometer unit (UIC CM5015) with titration cell (Figure 1)
- Acidification module (similar to UIC CM5030) (Figure 2)
- Dual balance system, motion-compensated, with control software
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Dual Balance System Hardware
A Cahn balance (Figure 3) and 2 Mettler Toledo XS204 (Figure 4) analytical balances with motion compensation software are used to measure the mass of samples and chemicals. The Cahn balance measures samples for the Coulometer and the Elemental Analyzer.
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Motion compensation software developed in house allows the user to weigh the mass of chemicals and samples at sea. Reagents and samples >250 mg must be measured on the Mettler-Toledo XS204 balance (Figure 5). Reagents and samples ≤ 250 mg must be measured on the Cahn balance (Figure 6).
Figure 5. Mettler-Toledo Dual Balance Control Software.
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- A scientist or staff member logs wet sample information into SampleMaster at the sampling table. The sample is given the name CARB to ensure proper routing.
- Freeze-dry the sample (Freeze-Drying the Sample).
- Homogenize (grind) the sample (Grinding the Sample).
- Weigh the sample, assign a container and code, and upload the mass data to LIMS (Weighing the Sample).
- Prepare the coulometer acidification for analysis (Preparing Acidification Module and Coulometer Cell).
Freeze-Drying the Sample
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- Add approximately 1/8- 1/4" layer of granular KI to the bottom of the side arm (small side, anode compartment) of the cell. The junction between the two compartments should be about halfway covered with KI. (Figure 7).
- Add a stir bar to large section (large side, cathode compartment) of the Carbon Coulometer Cell.
- Fill the large section of the Carbon Coulometer Cell with cathode solution to the 100mL line.
- Fill the small section of the Carbon Coulometer Cell with anode solution to just under the level of the cathode solution (about 20mL). Do this quickly (within 1 min) after filling the cathode cell, or else the cathode solution will start filtering through the junction between the cells and contaminate the anode solution.
- Press the cathode top on the cathode compartment and the anode top on the anode compartment. In the anode compartment, make sure the electrode is in the solution but not in contact with the granular KI.
- Fill the KOH pre-scrubber trap 1/2 full of 45% KOH solution.
- Fill the AgNO3 post-scrubber trap 1/2 full of 3% AgNO3 solution.
- Add 3 drops of 2N H2SO4 to the AgNO3 trap.
- Attach the input gas tube (carrier gas inlet) to the KOH trap.
- Turn on the gas flow and set to 100 cm3/min.
- Connect the KOH trap to the reaction flask.
- Connect the reaction flask to the thinner side of the AgNO3 trap.
- Connect the top of the AgNO3 trap to the Carbon Coulometer Cell (through the line entering the back of the Coulometer
Connect the anode/cathode to the titration cell ports next to the titration cell (Figure 8).
Figure 7. Acidification Module and Carbon Coulometer Cell.
Level of granular KI 
Figure 8. Titration cell ports.
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- Switch the cell to On on the main coulometer unit.
- Allow the cell to equilibrate for 30-45 minutes before continuing. The %T should be 29.6 and steady.
- Open the Coulometer software and log in.
- Heat the vial with the weighed standard (100% CaCO3) in the heating unit for 2-3 minutes before measuring.
- Attach the top of the reaction cell to the sample vial. Bubbles should appear in the pre and post scrubber traps. If not, make sure all connections are tight.
- Select the sample from the list you wish to measure on the Coulometer software (Figure 9).
Figure 9. Coulometer Software
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