Coulometer: User Guide

Manual Information

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User Guide Contents

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Introduction

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• Coulometer unit (UIC CM5011CM5015) 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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A Cahn balance and 2 Mettler Toledo XS204 analytical balances with motion compensation software are used to measure the mass of samples and chemicals. The Cahn balance (Figure 3) measures samples for the Coulometer.

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Motion compensation software developed in house allows the user to weigh the mass of chemicals and samples at sea. Reagents must be measured on the Mettler-Toledo XS204 balance using the Balance Master program (see Balance User Guide)(Figure 4). Sample material must be measured on the Cahn balance (unless the sample is larger than ~1 gram) (Figure 5Figure 4).

Figure 4. Mettler-Toledo Dual Balance Control Software.


 

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1. 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.
2. Freeze-dry the sample (Freeze-Drying the Sample).
3. Homogenize (grind) the sample (Grinding the Sample).
4. Weigh the sample, assign a container and code, and upload the mass data to LIMS (Weighing the Sample).
5. Prepare the coulometer acidification for analysis (Preparing Acidification Module and Coulometer Cell).

Freeze-Drying the Sample

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1. Log into the Dual Balance system for the Cahn Balance. Answer Yes or OK on all prompts that appear during the log-in process. The user's log-in ID must be same as the LIMS database ID.
2. Click Test Option, and enter a number (usually >100 based on sea state; see the technician for guidance). Click Save/Exit to return to the main window.
3. Fold a small piece of wax paper (~0.5 cm x 0.5 cm) on opposite edges to create a U-shaped wax paper sample boat. Place the wax paper boat on the left weighing pan. Place a similar size of paper on the tare pan (right). Close the door, click Tare, and then Start on the plot screen.
4. Once the measurement is finished and the value is acceptable, click Get Mass. The tare value will be changed and the display will clear.
5. Put the sample on the weighing pan (~7–13 mg) using the scoop.
6. Press Weigh on the screen and then Start on the plot panel. The Weigh measurement will not begin if you do not press Start.
7. Once the measurement is done and the value is acceptable, click Get Mass. Final mass value (under the weigh button) will be changed and the display will clear.
8. Select COULOMETER from the Objective from the list, then and enter a part of the text ID or label ID of the sample, then click Search.
9. Select a the appropriate sample from the list, then click Assign to return to the main window.
10. Enter a container number, and click Save to save the mass value into the LIMS. Write down on a piece of paper the mass, container number, and text_id. Keeping a good logbook of your experiments is highly recommended!

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1. Add granular KI to the empty small section of the Carbon Coulometer Cell (the anode cell) to a depth of 5 mm from the bottom of the cell (Figure 6, far right).
2. Fill the large section of the Carbon Coulometer Cell with cathode solution to a mark 4 cm from the base.
3. Fill the small section of the Carbon Coulometer Cell with anode solution to a mark 4 cm from the base.
4. Important! Do this Add the anode solution 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.
5. Fill the KOH pre-scrubber trap 1/2 full of 45% KOH solution.
6. Fill the AgNO₃ post-scrubber trap 1/2 full of 3% AgNO₃ solution.
7. Add 3 drops of 2N H₂SO₄ to the AgNO₃ trap.
8. Attach the input gas tube (carrier gas inlet) to the KOH trap.
9. Turn on the gas flow and set to 100 cm³/min.
10. Connect the KOH trap to the reaction flask.
11. Connect the reaction flask to the horizontal fitting on the AgNO₃ trap.
12. Connect the top of the AgNO₃ trap to the Carbon Coulometer Cell.
13. Connect the anode/cathode to the titration cell ports next to the titration cell.

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1. Turn on the heating unit and power to the main coulometer unit.
2. With titration cell in place, adjust transmittance to 100%.
3. Turn on power to the cell.
4. Calibrate the instrument (see Calibration) or verify calibration (Calibration Verification), as applicable.
5. Login to the Coulometer software using a LIMS login.
6. Highlight a sample to be measured. Replicates of a sample (same TEXTID) are stored within the same line of the sample list. A dropdown option appears over the sample name allowing the user to select the desired replicate.
7. Connect the sample vial to jacketed condenser component of the sample introduction system (Figure 6). Ensure the connection is airtight. Then slowly add 5 mL of 2N HCl using the connected repeater dispensette.
8. Quickly press Measure in the sample list page of the coulometer software. If the measurement is delayed the results may underestimate the calcium carbonate percentage. A measurement screen will appear displaying real time data acquisition, the options to abort or stop the measurement, and to save/not save the results. The slope threshold is a measurement of the µg carbon with respect to time, and may be adjusted to specify the stopping point of the titration. Setting the slope threshold too low increases measurement times with the possibility of including circuit noise in the results, whereas setting the threshold too high will cause the measurement to prematurely terminate. The default slope threshold is 0.1.
9. The cell solution will fade upon dissolution of carbon dioxide and will return to a blue color (i.e., the start point) during titration.
10. After the measurement is complete, press Save or Don't Save to keep or disregard the data. A few reasons to not save data:
    1. Sample powder coated the sides of the vial and was not dissolved by the acid.
    2. The amount of calcium carbonate was so low its signal is greatly influenced by instrument noise.
    3. The slope threshold was set incorrectly.
    4. There may be constituent siderite in the sample that confounds the results. Siderite tends to react with the acid less quickly than calcium carbonate
11. After saving the data the measurement screen will revert to the sample list screen.

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