Coulometer: User Quick Start Guide

Manual Information

User Guide 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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A variety of carrier gases can be used for coulometry (O₂, N₂, He, and dry air); the JRSO uses N₂ for the carrier gas. Interferences caused by compounds such as SO₂, SO₃, H₂S, HCl, HBr, HI, and Cl₂ are removed with KOH and AgNO₃ scrubbers.

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Apparatus, Reagents, & Materials

Hardware

• 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.
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Figure 3. Cahn Electrobalance. Figure 4. Mettler Toledo XS204.

Software

Dual Balance System Software

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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).

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Figure 5. Mettler-Toledo Dual Balance Control Software.
Image Modified

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Figure 6. Cahn Balance Control Software.
Image Modified

Laboratory Supplies

Apparatus

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• Nitrogen (99.995% or better) is used as carrier gas to minimize the amount of CO₂ the scrubber (KOH) must absorb

Reagent Solutions

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• Wiki Markup45% KOH (\45% KOH ([%w/v\]: add 90 g KOH pellets to water and make up to 200 mL once fully dissolved. <span style="color: #ff0000"><strong>Warning!</strong></span> This procedure liberates caustic fumes and heat. Perform in a fume hood.)
• Warning!This procedure liberates caustic fumes and heat. Perform in a fume hood.)

• 3% AgNO₃ (

  Wiki Markup3% AgNO{~}3~ (\

  [%w/v

  \

  ]:

  dissolve

  3

  g

  silver

  nitrate

  in

  water

  and

  make

  up

  to

  100

  mL

  when

  fully

  dissolved.)

• 2N H₂SO₄: add 55.5 mL concentrated sulfuric acid to water and make up to 1L

• 2N HCl: add 166 mL concentrated hydrochloric acid to water and make up to 1L

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Sample Preparation

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Liquid samples are pipetted directly into the sample tube. Most samples use 2 mL volume. If samples are suspected to contain high sulfur contents, use 0.5 mL.
Solid samples must be dried, ground, and weighed before introduction into the prepared Coulometer apparatus. The workflow for solid sample preparation is as follows:

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). AnchorRTF32383532373a203348656164RTF32383532373a203348656164

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1. ).

Freeze-Drying the Sample

1. Cut the sample bags or roll back the top to ensure an open orifice during the freeze-drying process.
2. Place the sample in the freeze-drier in the Chemistry Lab under vacuum for 12 hr. If sample is finely divided and is clumpy, freeze-drying may take >12 hr. Sample should appear dry and powder easily (in mortar and pestle).
3. Do not overload the freeze dryer.

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Grinding the Sample

1. Remove the freeze-dried sample from the sample bag and place in a mortar. If the sample volume is too large to be ground in the mortar, grind it in separate smaller portions and recombine.
2. Grind the sample with a pestle to a fine, powder-like consistency with no large clumps. If the sample is too hard to grind in a mortar and pestle, use the mixer mill (see the X-ray technician for assistance in operating the mixer mill).
3. Transfer the sample to a new bag or container.

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Weighing the Sample

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. The current mass shown in the software between the left and tare (right) weighing pan should be no more than 1.0 mg.
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 Objective from the list and enter a part of the text ID or label ID of the sample, then click Search.
9. Select a 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, text_id, and core information (ie. expedition, site, hole, section, and interval if applicable).

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Preparing Acidification Module and Coulometer Cell

1. 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).
2. Add a stir bar to large section (large side, cathode compartment) of the Carbon Coulometer Cell.
3. Fill the large section of the Carbon Coulometer Cell with cathode solution to the 100mL line.
4. 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.
5. 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.
6. Fill the KOH pre-scrubber trap 1/2 full of 45% KOH solution.
7. Fill the AgNO₃ post-scrubber trap 1/2 full of 3% AgNO₃ solution.
8. Add 3 drops of 2N H₂SO₄ to the AgNO₃ trap.
9. Attach the input gas tube (carrier gas inlet) to the KOH trap.
10. Turn on the gas flow and set to 100 cm³/min.
11. Connect the KOH trap to the reaction flask.
12. Connect the reaction flask to the thinner side of the AgNO₃ trap.
13. 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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Sample Analysis

Once the sample is placed in the reaction vial, acid is added to release CO₂ gas. This gas is carried through the coulometer cell and into the titration cell, where the sample is titrated by the coulometer automatically and the software plots µg carbon vs. time. The software evaluates the slope of the plot against a drift threshold and then compares the slope against $Threshold_slope (method-determined value equivalent to 29% transmittance) to determine when the titration is complete. When the threshold is reached, titration halts and the final result is expressed in µg C, from which weight percent (wt%) CaCO₃ can be calculated.

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Weight percent calcium carbonate is calculated from µg carbon measured during the titration as follows:
%CaCO₃ = µg C x 8.333/sample mass
Sample mass is stored in LIMS associated with the container ID that the coulometer analysis is associated with.

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Quality Assurance/Quality Control

QA/QC for Coulometer analysis consists of instrument calibration and continuing calibration verification using check standards, along with blanks and replicate samples.

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• In Control
• In Control (exceeds warning limit
• Out of Control (exceeds control limit)

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For a system to be considered in control, all QA/QC samples (blanks, calibration verification \ [CV\] standards, and replicate samples) must be in control.

In Control

A QA/QC sample is in control when the sample analysis result is within a certain tolerance of acceptable limits (usually 1¿). Calibration verification standards should be within acceptable limits of the actual value of carbonate, blanks should be within acceptable limits of background levels of carbonate, and replicate samples should be within acceptable limits of precision. When the system is in control, as indicated by acceptable results on QA/QC samples, analytical results for unknown samples are considered to be reliable.

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• If the blank result is <$WL and <$CL, the system is in control and analysis can continue.
• If the blank result is >$WL and <$CL, the system is flagged with warning limits, although analyses can proceed.
• If the blank result is >$CL, the system is out of control and samples in the analytical batch (between the previous blank and the current blank) are invalid and must be rerun.

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Calibration

The Coulometer instrument electronics are calibrated by the manufacturer. Each time the reagents are changed a calibration curve is constructed by running the following standards:

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The calibration curve is calculated using linear fit, least-squares method as measured CaCO₃ vs. STD CaCO₃:

A transfer function relates measured µg carbon from the instrument to normalized %CaCO₃. This transfer function is applied to all measurements in the range for which the calibration is valid.

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Calibration Verification

A check standard is run every 6 hr of Coulometer instrument operation or every 10 samples (whichever comes first). Check standards consist of a 100% CaCO₃ standard (reagent grade calcium carbonate).
The check standard result is evaluated against the threshold for %variance limits for calibration verification standard ($X) against true value as follows:
(834% x mass_C_normal/mass_normal) = m x (834% x mass_C_check/mass_check) + b
(834% x mass_C_normal/mass_normal) = normalized%CaCO3_

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• Carbonate carbon in calcium carbonate: 12.00%/12.00% ± 0.05%
• Titration accuracy is ±0.15% in samples with >1000 µg C.
• If sample volume limits CO₂ evolution to small amounts, accuracy is better than ~1 µg C. AnchorRTF38313031373a203248656164RTF38313031373a203248656164

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LIMS Integration

Sample Characteristics

• Analysis is typically performed on a homogenized powdered subsample
• Sample type can be homogenized powder or aqueous
• Analysis is destructive

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• Sample ID
• Instrument serial number
• Analysis timestamp
• µg carbon measured (measured)
• Slope threshold
• Analysis duration
• Method reference
• Calibration information
• Slope (m)
• Intercept (b)
• R²
• Timestamp

LIMS Analysis Components

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Health, Safety, & Environment

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Waste may be washed down drain with flowing water.

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Maintenance/Troubleshooting and Parts/Consumables

For maintenance and troubleshooting or parts and consumables, see the Coulometer User Guide.