Coulometer: User Guide

Manual Information

User Guide Contents

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Introduction

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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 measurement. 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 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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Figure 1. Model CM5011 CM5015 Coulometer.

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Figure 2. Acidification Module. Image Modified

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Dual Balance System Hardware

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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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Figure 3. Cahn Electrobalance.

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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 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).
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Figure 4. Mettler-Toledo Dual Balance Control Software.

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Laboratory Supplies

Apparatus

• KOH pre-scrubber trap
• AgNO₃ post-scrubber trap
• Reaction flask/reaction vial
• Bottle-top dispenser, 5 mL
• Agate mortar and pestle

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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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• • 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.) Wiki Markup3% AgNO{~}3~ (\
• • Warning!This procedure liberates caustic fumes and heat. Perform in a fume hood.)
  • 3% AgNO₃ ([%w/v
\
• • ]:
• • dissolve
• • 3
• • g
• • silver
• • nitrate
• • in
• • water
• • and
• • make
• • up
• • to
• • 100
• • mL
• • when
• • fully
• • dissolved.)
  Caution! Concentrated acids should always be added to water, and may generate significant heat.
• 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

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 to avoid overloading the AgNO₃ trap.
Solid samples must be dried, ground, and weighed before introduction into the prepared Coulometer apparatus. The workflow for solid sample preparation is as follows:

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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). AnchorRTF32383532373a203348656164RTF32383532373a203348656164

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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.
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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Preparing Acidification Module and Coulometer Cell

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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Figure 6. Acidification Module and Carbon Coulometer Cell.
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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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Figure 5: Coulometer software sample list screen. Options to refresh the list, append a new sample, edit an existing sample, or delete a sample or locate on the top right. The bottom left button allows the user to view the measurement history. The Measure button commences a measurement for the currently highlighted sample.

Running Samples

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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Figure 6: The sample measurement screen.

Shutting Down the Coulometer

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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

Common Problems

Poor Results

Instrument Not Operating Properly

Endpoint Never Reached

If the endpoint never seems to occur (the instrument continues to register small amounts of carbon long after the expended endpoint is reached), check the following:

Readings Are Too Low

Silver Nitrate Tube Clogged

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This tube is prone to clogging. To clean, use compressed air, then rinse with DI water. Note: Blow air through the tube over the sink to silver nitrate isn't blown all over the lab.

Display Not Lit

Coulometer Lamp Not Lit

No Cell Current

Low %T

A solution color change from the light blue at 29% transmittance to a royal dark blue at 0% indicates high silica in the sample, typical of a diatom mat. Ask the scientists to refrain from taking CARB samples from diatom layers.

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Cell Current Won't Shut Off

Low Maximum Current (less than 200 mA when %T is greater than 62)

Solution Rising in Anode Compartment

Measure Lamp Voltage

1. Remove cell from coulometer, turn off power, and remove left side panel.
2. Locate the carbon front end board (CM110-020).
3. Attach a volt meter to TP7 (red) and TP8 (black) on the CM110-020 board.
4. Turn voltmeter on in DC mode and record lamp voltage.
5. Adjust %T knob full clockwise and measure lamp voltage.
6. If lamp voltage is lower than the recommended range (<2.0–2.3 V), adjust the potentiometer marked RV4 to increase voltage. Do not increase voltage >2.5 V.

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Evaluate Electronics

Maximum/Minimum %T Test

1. Remove cell from coulometer.
2. Turn %T knob fully clockwise and record %T (should be >100%; factory setting = 110%).
3. Rotate %T knob fully counterclockwise and record minimum %T (factory setting = 12%).

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1. Switch the cell current switch to the OFF (center) position.
2. Disconnect and remove the cell from the cell compartment.
3. Turn on the main power supply and allow the instrument to warm up for a minimum of thirty (30) minutes.
4. From the Main Menu screen, touch System Parameters.
5. From the System Parameters screen, touch Change Settings.
6. Select the following parameters:
   
   

• • Analysis type = Carbon (CO₂ and CO₃ will be chosen in subsequent tests)
  • Calculation based on = Units Only
  • % Difference criteria = 0.1 (This value does not matter. It will not be used in any calculations.)
  • Factor = 1.0
  • Number of Readings = 2
  • Interval = 1.0
  • Timing Method = Fixed # of Readings
  • Sampling Method = Manual
  • Print Out Format = Cal. Test Format
  • Instrument ID = Default Value
  • Analyst ID = Default Value

1. From the Main Menu screen touch Run Cell Set-Up.
2. From the Cell Setup screen, make sure the value is stable. Touch Next to continue.
3. Note: the value will be less than 2700. Expect the value to be between 1200 and 1700.
4. From the Main Menu screen touch Run Analysis.
5. The Cell Activity screen will be presented. Touch Next to continue.
6. On the How Many Samples? screen enter 2 and touch Next.
7. On the Sample Entry screen enter BLANK for the first Sample Name and touch Enter (no Sample Size is required).
8. On the Sample Entry screen enter QC for the second Sample Name and touch Enter (no Sample Size is required).
9. The Begin Analysis/Monitor Cell Activity screen will be presented. Keep the Cell Current switch in the OFF (middle) position.
10. Touch Begin Analysis.
11. The Analyzing Sample screen will be presented. The %T should show 99.7—100.2 and the Cell I (cell current) should be 0.0—0.1.
12. After 1 minute the analysis will end and the Sample Complete screen will be presented momentarily as the data are written to the SD card.
13. The Begin Analysis/Monitor Cell Activity screen will be presented. Switch the Cell Current switch to the TEST (lower) position.
14. Touch Begin Analysis.
15. The Analyzing Sample screen will be presented. The %T should show 99.7—100.2 and the Cell I (cell current) should be 199.8—200.1.
16. After 1 minute the analysis will end and the Sample Complete screen will be presented.unmigrated-wiki-markup

17. Record

    the

    Result

    and

    Time

    values

    from

    the

    screen.

    These

    values

    will

    also

    be

    printed

    to

    the

    optional

    printer

    \

    [the

    JRSO

    does

    not

    have

    one

    \

    ],

    saved

    to

    the

    SD

    card,

    and

    transmitted

    through

    the

    serial

    and/or

    Ethernet

    ports

    for

    recovery

    later.

18. From the Sample Complete screen touch Done.
19. Repeat steps 4 through 22, selecting CO₂ and CO₃, successively as the Analysis Type.

20. Use the data that was collected from the three analyses to make the following calculations:
    
    

    Analysis Type	Theoretical Value	Actual Result	Time	Normalized Result	% Difference
    Carbon	1493.8

     

     

     

     

    CO2	5473.5

     

     

     

     

    CO3	7463.1

     

     

     

     

    
    
    

Actual Result = data collected from Step 22

Time = data collected from Step 22

Normalized Result = Actual Result / Time

% Difference = ((Normalized Result—Theoretical Value)/Theoretical Value)x100%

The calculated % Difference for any of the Analysis Types should be below ± 0.15%. If any of the values are > 0.15%, contact UIC for a bench calibration of the instrument.

Thorough Cleaning

At times, component parts may require a more thorough cleaning. To clean the frit, fill the cell with enough 1:1 concentrated nitric acid to water solution to cover the frit and allow the acid to clean the frit overnight. Dispose of the acid and rinse the cell and frit completely with water before re-use. If the potassium iodide solution turns brown after refilling the anode compartment, the frit has not been sufficiently rinsed.

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Figure 7. Coulometer Cell.

Chemicals

Expected Consumable Usage

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Expected usage levels of consumables are as follows. Actual usage levels will vary depending on sample load, type, matrix, carbon levels, and interfering substance levels.

Additional Consumables

• Silver Nitrate: 4 g per 200 samples
• KOH: 500 g/3000 samples
• Anode solution: 25 mL per 200 samples
• Cathode solution: 150 mL per 200 samples
• KI: 5 g per 200 samples

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