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.
IODP's UIC Coulometrics CM5011 CM5015 coulometer provides absolute determination of the concentration of carbon dioxide (CO2) evolved from an acidification process. The coulometer cell is filled with a proprietary solution containing monoethanolamine and a colorimetric pH indicator. A platinum cathode and silver anode are positioned in the cell, and the assembly is located between a light source and a photodetector. When a gas stream passes through the solution, CO2 is quantitatively absorbed, reacting with the monoethanolamine to form a titratable acid. This acid causes the color indicator to fade. A spectrophotometer monitors the change in the solution's percent transmittance (%T). As %T increases, the titration current is automatically adjusted to generate a base at a rate proportional to the reduction of %T. When the solution returns to its original color (original %T), the current stops. The amount of CO2 evolved is calculated from the duration and magnitude of the current required to balance the acid by CO2 evolution. Based on the principle of Faraday's Law of Electrolysis (the quantity of a substance produced by electrolysis is proportional to the quantity of electricity used), each mole of electrons added to the solution is equivalent to 1 mole of CO2 titrated.
Chemical reactions occurring in the coulometer cell follow:
Absorption of CO2 by the cathode solution (cathode reaction):
CO2 + HOCH2CH2NH2 —> HOCH2CH2NHCOOH
Electrochemical generation of OH– (cathode reaction):
2H2O + 2e– —> H2 (g) + 2OH–
Neutralization of absorbed CO2 reaction product by electrochemically generated OH–:
HOCH2CH2NHCOOH + OH– —> HOCH2CH2NHCOO– + H2O
Anode reaction:
AgO —> Ag+ + e–
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.
Figure 1. Model CM5011 CM5015 Coulometer. | Figure 2. Acidification Module. |
Figure 3. Cahn Electrobalance. | Figure 4. Cahn Balance Control Software. |
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.
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). Sample material must be measured on the Cahn balance (unless the sample is larger than ~1 gram) (Figure 4).
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) (to make 250 mL, dissolve 7.5 g silver nitrate in water and make up to 250 ml once fully dissolved)
2N H2SO4: add 55.5 mL concentrated sulfuric acid to water and make up to 1L (to make 100 mL, add 5.55 mL concentrated sulfuric acid to water and make up to 100 mL)
2N HCl: add 166 mL concentrated hydrochloric acid to water and make up to 1L
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:
Figure 5. Acidification Module and Carbon Coulometer Cell.
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.
Figure 6: Coulometer software sample list screen. Options to refresh the list, append a new sample, edit an existing sample, or delete a sample 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.
Prepare the coulometer cell, place it in the coulometer and connect the leads before turning on power.
The sample measurement screen
A completed check standard measurement
Shut down the instrument after each run.
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.
QA/QC for Coulometer analysis consists of instrument calibration and continuing calibration verification using check standards, along with blanks and replicate samples.
The working range of the CO2 coulometer is <1 to 10,000 µg C per sample (optimum range = 1000–3000 µg C). The coulometer cell solution can absorb >100 mg of C. Titrating at maximum current (200 mA), the coulometer can titrate 1500 µg of carbon (or 5500 µg CO2{~}) per min.
An analytical batch is a method-defined number of samples with which QC samples including calibration verification, blank check, and replicate samples are run. Because samples are grouped into QC batches, if problems arise, affected samples can be identified and reanalyzed. Analytical batches for the coulometer are typically 10 samples.
Each QA/QC sample has one the following results:
For a system to be considered in control, all QA/QC samples (blanks, calibration verification [CV] standards, and replicate samples) must be 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.
When QA/QC samples exceed the warning limits (generally 2¿ but ¿ to 3¿¿, the system is considered to be in danger of becoming out of control (but is not yet out of control). Typically, the warning situation indicates that the operator must decide whether to take action. The operator can continue the analysis if he or she does not think that the control limit will be exceeded.
If the control limits are exceeded (generally 3¿), the instrument system is considered out of control and all samples in the current analytical batch are invalid and should be reanalyzed once corrective action has been taken to put the system back in control.
A blank is run every N (defined by method) samples. The blank result is evaluated against $CL, the method-defined percent threshold that the measured blank value can deviate from standard value and still be considered in control, and $WL, the method-defined percent threshold that the measured blank value can deviate from the standard value before setting a warning flag.
The Coulometer instrument electronics are calibrated by the manufacturer. Each time the reagents are changed a calibration curve is constructed by running the following standards:
The calibration curve is calculated using linear fit, least-squares method as measured CaCO3 vs. STD CaCO3:
Variable | Calculation |
y = STD_CaCO3 | (mass_C_std/mass_std) x (100.087/12) x 100% = 834% x mass_C_std/mass_std |
m = slope | (STD_CaCO3/Sample_CaCO3) |
b = intercept | STD_CaCO3 |
x = meas_CaCO3 | (mass_C_sample/mass_sample) x (100.087/12) x 100% = 834% x mass_C_sample/mass_sample |
y = mx + b | (834% x mass_C_std/mass_std) = m x (834% x mass_C_sample/mass_sample) + b |
A transfer function relates measured µg carbon from the instrument to normalized %CaCO3. This transfer function is applied to all measurements in the range for which the calibration is valid.
A check standard is run every 6 hr of Coulometer instrument operation or every 10 samples (whichever comes first). Check standards consist of a 100% CaCO3 standard (reagent grade calcium carbonate).
The check standard result is evaluated against the threshold for %variance limits for calibration verification standard ($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_
Every N (defined by method) samples, a single sample is analyzed in replicate. The deviation between the two sample results is evaluated against $CL, the method-defined maximum percent deviation allowable for the precision to be considered in control, and $WL, the method-defined percent deviation allowable for the precision before setting a warning flag.
Typical accuracy using the UIC Coulometer is as follows:
Data have the following dependencies on weight analysis:
The following analysis components are uploaded from the coulometer into the LIMS with each sample result:
Analysis | Component | Definition | Unit |
COUL | calcium_carbonate_percent | Concentration of CaCO3 in sample | wt% |
carbon_mass | Mass of carbon in sample | µg | |
carbon_percent | Concentration of carbon in sample | wt% | |
container_number | |||
mass | Mass of sample | mg | |
COUL_QAQC | calcium_carbonate_expected_percent | Concentration of CaCO3 expected in standard | wt% |
calcium_carbonate_percent | Concentration of CaCO3 in sample | wt% | |
carbon_expected_mass | Mass of carbon expected in a standard | µg | |
carbon_expected_percent | Concentration of carbon expected in standard | wt% | |
carbon_mass | Mass of carbon found in standard | µg | |
carbon_percent | Percent carbon found in standard | wt% | |
container_number | |||
corr2 | Correlation coefficient R2 | ||
intercept | |||
mass | Mass of sample | mg | |
slope | |||
standard_percent | Percent of carbon expected in standard as determined from standard | wt# |
–Hazardous components: Dimethyl sulfoxide, Monoethanolamine, Tetraethylammonium bromide (TEAB)
–Hazards:
–Handling: absorbs CO2; keep tightly closed.
–Storage: keep away from oxidizers, heat, and ignition sources
–PPE: gloves, safety glasses
–Reactivity: stable; incompatible with oxidizers, acids, alkali metals, CO2
–Hazardous components: Dimethyl sulfoxide, potassium iodide
–Hazards:
–Storage: keep away from heat/ignition sources and oxidizing agents
–PPE: gloves, safety glasses
–Reactivity: stable; incompatible with oxidizers, acids, alkali metals, CO
–Hazards:
–Incompatible materials: alkaloid salts, chloral hydrate, potassium chlorate, metallic salts, tartaric and other acids, bromine trifluoride, fluorine perchlorate
Waste of cathode and anode solutions should be collected in a bottle until it can be removed during the next port call. The potassium hydroxide and silver nitrate solutions may be disposed of in the sink.
Potential explanation | Solution |
Non-coulometer malfunction | Inspect other components of the system for leaks, clogs, expended solutions or scrubber chemicals |
Clogged frit in cell | See Thorough Cleaning, below |
Silver electrode not in cell | Lower electrode into solution |
Excessive deposits on silver electrode | Clean electrode with saturated KI solution, rinse with water |
No excess KI in anode compartment | Add KI to anode compartment |
Excessive deposits on platinum electrode | Clean platinum electrode with 1:1 concentrated nitric acid to water solution, then rinse thoroughly with water |
Exhausted coulometer solutions | Replace coulometer solutions |
Improper cell alignment | Align cell and run new Cell Setup |
Faulty coulometer calibration | Perform Electronic Calibration Check (and contact UIC if it fails) |
No stir bar in cell | Place stir bar in cell |
Check | Specifications |
Age of titration solution | If >50 samples have been analyzed using current titration solution, make new |
Age of reagents in the traps | If >50 samples have been analyzed using reagent in traps, replace solutions |
Are the traps assembled correctly? | Verify that the traps are assembled correctly and in the proper order |
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:
Potential explanation | Solution |
Sample takes a long time to break down | Some samples take longer to break down than others |
Titration solution is old | Change titration solution and recalibrate the instrument |
KOH scrubber is exhausted | Change out all reagents in scrubber |
Fittings are leaking | Any leaks in fittings allows atmospheric air into the system |
Potential explanation | Solution |
Inadequate sample pickup | Check that inner plastic tubing in the sample is within 5 mm of bottom of glass sample tube |
Leaks | Check tubing connections for leaks |
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.
Potential explanation | Solution |
Power not on | Turn on power |
Blown fuse | Replace fuse |
Defective display | Contact UIC for repair |
Potential explanation | Solution |
Defective lamp | Replace lamp or contact UIC for repair |
Potential explanation | Solution |
Cell current switch in OFF position | Switch cell current switch to ON position |
Loose electrical connection | Check both red and black electrode connections; check electrode continuity |
Defective power supply | Contact UIC for repair |
Defective current source | Contact UIC for repair |
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.
Potential explanation | Solution |
Lamp brightness has deteriorated with age | Replace lamp (CM140-005) |
Path to detector is blockedLight path blocked | Check for physical blocking of the light path; you will need to run a new Cell Setup once the cell is moved |
Lamp voltage is incorrect | Measure lamp voltage (see Measure Lamp Voltage) |
Detector and/or filter are cloudedDefective photodiode | Replace filter (CM140-001) or photodiode (CM140-002). It is best to replace entire photodiode subassembly (CM101-178).Contact UIC for repair |
Detector is defectiveDefective amplifier circuit | See Evaluate Electronics Contact UIC for repair |
Loose connection on front end board | Locate the front end board (CM110-020). Ensure all connectors to the board are plugged in securely; reset connectors by pushing on them. |
Electronic problem on circuit board | Run electronics checks (see Evaluate Electronics) |
Potential explanation | Solution |
Defective main board | Contact UIC for repair |
Bubbles flowing through light path | Reposition cell and run new Cell Setup |
Cathode solution is expended | Clean and refill cell |
Potential explanation | Solution |
Clogged frit in cell | See Thorough Cleaning below |
Excessive deposits in silver electrode | Clean electrode with saturated KI solution, rinse with water |
Potential explanation | Solution |
Blocked vent cell tube | Clear or replace vent cell tube |
An electronic calibration check is performed to verify the proper operation of the internal components of the CM5015. This check does not verify the integrity of the analytical cell, the front-end system, or analytical standards.
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.
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.
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.
Figure 7. Coulometer Cell.
Part | Name | UIC Part Number |
1 | Cell with side arm | CM200-051 |
2 | Cathode top | CM192-005 |
3 | Platinum electrode, cathode | CM101-034 |
4 | Cell inlet tube | CM190-002 |
9 | Anode top | CM192-006 |
10 | Silver electrode, anode | CM101-033 |
11 | Stir bar, 1.5 in. | CM121-006 |
12 | Complete cell assembly | CM210-008015 |
Name | UIC Part Number |
Carbon cathode solution, 1 gallon | CM300-001 |
Carbon anode solution, 16 oz | CM300-002 |
Potassium iodide, 50 g | CM300-003 |
Calcium carbonate standard, 100 g | CM301-002 |
Carbon cell reagent kit | CM310-001 |
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.
UIC Part Number | Name | Estimated usage |
CM300-001 | Carbon cathode solution | 250 mL/wk |
CM300-002 | Carbon anode solution | 32 mL/wk |
CM300-003 | Potassium iodide | 3.2 g/wk |
CM101-033 | Silver electrode (anode) | 400 analyses |
CM101-034 | Platinum electrode (cathode) | Replace only when broken |
CM129-071 | Cell inlet tube fitting | 1/6 months |
CM140-005 | Lamp | 1/12 months |
45% solution | 15–25 mL/month | |
2N HCl solution | 10 mL/sample | |
CM210-022 | Pre-scrubber | 1/year |
CM192-003 | Check valve, pk/6 | 10 weeks per valve |
UIC Inc. 1225 Channahon Road Joliet, IL 60436 800-342-5842 uicsales@uicinc.com www.uicinc.com
Be certain that the CM5015 is running in CM5011 emulation mode for proper interface with the JRSO software. Also note that the "latch" commands used with the CM5011 are not applicable to the CM5015.
ANALYSIS | TABLE | NAME | ABOUT TEXT |
COUL | SAMPLE | Exp | Exp: expedition number |
COUL | SAMPLE | Site | Site: site number |
COUL | SAMPLE | Hole | Hole: hole number |
COUL | SAMPLE | Core | Core: core number |
COUL | SAMPLE | Type | Type: type indicates the coring tool used to recover the core (typical types are F, H, R, X). |
COUL | SAMPLE | Sect | Sect: section number |
COUL | SAMPLE | A/W | A/W: archive (A) or working (W) section half. |
COUL | SAMPLE | text_id | Text_ID: automatically generated database identifier for a sample, also carried on the printed labels. This identifier is guaranteed to be unique across all samples. |
COUL | SAMPLE | sample_number | Sample Number: automatically generated database identifier for a sample. This is the primary key of the SAMPLE table. |
COUL | SAMPLE | label_id | Label identifier: automatically generated, human readable name for a sample that is printed on labels. This name is not guaranteed unique across all samples. |
COUL | SAMPLE | sample_name | Sample name: short name that may be specified for a sample. You can use an advanced filter to narrow your search by this parameter. |
COUL | SAMPLE | x_sample_state | Sample state: Single-character identifier always set to "W" for samples; standards can vary. |
COUL | SAMPLE | x_project | Project: similar in scope to the expedition number, the difference being that the project is the current cruise, whereas expedition could refer to material/results obtained on previous cruises |
COUL | SAMPLE | x_capt_loc | Captured location: "captured location," this field is usually null and is unnecessary because any sample captured on the JR has a sample_number ending in 1, and GCR ending in 2 |
COUL | SAMPLE | location | Location: location that sample was taken; this field is usually null and is unnecessary because any sample captured on the JR has a sample_number ending in 1, and GCR ending in 2 |
COUL | SAMPLE | x_sampling_tool | Sampling tool: sampling tool used to take the sample (e.g., syringe, spatula) |
COUL | SAMPLE | changed_by | Changed by: username of account used to make a change to a sample record |
COUL | SAMPLE | changed_on | Changed on: date/time stamp for change made to a sample record |
COUL | SAMPLE | sample_type | Sample type: type of sample from a predefined list (e.g., HOLE, CORE, LIQ) |
COUL | SAMPLE | x_offset | Offset (m): top offset of sample from top of parent sample, expressed in meters. |
COUL | SAMPLE | x_offset_cm | Offset (cm): top offset of sample from top of parent sample, expressed in centimeters. This is a calculated field (offset, converted to cm) |
COUL | SAMPLE | x_bottom_offset_cm | Bottom offset (cm): bottom offset of sample from top of parent sample, expressed in centimeters. This is a calculated field (offset + length, converted to cm) |
COUL | SAMPLE | x_diameter | Diameter (cm): diameter of sample, usually applied only to CORE, SECT, SHLF, and WRND samples; however this field is null on both Exp. 390 and 393, so it is no longer populated by Sample Master |
COUL | SAMPLE | x_orig_len | Original length (m): field for the original length of a sample; not always (or reliably) populated |
COUL | SAMPLE | x_length | Length (m): field for the length of a sample [as entered upon creation] |
COUL | SAMPLE | x_length_cm | Length (cm): field for the length of a sample. This is a calculated field (length, converted to cm). |
COUL | SAMPLE | status | Status: single-character code for the current status of a sample (e.g., active, canceled) |
COUL | SAMPLE | old_status | Old status: single-character code for the previous status of a sample; used by the LIME program to restore a canceled sample |
COUL | SAMPLE | original_sample | Original sample: field tying a sample below the CORE level to its parent HOLE sample |
COUL | SAMPLE | parent_sample | Parent sample: the sample from which this sample was taken (e.g., for PWDR samples, this might be a SHLF or possibly another PWDR) |
COUL | SAMPLE | standard | Standard: T/F field to differentiate between samples (standard=F) and QAQC standards (standard=T) |
COUL | SAMPLE | login_by | Login by: username of account used to create the sample (can be the LIMS itself [e.g., SHLFs created when a SECT is created]) |
COUL | SAMPLE | login_date | Login date: creation date of the sample |
COUL | SAMPLE | legacy | Legacy flag: T/F indicator for when a sample is from a previous expedition and is locked/uneditable on this expedition |
COUL | TEST | test changed_on | TEST changed on: date/time stamp for a change to a test record. |
COUL | TEST | test status | TEST status: single-character code for the current status of a test (e.g., active, in process, canceled) |
COUL | TEST | test old_status | TEST old status: single-character code for the previous status of a test; used by the LIME program to restore a canceled test |
COUL | TEST | test test_number | TEST test number: automatically generated database identifier for a test record. This is the primary key of the TEST table. |
COUL | TEST | test date_received | TEST date received: date/time stamp for the creation of the test record. |
COUL | TEST | test instrument | TEST instrument [instrument group]: field that describes the instrument group (most often this applies to loggers with multiple sensors); often obscure (e.g., user_input) |
COUL | TEST | test analysis | TEST analysis: analysis code associated with this test (foreign key to the ANALYSIS table) |
COUL | TEST | test x_project | TEST project: similar in scope to the expedition number, the difference being that the project is the current cruise, whereas expedition could refer to material/results obtained on previous cruises |
COUL | TEST | test sample_number | TEST sample number: the sample_number of the sample to which this test record is attached; a foreign key to the SAMPLE table |
COUL | CALCULATED | Top depth CSF-A (m) | Top depth CSF-A (m): position of observation expressed relative to the top of the hole. |
COUL | CALCULATED | Bottom depth CSF-A (m) | Bottom depth CSF-A (m): position of observation expressed relative to the top of the hole. |
COUL | CALCULATED | Top depth CSF-B (m) | Top depth [other] (m): position of observation expressed relative to the top of the hole. The location is presented in a scale selected by the science party or the report user. |
COUL | CALCULATED | Bottom depth CSF-B (m) | Bottom depth [other] (m): position of observation expressed relative to the top of the hole. The location is presented in a scale selected by the science party or the report user. |
COUL | RESULT | calcium_carbonate_percent (wt%) | RESULT calcium carbonate (wt%): calcium carbonate concentration in sample expressed as weight percent; this is a calculated field. |
COUL | RESULT | carbon_mass (µg) | RESULT carbon mass (ug): mass of inorganic carbon in sample expressed as total micrograms of carbon (not a concentration) |
COUL | RESULT | carbon_percent (wt%) | RESULT inorganic carbon (wt%): concentration of carbon in sample expressed as weight percent; this is a calculated field. |
COUL | RESULT | container_number | RESULT container number: container number of the coulometer sample (used to keep samples straight) |
COUL | RESULT | mass (mg) | RESULT sample mass (mg): mass of sample weighed into the container for analysis expressed in mg |
COUL | SAMPLE | sample description | SAMPLE comment: contents of the SAMPLE.description field, usually shown on reports as "Sample comments" |
COUL | TEST | test test_comment | TEST comment: contents of the TEST.comment field, usually shown on reports as "Test comments" |
COUL | RESULT | result comments | RESULT comment: contents of a result parameter with name = "comment," usually shown on reports as "Result comments" |
Coulometer User Guide: 29th September 2022