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 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 %T. When the solution returns to its original color (original %T), the current stops. The amount of CO2 evolved is quantitated 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 carrier gas. Interferences caused by compounds such as SO2, SO3, H2S, HCl, HBr, HI, and Cl2 are removed with KOH and AgNO3 scrubbers.
Figure 1. Model CM5015 Coulometer. | Figure 2. Acidification Module. |
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.
Figure 3. Cahn Electrobalance.
Figure 4. Mettler Toledo XS204.
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.
Figure 6. Cahn Balance Control Software.
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.
Solid samples must be dried, ground, and weighed before introduction into the prepared Coulometer apparatus. The workflow for solid sample preparation is as follows:
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.
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.
Prepare the coulometer cell, place it in the coulometer and connect the leads before turning on power.
Figure 9. Coulometer Software
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 0.01 µg to 100mg 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 may be washed down drain with flowing water.
For maintenance and troubleshooting or parts and consumables, see the Coulometer User Guide.