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Gulf Coast Repository

Coulometer

Overview

Thumbnail image of coulometer Coulometer analysis determines carbonate concentration in a variety of samples, including sediments, 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.

Instrument Make & Model

UIC CM5017

Estimated Measurement Time

  • Sample Preparation: 1-3 days
  • Sample Measurement: ~8 samples/hour

Quantity of Sample Material

2–100 mg per sample; dependent on calcium carbonate content

Principles & Analytical Capabilities

The working range of the coulometer is <1 to 10,000 µg C per sample (optimal range = 1000–3000 µg C). The coulometer cell solution can absorb >100 mg of C.

The GCR uses N2 as the carrier gas.

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.

Data Generated

Data components:

  • Carbon (ug)
  • Carbon %
  • Calcium Carbonate %

Data files:

  • Tabular data (XLSX): compilation file of discrete measurements compiled into tabular format by the operator

Data Example

Graph of calcium carbonate data from Hole U1607A.
Calcium carbonate data from Hole U1607A of IODP Expedition 400 in the northwest Greenland glaciated margin. High calcium carbonate content in this environment is often found in the carbonate mud lithology. 1, 2

Procedures & Manuals

References

1  Knutz, P. C., Jennings, A. E., Childress, L. B., Bryant, R. M., Cargill, S. K., Coxall, H. K., Frank, T. D., Grant, G. R., Gray, R. E., Ives, L., Kumar, V., Le Houedec, S., Martens, J., Naim, F., Nelissen, M., Özen, V., Passchier, S., Pérez, L. F., Ren, J., … Zimmermann, H. H. (2025). IODP Expedition 400 Inorganic carbon (coulometer) [Data set]. International Ocean Discovery Program. https://doi.org/10.5281/zenodo.15366835

2  Knutz, P. C., Jennings, A. E., Childress, L. B., Bryant, R. M., Cargill, S. K., Coxall, H. K., Frank, T. D., Grant, G. R., Gray, R. E., Ives, L., Kumar, V., Le Houedec, S., Martens, J., Naim, F., Nelissen, M., Özen, V., Passchier, S., Pérez, L. F., Ren, J., … Zimmermann, H. H. (2025). IODP Expedition 400 GEODESC Description Files [Data set]. International Ocean Discovery Program. https://doi.org/10.5281/zenodo.15366849