Ocean Acidification

Total dissolved inorganic carbon (DIC) is a fundamental variable for characterizing the marine carbonate system and quantifying key biogeochemical processes, including ocean acidification, carbon storage, air-sea CO2 fluxes, and monitoring, reporting, and verification for marine carbon dioxide removal. Although standardized guidelines exist (e.g., IOCCP Report No. 8, SOP 2), routine oceanographic practice frequently relies on single-point calibrations using seawater reference materials (RMs) near ~2000 µmol/kg. While this practice has significantly improved inter-laboratory comparability, it often lacks formal metrological traceability to the International System of Units (SI), fails to verify system response linearity across the full working range, and leaves major components of the measurement uncertainty budget unconstrained.
To address these metrological limitations, the International Association for the Physical Sciences of the Oceans Best Practice Study Group (IAPSO-BP-SG) on ‘Calibrating Measurements of Total Dissolved Inorganic Carbon in Seawater’ was established in late 2023. This IAPSO-BP-SG presents a updated Standard Operating Procedure (SOP) based on recent inter-laboratory comparison results. This SOP unifies calibration guidelines across diverse analytical architectures. The recommended framework is structured around four primary operational pillars:

1. Daily single-point calibration and adjustment: Using a characterized seawater RM to adjust for daily slope drift and verify operational stability.
2. Periodic linearity verification: Assessing system linearity over the entire working range (0–2500 µmol/kg) using a minimum of six sodium carbonate standard solutions or calibrated gas loops to evaluate baseline intercepts and prevent scaling errors.
3. Repeatability quantification: Regularly determining system and method precision through replicate measurements under repeatable laboratory conditions.
4. Metrological uncertainty evaluation: Constructing a robust uncertainty budget following the Guide to the Expression of Uncertainty in Measurement (GUM), propagating contributions from standard materials, repeatability, density conversions, and potential instrumental drift.
   Additionally, the SOP outlines detailed procedures for volumetric and gravimetric preparation of primary standard solutions under CO2-free conditions, along with guidelines for density corrections and uncertainty propagation. Implementing this SOP ensures SI-traceable, high-precision DIC data suitable for long-term oceanographic monitoring and global data repositories.

García-Ibáñez M. I., Seitz S., Álvarez M., Cantoni C., Capitaine G., Easley-Vidal R. A., Humphreys M. P., Jenkins A., Knockaert M., Metzl N., Norbisrath M., Steinhoff T., Williams R., & Woosley R., 2026. Standard Operating Procedure – Calibration of measurement systems for total dissolved inorganic carbon in seawater. International Association for the Physical Sciences of the Oceans (IAPSO) Best Practice Study Group on ‘Calibrating Measurements of Total Dissolved Inorganic Carbon in Seawater’. SOP.