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1. Re-evaluation of carbonic acid dissociation constants across conditions and the implications for ocean acidification

   https://doi.org/10.1016/j.marchem.2023.104247  

   Woosley, Ryan J.; Moon, Ji-Young (March 2023, Marine Chemistry)

   With the increasing threat of ocean acidification and the important role of the oceans in the global carbon cycle, highly precise, accurate, and intercomparable determination of inorganic carbon system parameters is required. Thermodynamic relationships enable the system to be fully constrained using a combination of direct measurements and calculations. However, calculations are complicated by many formulations for dissociation constants (over 120 possible combinations). To address these important issues of uncertainty and comparability, we evaluated the various combinations of constants and their (dis)agreement with direct measurements over a range of temperature (−1.9–40 ◦C), practical salinity (15–39) and pCO2 (150–1200 μatm). The results demonstrate that differences between the calculations and measurements are significantly larger than measurement uncertainties, meaning the oft-stated paradigm that one only needs to measure two parameters and the others can be calculated does not apply for climate quality ocean acidification research. The uncertainties in calculated pHt prevent climate quality pHt from being calculated from total alkalinity (TA) and dissolved inorganic carbon (DIC) and should be directly measured instead. However, climate quality TA and DIC can often be calculated using measured pH and DIC or TA respectively. Calculations are notably biased at medium-to-high pCO2 values (~500–800 μatm) implying models underestimate future ocean acidification. Uncertainty in the dissociation constants leads to significant uncertainty in the depth of the aragonite saturation horizon (>500 m in the Southern Ocean) and must be considered when studying calcium carbonate cycling. Significant improvements in the precision of the thermodynamic constants are required to improve pHt calculations. 

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2. Apparent nitrous acid dissociation across environmentally relevant temperatures in freshwater and seawater

   https://doi.org/10.1002/lno.12714  

   Borer, Benedict; Bi, Eric; Woosley, Ryan J; Babbin, Andrew R (December 2024, Limnology and Oceanography)

   Abstract Nitrite is a ubiquitous compound found across aquatic systems and an intermediate in both the oxidative and reductive metabolisms transforming fixed nitrogen in the environment. Yet, the abiotic cycling of nitrite is often overlooked in favor of biologically mediated reactions. Here we quantify the apparent acid dissociation constant (pK_a) between nitrous acid and its conjugate base nitrite in both freshwater and seawater systems across a range of environmentally relevant temperatures (5–35°C) using potentiometric‐based titration. In freshwater, we measured a pK_a,NBSof 3.14 at 25°C and a pK_a,Tof 2.87 for seawater at the same temperature. We quantify substantial effects of both salinity and temperature on the pK_a, with colder and fresher water manifesting higher values and thus a greater proportion of protonated nitrite at any given pH. Because nitrous acid is unstable and decomposes to nitric oxide, the implications for the nitrous acid dissociation constant on ecosystem function are broad. 

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3. High‐resolution dataset of stable carbon isotope of dissolved inorganic carbon ( δ¹³C ‐ DIC ) from the North Atlantic Ocean

   https://doi.org/10.1002/lol2.70038  

   Sun, Zhentao; Gao, Hui; Dong, Bo; Hussain, Najid; Atekwana, Eliot A; Cai, Wei‐Jun (September 2025, Limnology and Oceanography Letters)

   Abstract The stable isotope ratio of dissolved inorganic carbon (δ¹³C‐DIC) is a valuable tracer for investigating carbon cycling in aquatic environments. However, its potential remains underutilized due to limited data availability. Fewer than 15% of cruise samples are analyzed forδ¹³C‐DIC, as isotope analysis using isotope ratio mass spectrometry is labor‐intensive and restricted to onshore laboratories. We present over 3500δ¹³C‐DIC measurements from the 2023 Global Ocean Ship‐based Hydrographic Investigations Program A16N cruise in the North Atlantic. Notably, three‐quarters of these measurements were conducted onboard using a CO₂extraction device coupled with cavity ring‐down spectroscopy, a more efficient and cost‐effective method. This extensive dataset providesδ¹³C‐DIC values with spatial resolution comparable to other ocean carbonate chemistry and biogeochemical parameters. This dataset supports improved quantification of anthropogenic CO₂uptake and storage, and may facilitate the development of algorithms to estimateδ¹³C‐DIC in under sampled regions. 

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4. NMR Study of CO ₂ Capture by Butylamine and Oligopeptide KDDE in Aqueous Solution: Capture Efficiency and Gibbs Free Energy of the Capture Reaction as a Function of pH**

   https://doi.org/10.1002/cphc.202300053  

   Yang, Kaidi; Schell, Joseph; Gallazzi, Fabio; Wycoff, Wei; Glaser, Rainer (June 2023, ChemPhysChem)

   Abstract We have been interested in the development of rubisco‐based biomimetic systems for reversible CO₂ capture from air. Our design of the chemical CO₂ capture and release (CCR) system is informed by the understanding of the binding of the activator CO₂(^ACO₂) in rubisco (ribulose‐1,5‐bisphosphate carboxylase/oxygenase). The active site consists of the tetrapeptide sequence Lys‐Asp‐Asp‐Glu (or KDDE) and the Lys sidechain amine is responsible for the CO₂ capture reaction. We are studying the structural chemistry and the thermodynamics of CO₂ capture based on the tetrapeptide CH₃CO−KDDE−NH₂ (“KDDE”) in aqueous solution to develop rubisco mimetic CCR systems. Here, we report the results of ¹H NMR and ¹³C NMR analyses of CO₂ capture by butylamine and by KDDE. The carbamylation of butylamine was studied to develop the NMR method and with the protocol established, we were able to quantify the oligopeptide carbamylation at much lower concentration. We performed a pH profile in the multi-equilibrium system and measured amine species and carbamic acid/carbamate species by the integration of ¹H NMR signals as a function of pH in the range 8 ≤ pH ≤ 11. The determination of ΔG₁(R) for the reaction R−NH₂ + CO₂ R−NH−COOH requires the solution of a multi‐equilibrium equation system, which accounts for the dissociation constants K₂ and K₃ controlling carbonate and bicarbonate concentrations, the acid dissociation constant K₄ of the conjugated acid of the amine, and the acid dissociation constant K₅ of the alkylcarbamic acid. We show how the multi‐equilibrium equation system can be solved with the measurements of the daughter/parent ratio X, the knowledge of the pH values, and the initial concentrations [HCO₃⁻]₀ and [R‐NH₂]₀. For the reaction energies of the carbamylations of butylamine and KDDE, our best values are ΔG₁(Bu) = −1.57 kcal/mol and ΔG₁(KDDE) = −1.17 kcal/mol. Both CO₂capture reactions are modestly exergonic and thereby ensure reversibility in an energy‐efficient manner. These results validate the hypothesis that KDDE‐type oligopeptides may serve as reversible CCR systems in aqueous solution and guide designs for their improvement. 

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5. Radiocarbon measurements of dissolved inorganic carbon (DIC) in sediment porewater and seawater at AWI MICADAS

   https://doi.org/10.1017/RDC.2024.139  

   Grotheer, Hendrik; Hammes, Jens S; Gentz, Torben; Castrillejo, Maxi; Wacker, Lukas; Hansman, Roberta L; Mollenhauer, Gesine (April 2025, Radiocarbon)

   Abstract Radiocarbon (¹⁴C) measurements on dissolved inorganic carbon (DIC) are a powerful tool to trace water masses and carbon cycling in the ocean. Existing methodologies to determine the¹⁴C content of seawater DIC requires large volumes of sample (usually >100 mL) and specialized graphitization techniques to achieve the accuracy and precision needed for meaningful data interpretation. The advancement of the CO₂gas ionization accelerator mass spectrometry (AMS) technique today allows routine¹⁴C measurements on small samples (<100 µgC) and may thus permit reducing the sample volumes needed to determine¹⁴C content of seawater DIC to ∼2 mL. The proposed method utilizes the carbonate handling system (CHS), gas interface system (GIS) and MICADAS AMS, and provides good accuracy but reduced precision compared to established methods. Good accuracy is shown by comparing results for a marine in-house DIC standard and a DIC seawater profile from Antarctica between the proposed CHS-GIS-MICADAS approach and reference measurements conducted on the same material at established laboratories (ETH and NOSAMS). Further, two sedimentary porewater profiles from a fjord system in Svalbard are presented. Despite good agreement, the precision of the CHS-GIS-MICADAS approach is reduced, potentially limiting possible interpretations on seawater DIC. Nonetheless, the reduction of sample volumes proves particularly helpful to analyze porewater DIC from sediment cores, where sample material is notoriously limited, reduces the required amounts of toxic HgCl₂and simplifies expedition logistics. 

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