Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

The aqueous carbon dioxide (CO₂) system stoichiometric dissociation constants K₁ and K₂ express the relative concentrations of CO₂, HCO₃⁻ (bicarbonate), and CO₃²⁻ (carbonate) in terms of pH. These constants are critical in the study of seawater and the oceans because any mathematical expression that relates the four major CO₂ system parameters requires the use of K₁ and K₂. 

The seawater CO₂ system can be described using four measurable parameters: total alkalinity (A_T), total dissolved inorganic carbon (C_T), pH (here expressed on the total hydrogen ion concentration scale, pH_T) and CO₂ fugacity (fCO₂). 

This dataset presents the first spectrophotometric data of K₂ at estuarine conditions (0≤S_p≤20; where S_p is practical salinity) and over a broad range of temperatures (275.15≤ T ≤308.15 K; where T is temperature in Kelvin). The low-salinity data were combined with the spectrophotometric data from Schockman and Byrne (2021) and Schockman et al. (2024). 

Seawater batches collected from surface waters of the northeastern Gulf of Mexico off the West Florida Shelf were diluted with Milli-Q water to produce salinities within the range of 0 to 20, representing conditions ranging from freshwater to brackish water, respectively. Each seawater batch was prepared immediately prior to use in the lab experiment. 

Spectrophotometric pH_T measurements were done following Clayton & Byrne (1993) and Dickson et al. (2007). Samples were measured in two-window 10cm cylindrical optical glass spectrophotometric cells using an Agilent 8453 diode array spectrophotometer with the UV light lamp turned off. Absorbance was taken at 434, 578, and 730 nanometers (nm) before and after adding 10 microliters (µL) of purified meta-Cresol Purple (mCP) indicator. The pH_T was calculated using 578/434 nm absorbance ratio, with 730 nm for baseline correction, using the model of Müller & Rehder (2018).

First, the pH_T was adjusted with HCl or NaOH to match the expected pH_T⁰ based on the sample's salinity (S_p) and temperature (T). Once pH_T was near pH_T⁰, five replicate pH_T readings were taken and averaged to get pH_initial. Then, a known amount of NaHCO₃ (or KHCO₃) was added to the sample and shaken thoroughly. Subsequently, the cell was placed back in the spectrophotometer and five pH_T replicates were recorded and averaged as pH_final. Next, the sample's temperature in the spectrophotometric cell was recorded using a digital thermometer (Ertco-Eutechnics Model 4400). Iterative adjustments were made if pH_final ≠ pH_initial. The next sample’s pH_initial was adjusted to better approximate pH_T⁰. 

After the daily samples were measured, seawater salinity for each individual sample was measured with a Guildline Portasal 8410A Laboratory Salinometer.  S_final denotes the salinity of each individual sample (i.e., salinity after bicarbonate salt addition), and S_initial is the salinity of diluted seawater batch. Note: NaHCO₃ additions increase salinity, especially when S_p < 5.

For experiments in which t<15 ºC (where t is temperature in ºC), the setup was moved to an environmental room for additional temperature control (Harris Environmental Systems). 

For a more detailed description of the methodology, please refer to Schockman and Byrne (2021) and Martin-Mayor et al. (2025, in review).

The resulting set of pK₂ values presented in this dataset was fitted as a function of S_P and T to obtain a new pK₂ parameterization for the salinity range 0≤ S_p ≤41 and temperature range 275.15≤T≤308.15 K (Martín-Mayor et al. 2025, in review). 

The resulting set of pK₂ values presented in this dataset was fitted as a function of  practical salinity (S_p) and temperature (T) to obtain a new pK₂ parameterization for the salinity range 0≤S_p≤41 and temperature range 275.15≤T≤308.15 K (Martín-Mayor et al. 2025, in review). 

- Imported data from source file "Martin-Mayor_Spec_pK2_S0-40_t2-35_FINAL.xlsx" sheet name "All data combined" into the BCO-DMO data system.
- Modified parameter (column) names to conform with BCO-DMO naming conventions.

NSF Award Abstract
Human health and well-being are linked in many ways to the health of our estuaries and coastal ocean waters. Yet surprisingly, we know less about some aspects of these important waters than we do the more distant waters of the deep ocean. This project will use state-of-the-art spectrophotometric methods (that is, light- and color-based methods) to advance our understanding of the fundamental and ever-changing chemistry of these waters and, eventually, the effects of these changes on marine life. The focus of this study will be to understand the chemistry of carbon dioxide in seawater. The new tools we will use are recently characterized pH indicators — chemicals that change color in seawater depending on the acidity of that water. These specially selected, purified indicators can be used to measure pH with unsurpassed precision and accuracy. We will use the indicators in laboratory experiments to determine how a critical parameter of the carbon dioxide system (a dissociation constant known as “K2”) changes depending on the temperature and salinity of the water. Characterizing K2 has been a goal of marine chemists for more than 50 years. The better we know K2, the better we can understand and predict how carbon moves through and cycles within natural waters. These measurements will expand our understanding of not only K2 but also the many other seawater characteristics that can be calculated from K2. Ultimately, this work will facilitate the interpretation and prediction of many ocean processes relevant to human health and coastal economies, such as ocean acidification (the lowering of ocean pH due to increasing carbon dioxide in the atmosphere) and calcium carbonate dissolution (the resulting dissolution of seashell material). The results will thus lay the groundwork for new perspectives on how ocean acidification affects the various shelled organisms that serve as food for economically important marine animals/fisheries and for people. The results of this work will also help to improve models of carbon dioxide dynamics in lakes, rivers, underground pore waters, and physiological fluids. As regards broader impacts, this work will help the PI continue to transfer his knowledge on this important topic to the next generation via his training of graduate, undergraduate, and high school students. This project would support one graduate and one undergraduate student, as well as help the current research projects of two minority doctoral students. Lastly, the PI plans to continue his involvement in the Bridge to the Doctoral Program aimed at getting minority students involved in the sciences.

In seawater, two carbonic acid dissociation constants (K1 and K2) describe the relationship between solution pH and the relative concentrations of dissolved carbonate ions, bicarbonate ions, and dissolved carbon dioxide. Accurate characterization of these CO2-system constants over broad ranges of environmental conditions has been a much sought-after goal for more than 50 years because knowledge of these terms is essential for quantitatively interpreting and predicting the biogeochemical cycling of carbon in all natural aqueous systems. The accuracy of CO2-system calculations is especially sensitive to uncertainties in K2, the equilibrium constant that describes the dissociation of bicarbonate ions to produce hydrogen ions and carbonate ions. This research project is designed to use spectrophotometric pH measurements (solely) to characterize this important constant. The purified pH indicators to be used in this work provide seawater pH measurements of unsurpassed precision and accuracy. Using select indicators whose properties have recently been characterized over freshwater-to-seawater ranges of salinity and temperature, we will determine K2 over similar ranges so as to improve the accuracy of CO2-system calculations in estuaries and coastal ocean waters. The resulting insight into equilibrium characteristics will facilitate interpretations and predictions of pH buffering in aqueous systems, provide an improved understanding of calcium carbonate solubility behavior, and lead to improved models of CO2-system behavior in freshwater lakes, rivers, soil and sediment pore waters, and physiological fluids. The longer-term benefits of this project will extend to assessments of the influence of ocean acidification on the life cycles of carbonate-bearing organisms that serve as food for economically important marine organisms.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.