Dataset: Spectrophotometric determination of bicarbonate dissociation constants (K2) for freshwater, estuarine, and marine waters over a wide range of temperatures

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).