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

This study presents the first spectrophotometric determination of K 2 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). Spectrophotometric data from Schockman and Byrne (2021) and Schockman et al. (2024) were combined with data obtained in the present study to produce a set of pK 2 (i.e., –log K 2 ) values for the salinity range 0 ≤ S p ≤ 41 and temperature range 275.15 ≤ T ≤ 308.15 K. The resulting set of pK 2 values was fitted as a function of S P and T to obtain a new pK 2 parameterization on the total pH scale: p K 2 = 5.1703 + 2136.77 T −1 – 177788 T −2 – 0.4457 S p 0.5 /(1 + 1.11* S p 0.5 ) + 0.0674 S p /ln T. – 0.0008238 S p 0.5 * T. The random uncertainty in this work is 0.0071 based on the standard deviation of the best-fit p K 2 parameterization. Residuals in the fit (i.e., measured minus parameterized p K 2 values) are approximately two times smaller in magnitude than the residuals obtained in past studies for similar S p and T ranges. The total combined standard uncertainty of p K 2 in this work is 0.0145. Assessments of high-quality total dissolved inorganic carbon ( C T ), total alkalinity ( A T ), and pH data obtained on oceanic research cruises indicate that internal consistency of CO 2 system calculations (comparing measured and calculated values; i.e., comparing measured pH with pH calculated from C T and A T ) is improved when calculations use this K 2 parameterization, especially for the Coastal Ocean Data Analysis Product in North America (CODAP-NA) data set (0.117 ≤ S p ≤ 36.980). As such, this new p K 2 parameterization provides an improved characterization of bicarbonate dissociation constant behavior that encompasses freshwater, estuarine, and open ocean conditions.