Spectrophotometric determination of the bicarbonate dissociation constant in seawater

The aqueous carbon dioxide (CO2) system stoichiometric dissociation constants K-1 and K-2 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 (pH, here expressed on the total hydrogen ion concentration scale, pH(T); total dissolved inorganic carbon, C-T; total alkalinity, A(T); and CO2 fugacity, fCO(2)) requires the use of K-1 and K-2. Uncertainties associated with current characterizations of pK(1) and pK(2) (where pK = -log K), on the order of 0.01 and 0.02, limit the accuracy of marine CO2 system calculations. This work reports the results of a spectrophotometric method to experimentally determine the product K1K2 over environmentally relevant ranges of temperature (288.15 <= T <= 308.15 K) and salinity (19.6 <= S-p <= 41) where S-p denotes the practical salinity scale. Using previously published parameterizations of K-1, values of pK(2) could then be calculated from the new K1K2 values. The resulting set of pK(2) values was fitted as a function of S-p and T to obtain a new pK(2) parameterization (denoted as (SW)pK(2)) calculated with the K-1 of Waters and Millero (2013) as revised by Waters et al. (2014): (SW)pK(2) = 116.8067 - 3655.02 T-1 - 16.45817 ln T + 0.04523 S-p - 0.615 S-p(0.5) - 0.0002799 S-p(2) + 4.969 (S-p/T) The average root mean square deviation between the equation and the observed data is 0.003. Residuals of this pK(2) fitting function (i.e., measured pK(2) minus parameterized pK(2)) are substantially smaller than the residuals obtained in previous works. Similarly, the total standard uncertainty in pK(2) is reduced from 0.015 (previous characterizations) to 0.010 (this work). Internal consistency assessments (comparisons of measured versus calculated values of AT, CT, pHT, and fCO(2)) were used to evaluate the computational utility of the new K-2 parameterization. Assessments from both laboratory and shipboard data indicate that the internal consistency of CO2 system calculations is improved using the K-2 parameterization of this work. This new K-2 parameterization provides the most precise, and potentially the most accurate, bicarbonate dissociation constant characterization presently available for open ocean conditions. (C) 2021 The Author(s). Published by Elsevier Ltd.

Automated summary

The study emphasizes the significance of CO2 system in seawater and oceans and the importance of dissociation constants related to pH. It improves the accuracy of calculations by experimentally determining K1K2 and obtaining a new pK(2) parameterization, which shows better consistency and reduced uncertainty compared to previous characterizations. The new K-2 parameterization provides the most precise characterization of bicarbonate dissociation constant for open ocean conditions.