Spectrophotometric Determination of the Bicarbonate Dissociation Constant in Seawater

Authors

Katelyn M. Schockman, University of South Florida
Robert H. Byrne, University of South FloridaFollow

Document Type

Article

Publication Date

2021

Keywords

CO2 system, Dissociation constant, Internal consistency, Seawater, Carbonate

Digital Object Identifier (DOI)

https://doi.org/10.1016/j.gca.2021.02.008

Abstract

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 (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, fCO₂) requires the use of K₁ and K₂. Uncertainties associated with current characterizations of pK₁ and pK₂ (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 K₁, values of pK₂ could then be calculated from the new K₁K₂ values. The resulting set of pK₂ values was fitted as a function of S_p and T to obtain a new pK₂ parameterization (denoted as _SWpK₂) calculated with the K₁ of Waters and Millero (2013) as revised by Waters et al. (2014): _SWpK₂ = 116.8067 − 3655.02 T ⁻¹ − 16.45817 ln T + 0.04523 S_p − 0.615 S_p^0.5 − 0.0002799 S_p² + 4.969 (S_p/T)

The average root mean square deviation between the equation and the observed data is 0.003. Residuals of this pK₂ fitting function (i.e., measured pK₂ minus parameterized pK₂) are substantially smaller than the residuals obtained in previous works. Similarly, the total standard uncertainty in pK₂ is reduced from 0.015 (previous characterizations) to 0.010 (this work). Internal consistency assessments (comparisons of measured versus calculated values of A_T, C_T, pH_T, and fCO₂) were used to evaluate the computational utility of the new K₂ parameterization. Assessments from both laboratory and shipboard data indicate that the internal consistency of CO₂ system calculations is improved using the K₂ parameterization of this work. This new K₂ parameterization provides the most precise, and potentially the most accurate, bicarbonate dissociation constant characterization presently available for open ocean conditions.

Rights Information

This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Geochimica et Cosmochimica Acta, v. 300, p. 231-245

Scholar Commons Citation

Schockman, Katelyn M. and Byrne, Robert H., "Spectrophotometric Determination of the Bicarbonate Dissociation Constant in Seawater" (2021). Marine Science Faculty Publications. 1593.
https://digitalcommons.usf.edu/msc_facpub/1593