Spectrophotometric determination of the bicarbonate dissociation constant in seawater

Highlights

  • K1K2 values were obtained via spectrophotometric pH measurements.
  • New method improved precision in CO2 system dissociation constants.
  • New K2 parameterization improves internal consistency of CO2 system calculations.

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, pHT; total dissolved inorganic carbon, CT; total alkalinity, AT; 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 ≤ Sp ≤ 41) where Sp 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 Sp 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 Sp – 0.615 Sp0.5 – 0.0002799 Sp2 + 4.969 (Sp/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 ATCT, pHT, 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.

Schockman K. M. & Byrne R. H., in press. Spectrophotometric determination of the bicarbonate dissociation constant in seawater. Geochimica et Cosmochimica Acta. Article.

0 Responses to “Spectrophotometric determination of the bicarbonate dissociation constant in seawater”



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