Spectrophotometric Determination of the Bicarbonate Dissociation Constant in Seawater

Katelyn M. Schockman, Robert H. Byrne

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Abstract

<p> <p id="x-x-sp0010"> The aqueous carbon dioxide (CO <sub> 2 </sub> ) system stoichiometric dissociation constants <em> K </em> <sub> 1 </sub> and <em> K </em> <sub> 2 </sub> express the relative concentrations of CO <sub> 2 </sub> , HCO <sub> 3 </sub> <sup> &minus; </sup> (bicarbonate), and CO <sub> 3 </sub> <sup> 2&minus; </sup> (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 CO <sub> 2 </sub> system parameters (pH, here expressed on the total hydrogen <a title="Learn more about ion concentration from ScienceDirect's AI-generated Topic Pages"> ion concentration </a> scale, pH <sub> T </sub> ; total <a title="Learn more about dissolved inorganic carbon from ScienceDirect's AI-generated Topic Pages"> dissolved inorganic carbon </a> , <em> C </em> <sub> T </sub> ; total alkalinity, <em> A </em> <sub> T </sub> ; and CO <sub> 2 </sub> fugacity, <em> f </em> CO <sub> 2 </sub> ) requires the use of <em> K </em> <sub> 1 </sub> and <em> K </em> <sub> 2 </sub> . Uncertainties associated with current characterizations of p <em> K </em> <sub> 1 </sub> and p <em> K </em> <sub> 2 </sub> (where p <em> K </em> =  &minus;log <em> K </em> ), on the order of 0.01 and 0.02, limit the accuracy of marine CO <sub> 2 </sub> system calculations. This work reports the results of a spectrophotometric method to experimentally determine the product <em> K </em> <sub> 1 </sub> <em> K </em> <sub> 2 </sub> over environmentally relevant ranges of temperature (288.15 &le; <em> T </em> &le; 308.15 K) and <a title="Learn more about salinity from ScienceDirect's AI-generated Topic Pages"> salinity </a> (19.6 &le; <em> S </em> <sub> p </sub> &le; 41) where <em> S </em> <sub> p </sub> denotes the practical salinity scale. Using previously published parameterizations of <em> K </em> <sub> 1 </sub> , values of p <em> K </em> <sub> 2 </sub> could then be calculated from the new <em> K </em> <sub> 1 </sub> <em> K </em> <sub> 2 </sub> values. The resulting set of p <em> K </em> <sub> 2 </sub> values was fitted as a function of <em> S </em> <sub> p </sub> and <em> T </em> to obtain a new p <em> K </em> <sub> 2 </sub> parameterization (denoted as <sub> SW </sub> p <em> K </em> <sub> 2 </sub> ) calculated with the <em> K </em> <sub> 1 </sub> of Waters and Millero (2013) as revised by Waters et al. (2014): <sub> SW </sub> p <em> K </em> <sub> 2 </sub> = 116.8067 &minus; 3655.02 <em> T </em> <sup> &minus;1 </sup> &minus; 16.45817 ln <em> T </em> + 0.04523 <em> S </em> <sub> p </sub> &minus; 0.615 <em> S </em> <sub> p </sub> <sup> 0.5 </sup> &minus; 0.0002799 <em> S </em> <sub> p </sub> <sup> 2 </sup> + 4.969 ( <em> S </em> <sub> p </sub> / <em> T </em> ) <p id="x-x-sp0015"> The average root mean square deviation between the equation and the observed data is 0.003. Residuals of this p <em> K </em> <sub> 2 </sub> fitting function (i.e., measured p <em> K </em> <sub> 2 </sub> minus parameterized p <em> K </em> <sub> 2 </sub> ) are substantially smaller than the residuals obtained in previous works. Similarly, the total standard uncertainty in p <em> K </em> <sub> 2 </sub> is reduced from 0.015 (previous characterizations) to 0.010 (this work). Internal consistency assessments (comparisons of measured versus calculated values of <em> A </em> <sub> T </sub> , <em> C </em> <sub> T </sub> , pH <sub> T </sub> , and <em> f </em> CO <sub> 2 </sub> ) were used to evaluate the computational utility of the new <em> K </em> <sub> 2 </sub> parameterization. Assessments from both laboratory and shipboard data indicate that the internal consistency of CO <sub> 2 </sub> system calculations is improved using the <em> K </em> <sub> 2 </sub> parameterization of this work. This new <em> K </em> <sub> 2 </sub> parameterization provides the most precise, and potentially the most accurate, bicarbonate dissociation constant characterization presently available for open ocean conditions. </p> </p></p>
Original languageAmerican English
JournalGeochimica et Cosmochimica Acta
Volume300
DOIs
StatePublished - Jan 1 2021

Keywords

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

Disciplines

  • Life Sciences

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