Preprints
https://doi.org/10.5194/bg-2020-307
https://doi.org/10.5194/bg-2020-307

  02 Sep 2020

02 Sep 2020

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Technical note: CO2 is not like CH4 – limits of and corrections to the headspace method to analyse pCO2 in water

Matthias Koschorreck1, Yves T. Prairie2, Jihyeon Kim2, and Rafael Marcé3,4 Matthias Koschorreck et al.
  • 1Department Lake Research, Helmholtz Centre for Environmental Research -- UFZ, Brückstrasse 3a, 39114 Magdeburg, Germany
  • 2Département des Sciences Biologiques, Université du Québec à Montréal, Montréal, Québec, Canada
  • 3Catalan Institute for Water Research (ICRA), Emili Grahit 101, 17003 Girona, Spain
  • 4University of Girona, Girona, Spain

Abstract. Headspace analysis of CO2 frequently has been used to quantify the concentration of CO2 in freshwater. According to basic chemical theory, not considering chemical equilibration of the carbonate system in the sample vials will result in a systematic error. In this paper we provide a method to quantify the potential error resulting from simple application of Henry's law to headspace CO2 samples. By analysing the potential error for different types of water and experimental conditions we conclude that the error incurred by headspace analysis of CO2 is less than 5 % for samples with pH < 7.5. However, the simple headspace calculations can lead to high error (up to −800 %) or even impossible negative values in highly undersaturated samples equilibrated with ambient air, unless the shift in carbonate equilibrium is explicitly considered. The precision of the method can be improved by lowering the headspace ratio and/or the equilibration temperature and use of a CO2 free gas for headspace creation. We provide a direct method to correct CO2 headspace results using separately measured alkalinity.

Matthias Koschorreck et al.

 
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Matthias Koschorreck et al.

Matthias Koschorreck et al.

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Short summary
The concentration of carbon dioxide (CO2) in water samples is often measured using a gas chromatograph. Depending on the chemical composition of the water this method can produce wrong results. We quantified the possible error and how it depends on water composition and the analytical procedure. We propose a method to correct wrong results by additionally analysing alkalinity in the samples. We provide an easily usable computer code to perform the correction calculations.
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