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

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.