Results from a Worldwide Proficiency Test on the Determination of Carbonic Species Concentration in Natural Waters

Abstract

The results of an international inter-laboratory proficiency test for the determination of carbonic species concentration are presented. Eight laboratories performed the analysis of twelve water samples (four synthetic waters, one lake water, four geothermal waters, one seawater, and two petroleum waters) by two methods: (a) individual laboratory analytical procedure and (b) acid-base titration curves in tabular form. From the titration curves the concentrations of carbonic species are calculated using the Hydrologist method, Geochemist’s method and initial pH and total alkalinity method. The titration curves of all laboratories yield very similar results (i.e., concentration of carbonic species for each sample). In contrast, the individual laboratory reported values showed high dispersion about the mean. This implies that there are some laboratories, which have problem in their concentration calculation procedure. To apply the Hydrologist and/or Geochemist methods it is necessary to locate always two equivalence points (e.g., NaHCO3EP and H2CO3EP) even for samples which have pH lower than that of NaHCO3EP. In addition, the backward titration curve with NaOH after complete removal of CO2 is strictly necessary to decide for the applicability of the Hydrologist method or the Geochemist method. In cases where the complete analyses of species that contribute to the alkalinity are known, the initial pH and total alkalinity method is appropriate.