A Revised Analytical Method for HCO3- and CO32-Determinations in Geothermal Waters: An Assessment of IAGC and IAEA Interlaboratory Comparisons

A statistical evaluation of the results of HCO₃^‐ determinations in geothermal waters during the interlaboratory comparison programmes of the International Association of Geochemistry and Cosmochemistry (IAGC) and International Atomic Energy Agency (IAEA) indicated that the analytical uncertainty increases with decreasing concentration of HCO_3‐ and was ? 25% for 50 μg ml^‐1 and ? 60% for 25 μg ml^‐1 of HCO₃^‐. The analytical method (Method 1) used by chemists and hydrologists works well for waters containing carbonic alkalinity, whereas Method 2 used by geochemists is conceptually incorrect. A stepwise comparison between the theoretical and experimental titration results for a given concentration Na₂CO₃ (0.0988 mol l^‐1) solution was performed to understand the limitations of the titration method for geothermal water analysis. Backward titration from the carbonic acid equivalence point (H₂CO₃EP) to the original pH after CO₂ removal, as had been practised earlier in the geothermal industry, in order to estimate the contribution of silicic and boric alkalinities to the total alkalinity, is incorrect because the amount of standard base (NaOH) added is equivalent to silicic and boric alkalinities plus some OH^‐ alkalinity. In a Na₂CO₃ solution, the added NaOH is equivalent to OH^‐ alkalinity only. Backward titration is only needed from the forward titration end point to the H₂CO₃EP in order to correct the total alkalinity for the excess of standard acid (HCl) added during the forward titration. In the case of a Na₂CO₃ solution, the H₂CO₃EP, after removal of CO₂ during the forward titration, is at pH = 7, not at pH = 4.5 (3.8) as has been considered in literature. Similarly, the liberation of CO₂ during titration occurs well before the point expected theoretically and it is less for shorter titration time. The revised procedure for the determination of carbonic species concentration is presented and illustrated for a water sample from Alchichica Lake, Puebla, Mexico.