Revised Analytical Methods for the Determination of Carbonic Species in Rain, Ground and Geothermal Waters

Abstract

The concentration of carbonic species (H2CO3, HCO3- and CO32-) in natural waters is determined by the volumetric acid-base titration. In the case of low concentration carbonic water (e.g. rainwater), the Gran titration method is used. The application and limitations of the Gran titration method are discussed. The Gran titration method is based on locating the linear tendency of Gran functions F1 to F6. However, it is impossible to find the linear tendency of functions F2, F3, F5 and F6. Thus, in the case of rainwater, the Gran titration method is modified using only F1 and F4.
The titration methods for ground and geothermal waters are revised. The pH of the equivalence points, H2CO3EP, NaHCO3EP, and Na2CO3EP depends on the total dissolved carbonic concentration CT in a solution. Therefore, the location of the equivalence points through the corresponding points of inflexion is suggested. The procedure for groundwater containing only carbonic alkalinity is called here as Method 1.
The titration procedure for geothermal waters, Method 2, used by geochemists is conceptually incorrect. It is shown theoretically that there is an error of 14.59, 14.00, 17.84 and 19.92 % in the determination of carbonic species of pure Na2CO3 solutions CT=0.05, 0.03, 0.01 and 0.005 m, respectively. The backward titration from H2CO3EP to the original pH after CO2 removal, as had been practiced earlier in 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. The 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, but not at pH=4.5 (3.8) as has been considered in literature. The revised procedure for the determination of carbonic species concentration is presented and illustrated for natural waters.