Acid neutralizing capacity of solutions containing organic acids isolated from Finnish lakes

Organic vs. bicarbonate contribution to Gran alkalinity was studied using empirical measurements. Distilled water with various amounts of bicarbonate and organic acids was equilibrated with synthetic air during Gran titrations. Natural organic acids isolated from Finnish lakes were used as a carbon source for these solutions. The experimental design corresponded to computer simulations made by Cantrell et al. (1990). The results indicate that the discrepancy between the Gran ANC and the ANC calculated from anion deficit can be explained partly by the organic acids, which remain unprotonated in the titration. The unprotonated fraction of organic acids was on average 5.5 and 6.2 eq per mg of dissolved organic carbon (DOC), when pH ranges of 3.0–4.0 and 3.5–4.5 for Gran plot regression were used, respectively. The acid neutralizing capacity provided by organic acids varied from 18 to 75 eq/L, depending on the initial pH and DOC concentration and the pH range of the Gran plot regression. The results of empirical Gran titrations agree with the theoretical calculations made by Cantrell et al. (1990).