Influence of organic acid site density on pH modeling of Swedish lakes

A semidirect titration method was used to determine individual site densiti es of carboxylic acids present in natural organic matter originating from 7 4 Swedish lakes. The average carboxylic acid site density of natural organi c matter of the studied lakes was 8.6 +/- 1.6 mu equiv.mg total organic car bon(-1), which is more variable than the site densities observed earlier in a set of streams. These data were then used to assess the effect on modeli ng pH and organic anion charge contribution caused by the natural variabili ty in site density. Air-equilibrated pH was modeled with a median error of 0.08 pH unit, equivalent to a mean error in the charge balance of 9 mu mol. L-1, using measured C-b-C-a and individually determined site densities. A b etter prediction of pH, however, was made using this average site density a nd the CBALK approach, which is a different method for estimating total buf fering capacity. The mean error in pH prediction was decreased to 0.05 pH u nit, equivalent to a mean error in the charge balance of 5 mu mol.L-1. Thes e uncertainties in pH prediction are significantly smaller than reported in other studies. This implies that the existing natural variation in site de nsity does not lead to significant effects on modeling pH or anion charge c ontribution in surface waters in the pH range of 4.5-8.5. Filtered samples (0.45 mu m) did not show any significant change in pH, total organic carbon , or site density compared with unfiltered samples.