A semidirect titration method was used to determine individual site densities of carboxylic acids present in natural organic matter originating from 74 Swedish lakes. The average carboxylic acid site density of natural organic matter of the studied lakes was 8.6 ± 1.6 µequiv.·mg total organic carbon-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 modeling pH and organic anion charge contribution caused by the natural variability 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 µmol·L-1, using measured Cb-Ca and individually determined site densities. A better prediction of pH, however, was made using this average site density and the CBALK approach, which is a different method for estimating total buffering capacity. The mean error in pH prediction was decreased to 0.05 pH unit, equivalent to a mean error in the charge balance of 5 µmol·L-1. These uncertainties in pH prediction are significantly smaller than reported in other studies. This implies that the existing natural variation in site density does not lead to significant effects on modeling pH or anion charge contribution in surface waters in the pH range of 4.5-8.5. Filtered samples (0.45 µm) did not show any significant change in pH, total organic carbon, or site density compared with unfiltered samples.
