Estimating organic acid dissociation in natural surface waters using total alkalinity and TOC

Organic acid dissociation has usually been estimated from the difference between measured cations and anions (the anion deficit approach). An alternative is to use alkalinity titrations and an estimate of strong organic acids from TOC as an independent measure of organic acid dissociation (the CBALK approach). This approach may reduce the potential for errors relative to the anion deficit approach, which relies on summing up all measured constituents. Organic acid dissociation in a set of Swedish surface water samples from 700 different lakes, as well as 200 samples from 11 streams was predicted using a triprotic analogue model with buffering characteristics similar to those of the widely used “Oliver” model (OTM), but with a lower site density of organic acids. The average equilibrium pH of the samples in the weakly buffered pH range of 4.7–6.7 could be predicted more precisely (±0.16 pH units) using the CBALK approach, than when using the usual anion deficit approach (±0.41 pH units). The study supports the general applicability of models similar to the empirical OTM for predicting the dissociation of natural organic acids on a regional scale.