Estimating organic acid dissociation in natural surface waters using totalalkalinity and TOC

Organic al:id dissociation has usually been estimated from the difference b etween measured cations and anions (the anion deficit approach). An alterna tive is to use alkalinity titrations and an estimate of strong organic acid s from TOC as an independent measure of organic acid dissociation (the CBAL K approach). This approach may reduce the potential for errors relative to the anion deficit approach, which relies on summing up all measured constit uents. Organic acid dissociation in a set of Swedish surface water samples from 700 different lakes, as well as 200 samples from 11 streams was predic ted using a triprotic analogue model with buffering characteristics similar to those of the widely used "Oliver" model (OTM), but with a lower site de nsity of organic acids. The average equilibrium pH of the samples in the we akly buffered pH range of 4.7-6.7 could be predicted more precisely (+/-0.1 6 FH units) using the CBALK approach, than when using the usual anion defic it approach (+/-0.41 pH units). The study supports the general applicabilit y of models similar to the empirical OTM for predicting the dissociation of natural organic acids on a regional scale, (C) 2000 Elsevier Science Ltd. All rights reserved.