Ion balance calculations were used to estimate the charge density of total organic carbon (TOC) and for calibrating the model of Oliver et al. (1983) for Finnish lakes. The database consisted of samples collected in the autumn of 1987 from 955 randomly selected lakes covering the whole country. Organic anion concentrations were estimated as the difference between Σ cations and Σ inorganic anions. The charge density of TOC was estimated by dividing the organic anion concentration by TOC. The median charge density was 7·5 μeq (mg TOC)-1. In lakes with pH values lower than 4·5 the average charge density was 5·2 μeq (mg TOC)-1and it increased to 10·3 μeq (mg TOC)-1in lakes with pH higher than or equal to 7·0. The calibrated model explained 79% of the observed variability in organic anion concentration. The value for the total amount of acidic functional groups per mol organic carbon in the model of Oliver et al. (1983) was slightly higher compared to the Finnish calibration. At pH values higher than 4·4 the Finnish calibration gives lower organic anion concentrations compared to the model of Oliver et al. (1983). © 1992.
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