Estimation of organic acid anion concentrations and evaluation of charge balance in atmospherically acidified colored waters

Organic acid anion concentrations (R−) represent an important part of the anion pool in colored surface waters. When the estimation of R− is based on a charge balance approach, R− cannot be used for the ionic balance control of the correctness of the analyses. We provide here another simple approach for R− estimation, which is independent of the charge balance. In this approach, R− was estimated using the concentration of ionizable carboxylic groups per mass of DOC (XI), DOC concentration and pH. The XI value was estimated as the difference between the average total concentration of carboxylic groups per 1 mg of DOC of natural fulvic and humic acids (10 μeq mg−1) and the concentration of metal-complexed carboxylic groups. The latter concentration was assumed to be equal to organically-bound Al and Fe. When applied to the Bohemian Forest stream waters, the average (±standard deviation) XI was 6.2±0.8 μeq mg−1, R− varied from 2 to 146 μeq l−1, and the average percent difference between the sums of cations and anions was 0.7%. The R− values, obtained using the charge balance approach, were strictly comparable (P<0.001) to the values estimated from the proposed approach.