ACID neutralizing capacity (ANC) is used to quantify the acid-base status of surface waters. Acidic waters have been defined as having ANC values less than zero, and acidification is often quantified by decreases in ANC. Measured and calculated values of ANC generally agree, except for low-ANC waters. These waters, however, are of primary interest in lake-acidification studies. The discrepancy in ANC values is greatest for waters with high concentrations of aluminium and/or dissolved organic carbon (DOC). The discrepancy due to aluminium increases with increasing concentration of dissolved monomeric aluminium (Alm) and can exceed 50 μeq 1-1at low pH and high Alm values. Here we show that this error can be minimized by using a proton reference level for aqueous aluminium of 2+, rather than 0 or 3+, as is done usually. The value of 2+ is consistent with the mean charge on aluminium at the equivalence point of strong-acid titrations. The discrepancy between calculated and measured ANC also increases with increasing DOC concentrations, exceeding 50 μeq 1-1 for waters with DOC concentrations greater than 800 μmol C 1-1 (1000 μmol C 1-1 = 12 mg 1-1 DOC). This error can be decreased by titrating to a low consistent pH value (for example, 3.0). This reduces the systematic underestimation of ANC due to curvature in Gran1 plot analysis, while still allowing accurate measurement of increments in hydrogen-ion concentration. ANC should not be used as a single parameter for characterizing the chemical suitability of surface waters for biota or for assessing the susceptibility of low-ANC waters to acidification by acid deposition.
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