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Abstract Net Alkalinity and Hot Acidity: How to Get the Right Answer Carl S. Kirby, Department of Geology, Bucknell University Charles A. Cravotta, III, US Geological Survey, Water Resources Division
“Net alkalinity” is poorly defined but used as a critical decision parameter in designing mine drainage treatment. We employ theory, modeling, and titration of field and synthetic samples to establish easily understood methods for calculating net alkalinity. Water from numerous large mine discharges in Pennsylvania has pH near 6-7 and contains dissolved iron; these waters have both alkalinity and acidity yet ultimately acidic pH (< 4.5). The use of net alkalinity = measured alkalinity – measured hot acidity to design treatment can lead to insufficient alkalinity to neutralize acidity and is not recommended. For samples with alkalinity, Standard Method “hot” H2O2 acidity titrations present significant interpretation problems. In such samples, acidity is neutralized by HCO3- before the titration itself begins, causing acidity to be underestimated. The Standard Method hot acidity titration returns a net acidity value rather than an acidity value. Laboratories need to report negative acidity values for hot acidity. Calc Acidity = 50[(2(Fe2+)/56) + (3(Fe3+)/56) + (3(Al)/27) + 2(Mn)/55 + 1000(10^-pH)] provides a good estimate of positive acidity for most mine waters but does not equal “hot” acidity titration values. Net acidity (= - net alkalinity) should be calculated by 1) = value obtained from Standard Method hot H2O2 acidity titration with negatives reported, or 2) = measured alkalinity – calculated acidity.
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