Validating a simple equation to predict and analyze organic anion charge in Swedish low ionic strength surface waters

Acid neutralising capacity (ANC) is substituted by CBALK, a term that has b een theoretically introduced in the early 1990s, to evaluate a simple pH eq uilibrium model used for Swedish surface waters. The CBALK approach with it s simple three pK(a) organic acid analogue is re-evaluated with a new data set of 900 stream water samples. End-point alkalinity and TOC suffice to pr edict air equilibrated pH in the range 4.5 to 7.5 within 0.08 pH units. Thi s is equivalent to an error in the charge balance of about 5 ueq L-1. In th e studied pH range it renders more precise predictions than when using ANC. The model equations can be used to estimate the effect of organic carbon o r carbon dioxide on ambient pH during episodes as well as for sample qualit y control.