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 been theoretically introduced in the early 1990s, to evaluate a simple pH equilibrium model used for Swedish surface waters. The CBALK approach with its 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 predict air equilibrated pH in the range 4.5 to 7.5 within 0.08 pH units. This is equivalent to an error in the charge balance of about 5 ueq L-1. In the 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 or carbon dioxide on ambient pH during episodes as well as for sample quality control.