Accurate approach for determining fresh-water carbonate (H ₂CO₃^*) alkalinity, using a single H ₃PO₄ titration point

A new, simple and accurate method is introduced for determining H ₂CO₃^* alkalinity in fresh waters dominated by the carbonate weak-acid system. The method relies on a single H₃PO₄ dosage and two pH readings (acidic pH value target: pH∼4.0). The computation algorithm is based on the concept that the overall alkalinity mass of a solution does not change upon the addition of a non-proton-accepting species. The accuracy of the new method was assessed batch-wise with both synthetic and actual tap waters and the results were compared to those obtained from two widely used alkalinity analysis methods (titration to pH∼4.5 and the Gran titration method). The experimental results, which were deliberately obtained with simple laboratory equipment (glass buret, general-purpose pH electrode, magnetic stirrer) proved the method to be as accurate as the conventional methods at a wide range of alkalinity values (20-400 mg L^-1 as CaCO₃). Analysis of the relative error attained in the proposed method as a function of the target (acidic) pH showed that at the range 4.0<pH<4.5 the error was minimal. A suggested experimental setup for continuous alkalinity measurement is also described.