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How can total inorganic C or titration alkalinity be input?


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We are modeling the solubility and speciation of metals released from drinking water/plumbing interactions. We directly analyze for total inorganic carbon in the water by coulometric titration. We also analyze what would be termed "titration alkalinity" to the carbonic acid equivalence point by sulfuric acid titration. Drinking waters can have weak acid contributions from such anions as aqueous ammonia, hypochlorite ion, or various phosphoric acid anions, depending on the pH of the water. So it would not be proper to assume this titration alkalinity to be the system parameter carbonate alkalinty. While there is an analyte for titration acidity in the system parameters, I don't see one for titration alkailnty. And while we can express the titration alkainitiy in many varied units, I don't know what the best way to input that analytical data would be. Must we define a complicated new analyte of our own? Directly inputting the total concentrations of the other weak acids and the analyzed TIC would likely be the most accurate. Can TIC be properly input using HCO3- as the analyte? Or do we define something else?

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You're in the lucky position of having more information than you need. Since you have specifically measured TIC, you can enter that value directly for the HCO3- entry. I would first decouple any other types of carbon from HCO3- (you can decouple individual redox pairs like methane or acetate, or just use the command "decouple Carbon" to decouple them all).

The titration alkalinity is commonly reported in lieu of directly analyzing TIC. Section 15.1 of the Geochemical and Biogeochemical Reaction Modeling text includes an example in which the modeler reproduces a laboratory titration in order to determine the contribution of carbonate to the overall measured alkalinity. In the example, the concentration of other species contributing alkalinity are known, but the concentration of carbonate is adjusted by trial and error until the model reaches the correct end-point pH.

Since you have measured titration alkalinity, the TIC, and the concentration of species like ammonia, hypochlorite, and so on, you could compare the titration alkalinity with the sum of the carbonate, ammonia, and so on to determine the quality of your analyses. If there is a disagreement, it's possible that some additional analyte contributes to the alkalinity but was not measured.

Hope this helps,

Brian Farrell


Aqueous Solutions LLC
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