Total Alkalinity Determined by Titration in the Presence of Dissolved Organic Matter

Jonathan David Sharp, University of South Florida, College of Marine Science, Saint Petersburg, United States and Robert Byrne, University of South Florida, College of Marine Science, St. Petersburg, United States
Abstract:
Total alkalinity, a quantitative description of charge balance in a mixed electrolyte solution, is one of the most important measured parameters pertaining to the chemistry of natural waters. Total alkalinity mixes conservatively and is insensitive to air–sea gas exchange. It is useful in evaluating calcium carbonate formation and dissolution dynamics, characterizing marine carbonate system chemistry, and tracking movements and mixing of water masses.

Total alkalinity is measured by acidimetric titration and quantification of the amount of hydrogen ions consumed by operationally-defined proton acceptors. In aquatic systems with exclusively inorganic acid–base chemistries, definition and measurement of alkalinity is unambiguous. However, in systems that may be influenced by organic species, such as rivers, estuaries, and coastal ocean regions (and possibly even the open ocean), the effect of dissolved organic matter on titration alkalinity cannot be ignored.

We use numerical modeling to elucidate issues that may arise when measuring total alkalinity by titration in organic-influenced systems. We highlight the effects of data analysis method, organic acid dissociation behavior, and initial carbonate chemistry. We also propagate potential errors in measured alkalinity values to calculations of other carbonate system parameters.

We find that in the presence of organics with relatively low pKA values (such as carboxylic acids), significant differences in measured alkalinity values can result from different methods of titration and data analysis. Measured values can also differ significantly from inorganic alkalinity, which is the desired quantity when using alkalinity to make carbonate system calculations. Discrepancies between measured and inorganic alkalinity values can produce computed carbonate system parameters that differ significantly from their true values in organic-influenced systems when total alkalinity measured by titration is used as an input parameter.

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