New Approaches in Modeling Carbonate Alkalinity and Total Alkalinity

Abstract

Alkaline measurements (carbonate alkalinity and total alkalinity) are of great importance in analyses of ocean, marine, lake and river waters, and other samples. Carbonate alkalinity (CA) refers to the cases in which only bicarbonate and carbonate species are considered. Total alkalinity (Alk) implies additional species having acid-base properties. Biochemical processes give rise to the organic alkalinity (OA) in natural waters. The value of these topics is obvious if we refer to the plethora of publications in this field. As a matter of fact, the approaches towards Alk provide ambiguous results and those for CA arouse some reservations as well. Having this in mind, a new approach for alkalinity determination, presented in recent review paper published in this journal (Crit. Rev. Anal. Chem. 2011, 41, 151), was developed in a more systematic and complete way, and new acronyms, TAL for total alkalinity and CAM for carbonate alkalinity, were introduced in that article. The methods proposed there and here are based on potentiometric pH titration, as the most common method applied for studying acid-base properties of humic substances (humic and fulvic acids). The parameters involved with CAM are resolved with the use of an Excel spreadsheet for the resolution of linear equations, whereas the parameters of a nonlinear regression equation referring to the TAL method are obtainable according to a more advanced, iterative computer program. A compact approach to CAM, based on the modified Gran II method, is carried out, but in a manner quite different and more thorough than the ones presented hitherto. Except for equivalence volumes, the CAM method enables evaluating the activity coefficient of hydrogen ions and values for (hybrid and concentration) dissociation constants of carbonic acid and ionic product of water. The possibilities and limitations of this method are tested by using a simulation procedure. The results obtained from simulated titrations provide the reference level related to error-free V and pH values at the points of titration curve ph = ph(V), ph = – log h, h – activity of hydrogen ions. The TAL model with Simms constants involved allows “homogenizing” the treatment of mono- and polyprotic acids and bases, especially those of undefined composition, i.e., carbonate species, fulvic acids, and their soluble complexes with metal ions. The TAL model may also be extended to the option where an approach based on a pre-dose addition is applied. pH and acid-base equilibria are very important topics and some aspects of them have been treated in this journal. Alkalinity determinations are of value in anaerobic digesters and anaerobic wastewater treatment, mine drainage and associated water, coastal seawater, marine aquaculture, estuarine water, and food and beverages, to point out a few recent examples.