Alkalinity - Theoretical Treatment of Alkalinity

Theoretical Treatment of Alkalinity

In typical groundwater or seawater, the measured alkalinity is set equal to:

A_T = _T + 2_T + _T + _T + 2_T + _T + _T − _sws −

(Subscript T indicates the total concentration of the species in the solution as measured. This is opposed to the free concentration, which takes into account the significant amount of ion pair interactions that occur in seawater.)

Alkalinity can be measured by titrating a sample with a strong acid until all the buffering capacity of the aforementioned ions above the pH of bicarbonate or carbonate is consumed. This point is functionally set to pH 4.5. At this point, all the bases of interest have been protonated to the zero level species, hence they no longer cause alkalinity. For example, the following reactions take place during the addition of acid to a typical seawater solution:

HCO₃− + H+ → CO₂ + H₂O

CO₃−2 + 2H+ → CO₂ + H₂O

B(OH)₄− + H+ → B(OH)₃ + H₂O

OH− + H+ → H₂O

PO₄−3 + 2H+ → H₂PO₄−

HPO₄−2 + H+ → H₂PO₄−

+ H+ →

It can be seen from the above protonation reactions that most bases consume one proton (H+) to become a neutral species, thus increasing alkalinity by one per equivalent. CO₃−2 however, will consume two protons before becoming a zero level species (CO₂), thus it increases alkalinity by two per mole of CO₃−2. and decrease alkalinity, as they act as sources of protons. They are often represented collectively as _T.

Alkalinity is typically reported as mg/L of CaCO₃. This can be converted into milliEquivalents per Liter (mEq/L) by dividing by 50 (the approximate MW of CaCO₃/2).