What does total alkalinity of water mean?
PH is a measure of the concentration of acids (protons) or bases (hydroxides) in water. The total alkalinity of water is a measure of its ability to neutralize acids. Indicates how much acid a solution can absorb without changing the pH. In other words, alkalinity reflects the buffering capacity of a solution. If a water has a low alkalinity, its pH will drop rapidly by adding an acid.
Rainwater absorbs carbon dioxide found in the biosphere. This is dissolved in water, like carbonic acid (H2CO3). Since it is a weak acid, it lowers the pH, albeit by a small value. Carbonated water reacts with some minerals present in rocks. The most common is calcium carbonate (CaCO3). The reaction is
H2CO3 + CaCO3 🡪 2 HCO3– + Ca+2
The anions that are part of the total alkalinity are bicarbonates (HCO3–), carbonates (CO3-2) and hydroxides (OH– ). The most common is the bicarbonate ion.
All of these react with the hydrogen ion.
H+ + HCO3 🡪 H2CO3
H2CO3 🡪 CO2↑ + H2O
2 h+ + CO3-2 🡪 2 H2CO3
2 h2CO3 🡪 2 CO2↑ + H2O
H+ + OH– 🡪 H2O
As it is observed that in the three cases, the only residue that remains in the water is the cation of the acid that contributed the proton.
A problem related to alkalinity in waters that also contain hardness is that it generates particularly encrusting salts. Not only are they characterized by embedding more than other salts: unlike the rest, they are less soluble the higher the temperature.