What does total alkalinity of water mean?
The pH is a measure of the concentration of acids (protons) or bases (hydroxides) in water. The total alkalinity of water is the measure of its ability to neutralize acids. It 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 decrease rapidly when an acid is added.
Rainwater absorbs carbon dioxide found in the biosphere. It is dissolved in water, as carbonic acid (H2CO3). Since it is a weak acid, it lowers the pH, albeit by a small value. Carbonate water reacts with some minerals present in the rocks. The most common is calcium carbonate (CaCO3). The reaction is
H2CO3 + CaCO3 ? 2 HCO3- + Ca+2
The anions that make up 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
2H2CO3 ? 2 CO2↑ +H2O
H+ + OH- ?H2O
As can be seen in all three cases, the only residue left 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 fouling salts. They are not only characterized by being more incrusting than other salts: unlike the rest, they are less soluble the higher the temperature.