Alkalinity is defined as water capacity to neutralize acid, or the quantity of anions in water which can neutralize hydrogen cations. Composer alkalinity water are bicarbonate anion (HCO3-), carbonate (CO32-) and hydroxide (OH-). Borate (H2BO3-), silicate (HSiO3-), phosphate (HPO42- and H2PO4-), sulfide (HS-), and ammonia (NH3) also contribute to alkalinity. However, the main formers of alkalinity are bicarbonate, carbonate, and hydroxide. Among the three ions, the bicarbonate present in most natural water.
Alkalinity is the water defenses against acidification. In natural water, alkalinity mostly due to the presence of bicarbonate and the remaining is caused by carbonate and hydroxide.
Some levels of alkalinity content are required in boiler water, so the complete removal of alkalinity in the boiler is rare to be performed except in demineralization treatment. Some of alkalinity is also required to provide optimum pH in feedwater to prevent corrosion in piping and equipment.
Alkalinity form is in HCO3-, CO32-, or OH-. If water city is made in softening, alkalinity is usually formed in bicarbonate (HCO3-) form; if lime adhesive is reduced, its content usually excess carbonate (CO32-), but water may also contain some hydroxides (OH-). When the bicarbonate and carbonate having heat in the boiler, bicarbonate and carbonate rupture to release CO2 as in the following equation:
2NaHCO3 → Na2CO3 + H2O + CO2 ↑
Natrium carbonate is then split again into:
Na2CO3 + H2O → 2NaOH + CO2 ↑
Carbon dioxide gas does not dissolve when the steam becomes solid, producing corrosive carbonic acid:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
The amount of CO2 produced is proportional to alkalinity. Because of an alkalinity that delivers twice as much CO2 formed from HCO3- by CO32- due to interference of bicarbonate is the sum of reactions above. Carbonic acid is usually neutralized with chemical treatment at each steam directly or indirectly through boiler to produce pH around 8.5 to 9.0. Reduction of alkalinity in the boiler feed water is necessary to minimize the formation of CO2 and reduce costs of chemical treatment.