alkalinity

Alkalinity is the capacity of water to neutralize added acid. It reflects the buffering systems in solution rather than the concentration of alkaline substances itself, and is commonly discussed in the context of carbonate chemistry and acid–base balance in natural waters.

In most natural waters, the major contributors are carbonate species and hydroxide ions. Dissolved carbon dioxide reacts to form bicarbonate (HCO3−) and carbonate (CO3^2−), with the proportions governed by pH. At typical environmental pH, bicarbonate is often the dominant contributor, while carbonate becomes more important at higher pH and hydroxide appears mainly in very alkaline conditions.

Alkalinity is measured by titration with a strong acid to a set pH. Total alkalinity (TA) is the amount of acid required to reach pH 4.5 and is reported as milligrams per liter of calcium carbonate (mg/L as CaCO3) or as milliequivalents per liter (meq/L). Phenolphthalein alkalinity (PA) is the acid needed to reach pH 8.3, representing OH− and CO3^2−; the difference between TA and PA approximates bicarbonate alkalinity (HCO3−).

Significance: Alkalinity buffers pH change, helping aquatic organisms withstand acid inputs and influencing treatment processes such as coagulation and lime dosing. It also reflects geochemical processes like mineral weathering and gas exchange with the atmosphere.

Typical values vary widely: freshwater systems can range from a few to several hundred mg/L as CaCO3, while seawater generally exceeds 2,000 mg/L as CaCO3, providing strong buffering against acidification.