Alkalinity

In-depth explanation of Alkalinity

Alkalinity is the capacity of natural water to neutralize added acid, arising mainly from bicarbonate, carbonate and hydroxide ions. High alkalinity makes rivers and lakes resistant to acidification because these ions consume hydrogen ions from acid rain. It is measured by titration with a strong acid to a defined endpoint, usually pH 4.5, which gives total alkalinity. Alkalinity is distinct from pH, since water with high alkalinity can have a moderate pH and still resist change. It is an important parameter in environmental monitoring and water treatment.

Going deeper: Alkalinity

Key ideas: Alkalinity is the capacity of water to neutralize acid, reported in mg/L as CaCO3. Major contributors: HCO3-, CO3 2-, OH-. Alkalinity = [HCO3-] + 2[CO3 2-] + [OH-] - [H+] (in eq/L). Measured by titrating with strong acid to pH 8.3 (phenolphthalein, carbonate to bicarbonate) and then 4.5 (total, methyl orange).

Common mistakes and connections: Mistakes: confusing alkalinity with pH (a water with pH 8 may have low or high alkalinity) and with hardness (they overlap but differ). High alkalinity buffers against acid rain. Alkalinity is unchanged by CO2 dissolving or escaping, which makes it a conservative quantity for modeling. Links to the carbonate buffer system and titration curves.

Visualization of Alkalinity

Alkalinity is the water capacity to absorb acid, mainly from HCO3- and CO3(2-); above pH 8.3 the carbonate fraction grows toward the HCO3-/CO3(2-) pKa of 10.33. The interactive lesson shows this as a live animation or chart.

Worked example and use case

Lakes in limestone regions are relatively resistant to acidification because their high alkalinity neutralizes incoming acid.

Where Alkalinity fits in Acids, Bases & Electrochemistry

Alkalinity is topic 105 of 105 in Acids, Bases & Electrochemistry.

Related topics

• Acid AnhydridesIntermediate
• Basic AnhydridesIntermediate
• Water HardnessIntermediate

Browse all 105 topics in Acids, Bases & Electrochemistry