Alkalinity is mainly due to which species?

• A. Chlorides and sulfates.
• B. Nitrates and phosphates.
• C. Organic matter.
• D. Carbonates, bicarbonates, and hydroxide ions.

Answer: (D)

Alkalinity is the water’s capacity to neutralize added acid, i.e., its buffering ability. In most natural waters this buffering mainly comes from the carbonate system: carbonate, bicarbonate, and hydroxide ions. These species react with acids to form carbonic acid and CO2, or to remove H+ directly, helping resist pH changes. Bicarbonate is usually the dominant contributor at typical environmental pH, with carbonate and hydroxide becoming more important at higher pH. That’s why the primary alkalinity components are carbonates, bicarbonates, and hydroxide ions. Chlorides and sulfates mainly affect salinity and conductivity rather than buffering capacity; nitrates, phosphates, and organic matter can influence pH and buffering only to a minor extent compared with the carbonate system.