Alkalinity definition and units

• A. Chloride concentration; mg/L
• B. The acidity of water due to organic acids; expressed as pH
• C. The capacity of water to neutralize acids, caused by carbonate, bicarbonate, hydroxide, and occasionally borate, silicate, and phosphate; expressed in mg per liter of CaCO3 equivalent
• D. Total dissolved solids; expressed as mg/L

Answer: (C)

Alkalinity is the water’s ability to neutralize acids, i.e., its buffering capacity. This buffering comes mainly from carbonate and bicarbonate ions and hydroxide, which react with added acids to prevent large drops in pH. Sometimes other bases like borate, silicate, or phosphate can contribute as well. Because chemists express this buffering capacity in terms of the amount of calcium carbonate that would neutralize the acid, alkalinity is given in milligrams per liter as CaCO3 (mg/L as CaCO3).

This concept is different from chloride concentration (which measures a dissolved salt and affects taste and corrosion potential, not buffering), pH (which reflects the current hydrogen-ion activity rather than how much acid the water can neutralize), and total dissolved solids (which sums all dissolved substances without specifically addressing acid–base buffering).