Alkalinity and Acid Neutralizing Capacity

Alkalinity and acid neutralizing capacity (ANC) are typically defined as a measure of the capacity of water to neutralize a strong acid. Frequently, the terms alkalinity and ANC are used interchangeably. Technically, alkalinity is determined from filtered samples (using at least a 0.45 µm filter) whereas ANC is determined from a non-filtered or whole water sample. Although alkalinity or ANC consists of the sum of titratable carbonate and non-carbonate chemical species in a sample which act to neutralize acid, in most natural waters alkalinity or ANC is attributed to the dissolved carbon dioxide species, bicarbonate and carbonate. Carbonate alkalinity therefore specifically refers to acid neutralizing capacity attributed to bicarbonate (HCO₃^-) and carbonate (CO₃^-2) species and is typically reported in terms of milliequivalents or microequivalents per liter or milligrams per liter as a carbonate species and titrated on a filtered sample. In many aqueous systems, alkalinity is attributed to carbonate chemistry and more commonly attributed to bicarbonate and less commonly to carbonate.

Waters highly buffered by calcium carbonate may have somewhat elevated hardness. Hardness is a measure of the abundance of cations, primarily calcium and magnesium which react with soap to form insoluble compounds or precipitate from heated water to form encrustations (Hem, 1985). Other divalent cations including strontium, iron, and manganese can contribute to hardness. Hard water requires considerable amounts of soap to produce lather and is the cause of scale wherever water is heated. Water is considered soft if it contains 0 to 60 mg/L of hardness, moderately hard if it contains more than 120 mg/L of hardness, and very hard if it contains more than 180 mg/L of hardness (Hem, 1985).

Reference

Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water (3rd edition): U.S Geological Survey Water-Supply Paper 2254, 263 p.

Last updated on January 13, 2009