National Field Manual for the Collection of Water-Quality Data
TWRI Book 9
Alkalinity and acid neutralizing capacity (ANC) are measures of the ability of a sample to neutralize strong acid. They are determined using identical electrometric procedures involving the acidimetric titration of a sample; the only difference is that the alkalinity sample is filtered, whereas the ANC sample is not filtered (raw, or unfiltered). The terms alkalinity, ANC, and carbonate alkalinity are used in this manual as follows:
|Alkalinity is the acid neutralizing capacity of solutes in a water sample, reported in equivalents per liter (or milliequivalents or microequivalents per liter). Alkalinity consists of the sum of titratable carbonate and noncarbonate chemical species in a filtered water sample (filter membrane of 0.45-µm pore size or smaller).|
|ANC is the acid-neutralizing capacity of solutes plus particulates in an unfiltered water sample, reported in equivalents per liter (or milliequivalents or microequivalents per liter). ANC is equivalent to alkalinity for samples without titratable particulate matter.|
|Carbonate alkalinity is the acid-neutralizing capacity attributable to carbonate solutes (bicarbonate and carbonate), reported either in equivalents per liter (or milliequivalents or microequivalents per liter) or in milligrams per liter as a carbonate species, and titrated on a filtered water sample. In many aqueous systems, alkalinity is controlled by carbonate chemistry and most commonly is attributable to bicarbonate (HCO3-) and less frequently to carbonate (CO32-).|
Alkalinity is used routinely to check the charge balance of a solution and to gain insights on the evolution of aqueous systems. Alkalinity and ANC provide information on the suitability of water for uses such as irrigation, determining the efficiency of wastewater processes, determining the presence of contamination by anthropogenic wastes, and maintaining ecosystem health.
Any substance in the water sample that reacts with strong acid titrant can contribute to the acid neutralizing capacity.
|Important noncarbonate contributors may include organic ligands (especially acetate and propionate) as well as hydroxide, silicate, borate, and less commonly ammonia and sulfide (Hem, 1985). When found in unusually high concentrations, phosphate and arsenate also may contribute to the acid neutralizing capacity of the sample (Stumm and Morgan, 1981).|
|Except for unusual natural waters and waters substantially affected by human activity, noncarbonate ionized contributors generally are not present in large enough quantities to affect alkalinity or ANC determinations.|
|Particulate matter can be an important contributor, and must be removed by filtration before titrating the sample for the alkalinity determination.|
|Alkalinity is determined on a filtered sample. ANC is determined on an unfiltered sample.|
TECHNICAL NOTE: Alkalinity
and ANC are measured relative to a solution of carbon dioxide in water; therefore,
they are independent of any exchange of carbon dioxide or other common gases
between the sample and the atmosphere. However, atmospheric gas exchange can
alter the concentrations of individual species, such as bicarbonate. Also, aeration
of a sample during filtration can cause mineral precipitation on the filterthis
may alter the alkalinity, especially in water systems closed to the atmosphere
under ambient conditions.