Water Research Center
The Role of Alkalinity
Alkalinity is the water's capacity to resist changes in pH that would make the water more acidic. This capacity is commonly known as "buffering capacity." For example, if you add the same weak acid solution to two vials of water - both with a pH of 7, but one with no buffering power (e.g. zero alkalinity) and the other with buffering power (e.g. an alkalinity of 50 mg/l), - the pH of the zero alkalinity water will immediately drop while the pH of the buffered water will change very little or not at all. The pH of the buffered solution would change when the buffering capacity of the solution is overloaded.
Alkalinity refers to the capability of water to neutralize acid. This is really an expression of buffering capacity. A buffer is a solution to which an acid can be added without changing the concentration of available H+ ions (without changing the pH) appreciably. It essentially absorbs the excess H+ ions and protects the water body from fluctuations in pH. In most natural water bodies in Pennsylvania the buffering system is carbonate-bicarbonate ( H2CO3, HCO3, and CO3).
Alkalinity of natural water is determined by the soil and bedrock through which it passes. The main sources for natural alkalinity are rocks which contain carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates also may contribute to alkalinity. Limestone is rich in carbonates, so waters flowing through limestone regions or bedrock containing carbonates generally have high alkalinity - hence good buffering capacity. Conversely, areas rich in granites and some conglomerates and sandstones may have low alkalinity and therefore poor buffering capacity.
The presence of calcium carbonate or other compounds such as magnesium carbonate contribute carbonate ions to the buffering system. Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks (limestone) which are mostly CaCO3. If CaCO3 actually accounts for most of the alkalinity, hardness in CaCO3 is equal to alkalinity. Since hard water contains metal carbonates (mostly CaCO3) it is high in alkalinity. Conversely, unless carbonate is associated with sodium or potassium which don't contribute to hardness, soft water usually has low alkalinity and little buffering capacity. So, generally, soft water is much more susceptible to fluctuations in pH from acid rains or acid contamination.
Alkalinity is important for fish and aquatic life because it protects or buffers against rapid pH changes. Living organisms, especially aquatic life, function best in a pH range of 6.0 to 9.0. Alkalinity is a measure of how much acid can be added to a liquid without causing a large change in pH. Higher alkalinity levels in surface waters will buffer acid rain and other acid wastes and prevent pH changes that are harmful to aquatic life.
Acid shock may occur in spring when acid snows melt, thunderstorms, natural discharges of tannic waters, "acid rain", acidic dryfall, and other discharges enter the stream. If increasing amounts of acids are added to a body of water, the water's buffering capacity is consumed. If additional buffering material can be obtained from surrounding soils and rocks, the alkalinity level may eventually be restored. However, a temporary loss of buffering capacity can permit pH levels to drop to those harmful to life in the water.
The pH of water does not fall evenly as acid contamination proceeds. The natural buffering materials in water slow the decline of pH to around 6.0. This gradual decline is followed by a rapid pH drop as the bicarbonate buffering capacity is used up. At a pH of 5.5, only very weak buffering materials remain and pH drops further with additional acid. Sensitive species and immature animals are affected first. As food species disappear, even larger, resistant animals are affected.
For protection of aquatic life the buffering capacity should be at least 20 mg/L. If alkalinity is naturally low, (less than 20 mg/L) there can be no greater than a 25% reduction in alkalinity.
Testing Methodology: Alkalinity is an electrometric measurement which is performed using a titrant and a pH electrode. A potentiometric titration is taken to an end-point reading of pH 4.5. The amount of acid required to reach a pH of 4.5 is expressed in milliliters. The calcium ions (CO3) neutralize the acid in this reaction, and show the buffering capacity of the sample. From the amount of acid used, a calculation will indicate the amount of carbonate (CO3) involved in the reaction. This then is expressed as mg of CaCO3/L even though actually part of the alkalinity may be contributed by MgCO3 , Na2CO3 or K2CO3.
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Alkalinity Speciation Calculator
For More information, please contact:
Attn: Mr. Brian Oram, Professional Geologist (PG)
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