Water Alkalinity
Alkalinity is produced by substances that yield hydroxyl ions or by definition, a substance is alkaline if it will neutralize hydrogen ions. Alkalinity is caused by strong bases and the salts of strong alkalis and weak acids. In water chemistry, alkalinity is defined as the capacity of the solution to neutralize acid. To complete this definition, some end point pH needs to be specified. Normally, the pH designated is a selected value between 5.1 and 4.5, or that of the methyl orange end point (about pH 4.0 to 4.6).
Sometimes, however, an alkalinity above the phenolphthalein end point (about pH 8.3) is specified. Thus, we find terms such-as methyl orange alkalinity or equivalent total alkalinity and phenolphthalein alkalinity as reported values.
Several different solute species contribute to the alkalinity of water as defined above. Alkalinity is often reported in terms of an equivalent quantity of calcium carbonate. However, a more meaningful and useful statement of the alkalinity is obtained by expressing the results of the determination as concentrations of bicarbonate and carbonate. Because in most natural water the alkalinity is practically all produced by dissolved carbonate and bicarbonate ions.
The alkalinity defined here is a capacity function which therefore has a different chemical basis than the intensity function pH. A solution whose pH is neutral may have considerable titratable alkalinity. For the most part, alkalinity is produced by anions or molecular species of weak acids which are not fully disassociated above of pH of 4.5. The most common weak acid in natural water is carbonic acid. This is formed when
carbon dioxide is dissolved in water.
The hydrogen ions produced by the disassociation of dissolved carbon dioxide species are the principle sources of the [H+] where the dissolved carbon dioxide species are relatively plentiful. They are instrumental in maintaining the pH of a solution at a fairly constant value, although the pH is also influenced by many other reactions involving solids and other solutes with the hydrogen ion.
Other weak acids which may contribute species to titratable alkalinity include silicic, phosphoric, and boric acids. In some waters there may also be contribution from organic acids or a wide variety of dissolved or suspended materials. The direct contribution of hydroxide is determinable from the pH.
In general, alkalinity is not a specific polluting substance, but rather a combined effect of several substances and conditions. Some natural waters, especially those in the Southwest, are highly alkaline, while others such as those in Northwest and Northeast are low in alkalinity.
Alkalinity in domestic water supplies is not considered to be detrimental to humans, but is generally associated with high pH values, hardness and excessive dissolved salts, all of which may be deleterious.
As for irrigation, excessive alkalinity is detrimental in that it adds to the total salinity and is frequently accompanied by high pH values. In conventional chemical analysis of irrigation waters, however, alkalinity is frequently not listed.
None of the strong alkalines such as calcium, potassium and sodium hydroxide have been shown clearly to be lethal to fully developed fish in natural waters. When the concentration is insufficient to raise the pH above 9.0, interference with normal development and other damage to fish life sometimes may occur. However, at lower pH values, alkalinity does not seem to have any harmful effect upon plankton and other aquatic life.
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