2012-11-05_HYDROLOGY - C1994082 (2)

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The term "pH" is a mathematical transformation of the hydrogen ion (H +) concentration; it conveniently expresses the acidity or basicity of water. The lowercase letter "p" refers to "power" or exponent, and pH is defined as the negative logarithm of the hydrogen ion concentration. Each change of one pH unit represents a ten -fold change in hydrogen ion concentration. The pH scale is usually represented as ranging from 0 to 14, but pH can extend beyond those values. At 25 °C, pH 7.0 describes the neutral point of w=ater at which the concentrations of hydrogen and hydroxyl ions (OH-) are equal (each at 10.7 moles /L). Conditions become more acidic as pH decreases and more basic as pH increases. The pH of freshwater ecosystems can fluctuate considerably within daily and seasonal timeframes, and most freshwa- ter animals have evolved to tolerate a relatively wide environmental pH range. Animals can, however, become stressed or die when exposed to pH extremes or when pH changes rapidly, even if the change occurs within a pH range that is normally tolerated. In addition to the direct effects of pH on aquatic animals, the hydrogen ion concentration affects aqueous equilibria involving ammonia, hydrogen sulfide, chlorine and dissolved metals. The interactions of pH with these variables are often more important than the direct effects of pH on aquatic 'Mississippi State University Southern Regional Aquaculture Center July 2008 OEM Craig S.Tucker and Louis R. D'Abramo' animals. Direct "pH toxicity" is relatively rare in aquaculture ponds because farm sites and water supplies are selected to provide a desirable environment for culture, which should include a pH of approximately 6 to 9. However, certain conditions may cause pH to rise or fall outside the tolerable range, killing the animals being cultured. This publication addresses the most common of these situations —when excessive underwa- ter photosynthesis causes pH to rise to high, basic levels. There are no precise guidelines for high pH tolerance, but pH values above 9.5 or 10 are generally considered undesirable in aquaculture ponds. pH of natural waters Pure water exposed to air has an acidic pH of about 5.6 because carbon dioxide hydrates in water to form carbonic acid, which dissociates to hydrogen ion and bicarbonate (HCO; ): COZ + H_,O = H2CO3 ;� H' + HCO3 Natural waters are never pure, though, because water is a powerful solvent. Water dissolves some of every gas or solid it contacts, and some of these dissolved substances affect the water's pH. Bicarbonate and carbonate (CO;' -) are negatively charged ions (anions) common in most waters. These basic anions are derived from the dissolution of limestone and they increase the pH of water. Bicarbonate and carbonate are also the anions primarily responsible SRAC Publication No. 4604 •VI for the property of water called "alkalin- ity," which is the capacity of water to neutralize acid. Chemical interactions among carbon dioxide, hydrogen ions, and the anions that produce alkalinity buffer the pH of most natural waters in a range of about 6 to 5.5. In the absence of processes that add or remove carbon dioxide, the initial pH of water in contact with air de- pends on its alkalinity, Waters with low alkalinities have an initial pH at the low end of that range, while waters of higher alkalinities have higher pH. Although alkalinity establishes the initial pH of water, adding or removing carbon dioxide causes pH to rise or fall from that initial value. Adding carbon dioxide "pushes" the previously defined chemi- cal reaction toward the right -hand side, forming carbonic acid and hydrogen ions and causing pH to decrease. Remov- ing carbon dioxide "pulls" the reaction to the left, thereby removing hydrogen ions and causing pH to increase. The magnitude of variation from the initial pH depends on 1) the amount of carbon dioxide added or removed and 2) alkalin- ity, which tends to buffer, or reduce, the effect of changes in carbon dioxide concentrations. gh pH problems in ponds Undeiwater biological activity controls carbon dioxide concentrations in most surface waters, including aquaculture ponds. All living organisms continuously