pH

pH is defined as the negative log-base 10 of the hydrogen ion concentration:

pH = - log10 [H+]

The pH is a log-base 10 scale that measures acidity of a solution on a scale of 0 to 14. The pH of neutral solutions, such as pure water, is equal to 7. Alkaline solutions will have high pHs (8-14) and acidic solutions will have low pHs (1-6).

For an example of the pH scale and examples of different acids and bases, check the following link:

http://www.r-can.com/images/ph-scale.gif

Since the pH is a log-base-10 scale, the pH changes 1 unit for every power of ten change in [H+]. For example, a water with a pH of 3 has 100 times the amount of [H+] that is found in a pH 5 water. Remember that because pH = - log10 [H+], the pH will decrease as the [H+] increases (Zumdahl, 1989).

The pH of water is controlled by the equilibrium achieved by dissolved compounds in the system. In natural waters, the pH is primarily a function of the carbonate system, which is composed of carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO3-) and carbonate (CO32-) (AWWA, 1990; EPA, 1986). The applicable equilibrium equations and the estimated pH ranges at which each are present are:

CO2 + H2O <--> H2CO3 (pH: < 6.4)
H2CO3 <--> (H+) + HCO3- (pH: 6.4 - 10.33)
HCO3- <--> (H+) + CO32- (pH: > 10.33)

Acid inputs to a water system may substantially alter the pH. The main sources of acid include acid mine drainage and atmospheric acid deposition.

1. Acid Mine Drainage: Coal mining and coal use pose a threat to water quality. Generally, coal contains 0.2 - 0.7 percent sulfur, half of which is held within the organic coal nodules and half of which is present as pyrite (FeS), iron disulfide (FeS2), and marcasite (Craig et al., 1988). When the sulfur is exposed to oxygen during mining practices, the sulfide compounds are converted to ferrous sulfate (FeSO4) and sulfuric acid (H2SO4).

   The exposed Fe and S compounds in the mines and in the mine spoils (piles of waste rock) are readily washed into surface water bodies by rainfall and ground water seepage. Often, surface water bodies impacted by mine drainage will contain gelatinous, reddish-brown precipitates of iron hydroxides on rocks and plants. The acidic water inputs can lower the pH of some streams to completely kill all life. In the United States alone, acid mine drainage is estimated to have affected almost 10,000 stream miles (Craig et al., 1988).

2. Acid Deposition: Large amounts of nitrogen oxides and sulfur dioxides (NOxs and SO2) have been emitted into the atmosphere for the past century. Roughly 75 to 100 million tons per year of the sulfur dioxides injected into the atmosphere are anthropogenic in origin. Another 75 to 100 million tons per year are emitted naturally by earth processes.

   The nitrogen oxides and sulfur dioxides injected into the atmosphere take one of two general paths. The emissions may return to earth quickly as dry deposition or remain in the atmosphere for an undetermined time. While circulating in the atmosphere, the NOxs and SOxs and their oxidative products react to form nitrates and sulfates. Ultimately the pollutants fall to earth during a rainfall event (wet deposition) as weak nitric or sulfuric acid, often called acid rain.

   Both wet and dry acid deposition can severely impact aquatic systems. Acid deposition has been implicated in the acidification of mountain streams and lakes, especially in the northeastern United States where the lakes have a poor ability to buffer against acid inputs because of a lack of alkalinity or acid neutralizing capacity anions with which the H+ could complex.

   In areas of acid rain, surface water systems will receive acidic inputs in rain, snow, and ground water seepage. Spring snow melt is a big concern because when the accumulated snow melts, a large slug of acidified water will flow directly into surface water systems. Also, if an extended period of dry acid deposition is followed by a heavy rainfall, a similar slug of acidic water may occur (Smith, 1990).

Health Effects: Low pH water may corrode distribution pipes in potable water plants. The pipes may be costly to replace and the corrosion may release metal ions such as copper, lead, zinc, and cadmium into the treated drinking water (EPA, 1986). Ingestion of heavy metals may pose substantial health risks to humans.

Industrial Effects: Although near-neutral pH values are preferred, industry as a whole can tolerate a wide pH range, depending on the intended water use. The Environmental Protection Agency reports that the widest pH range tolerable for process waters is pH = 3.0 - 11.7, and for cooling waters is pH = 5.0 - 8.9. Specific industries will require more limited ranges.

Deleterious effects may result from pH values occurring at the extreme ends of the ranges. Low pH water may corrode system pipes. Not only will the pipes be costly to replace, but the corrosion may also release metal ions such as copper, lead, zinc, and cadmium into the water (EPA, 1986).

Because pH is easily altered during water treatment, an industry can usually prepare water of the proper pH to meet its needs (EPA, 1986).

Environmental Effects: A reduction in pH (more acidic) may allow the release of toxic metals that would otherwise be absorbed to sediment and essentially removed from the water system. As the hydrogen ion concentration increases, the metal cations experience greater competition from H+ ions for binding sites. Some metal cations will be out-competed and liberated into overlying water. For example, a decrease in pH values may release aluminum ions from complexation with other cations. Aluminum concentrations of 0.1 - 0.3 mg/l will increase mortality, retard growth, gonadal development, and egg production of fish. Even if the aluminum availability is low, recent studies have shown that acidity alone may cause mortality in developing brook trout (Smith, 1990).

Once mobilized, these metals are available for uptake by organisms. For many metals, the rate of uptake is directly proportional to the levels of metal availability in the environment. Thus, a decrease in pH increases metal availability, lending itself to greater metal uptake by organisms. Metal uptake can cause extreme physiological damage to aquatic life (Connell et al., 1984).

An increase in pH may cause heightened ammonia concentrations (EPA, 1986). At low pH, ammonia combines with water (H2O) to produce an ammonium ion (NH4+) and a hydroxide ion (OH-). The ammonium ion is non-toxic and not of concern to organisms. Above a pH of 9, ammonia (un-ionized) is the predominant species (Morgan et al., 1981). The un-ionized ammonia (NH3) is very toxic to organisms. Thus, organisms experience ammonia toxicity more readily at higher pH (NRC, 1979).

Experiments have shown that a pH decrease of 1.4 units of pH can disturb the aquatic community. After acidification of a test area, the water column concentrations of aluminum, calcium, magnesium, and potassium increased; the downstream drift of immature insect larvae increased; emergence of mature stoneflies and mayflies decreased; periphyton (attached algae) biomass increased; and trout migrated to areas of higher pH (Smith, 1990).

Acidification of aquatic systems also inhibits microbial activity in the benthos, reducing decomposition and nutrient cycling. This may lead to a reduction of the invertebrates and plankton that are a vital part of the food chain. Eventually, a shift in community structure may occur (Smith, 1990).

Some amphibians are very sensitive to acid inputs, especially during reproductive periods. The fertilization stage is most noticeably affected by acid because disintegration of amphibian sperm occurs at low pH. The embryonic stages of the leopard frog (Rana pipens) and the spotted salamander (Ambystoma maculatum) have suffered 100% mortality at pH 4.0 - 5.0 (Smith, 1990).

http://www.redpath-museum.mcgill.ca/Qbp/Images/Herps/leopard%20frog.jpg

http://www.uga.edu/srelherp/images/spottedsal.jpg

Sources of pH Change:

1. Nonpoint sources: Acid inputs from acid mine drainage and wet/dry acid deposition can substantially lower the pH of a system.
2. Point sources: Effluent water that is treated for industrial use may be more acidic or more alkaline than the receiving water body.

1. Electronic pH Meter: A probe containing an acidic aqueous solution encased in a special glass membrane allows migration of hydrogen ions (H+). If the water has a pH different from that of the solution within the probe, an electric potential results that registers on the meter.

2. Mode of Transport: Acid may be carried to surface water bodies via overland or subsurface flow. Weak acids of nitrogen and sulfur may be deposited from the atmosphere in a dry (fallout) or wet (rainfall) form.

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NCSU Water Quality Group

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