7-كتالوج اجهزة معمل تحاليل المياه والهندسة الصحية (القلوية)

chemical parameters of water

Alkalinity

Alkalinity refers to how well a water body can neutralize acids.

Alkalinity measures the amount of alkaline compounds in water, such as carbonates (CO 3 2- ), bicarbonates (HCO 3 - ), and hydroxides (OH - ).

These compounds are natural buffers that can remove excess hydrogen ions that have been added from sources such as acid rain or acid mine drainage.

Alkalinity mitigates or relieves metals toxicity by using available HCO 3 - and CO 3 2- to take metals out of solution, thus making it unavailable to fish.

Alkalinity is affected by the geology of the watershed; watersheds containing limestone will have a higher alkalinity than watersheds where granite is predominant.

Alkalinity is a measure of the buffering capacity of water, or the capacity of bases to neutralize acids.

Measuring alkalinity is important in determining a stream's ability to neutralize acidic pollution from rainfall or wastewater.

Alkalinity does not refer to pH, but instead refers to the ability of water to resist change in pH.

The presence of buffering materials help neutralize acids as they are added to the water.

These buffering materials are primarily the bases bicarbonate (HCO3-), and carbonate (CO32-), and occasionally hydroxide (OH-), borates, silicates, phosphates, ammonium, sulfides, and organic ligands.

Waters with low alkalinity are very susceptible to changes in pH.

Waters with high alkalinity are able to resist major shifts in pH.

As increasing amounts of acid are added to a water body, the pH of the water decreases, and the buffering capacity of the water is consumed.

If natural buffering materials are present, pH will drop slowly to around 6; then a rapid pH drop occurs as the bicarbonate buffering capacity (CO32- and HCO3-) is used up.

At pH 5.5, only very weak buffering ability remains, and the pH drops further with additional acid.

A solution having a pH below 4.5 contains no alkalinity, because there are no CO32- or HCO3- ions left.

Alkalinity not only helps regulate the pH of a water body, but also the metal content.

Bicarbonate and carbonate ions in water can remove toxic metals (such as lead, arsenic, and cadmium) by precipitating the metals out of solution.

The natural buffering capacity of a stream may mask the presence of acidic or basic pollutants.

Therefore simply measuring a stream's pH may not be sufficient.

For a more complete assessment of water quality, most scientists also measure streamwater alkalinity.

Alkalinity is measured to determine the ability of a stream to resist changes in pH.

That is to say alkalinity allows scientists to determine a stream's buffering capacity.

Alkalinity values of 20-200 ppm are common in freshwater ecosystems.

Alkalinity levels below 10 ppm indicate poorly buffered streams.

These streams are the least capable of resisting changes in pH, therefore they are most susceptible to problems which occur as a result of acidic pollutants.
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The Chemistry of Alkalinity

Alkalinity results from the dissolution of calcium carbonate (CaCO3) from limestone bedrock which is eroded during the natural processes of weathering.

The carbon dioxide (CO2) released from the calcium carbonate into the streamwater undergoes several equilibrium reactions.

Equilibrium reactions are those that go in either direction, depending on what compounds are present in the largest concentrations.

The following set of reactions is referred to as the bicarbonatebuffering system:

CO2 + H2O H2CO3 H+ + HCO3- 2H+ + CO32-

As depicted in the above set of reactions, carbonate (CO32- ) and bicarbonate (HCO3-) ions act as hydrogen ion absorbers.

This causes the reactions of the bicarbonate buffering system to shift left or right while maintaining a relatively constant pH.

If hydrogen ions are added to the solution, they combine with available bicarbonate or carbonate ions, causing the reactions to shift to the left and eventually liberate carbon dioxide and water molecules.

The addition of carbonate to the solution causes the hydrogen ions to be occupied and shifts the reactions to the right.

In this way, an alkaline stream with a large buffering capacity is able to "hold" more acidic pollutant without displaying a significant decrease in pH.

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Measurement of Alkalinity

Alkalinity is measured by titration.

An acid of known strength (the titrant) is added to a volume of a treated sample of water.

The volume of acid required to bring the sample to a specific pH level reflects the alkalinity of the sample.

The pH end point is indicated by a color change.

Alkalinity is expressed in units of milligrams per liter (mg/l) of CaCO3 (calcium carbonate).

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Factors Affecting Alkalinity

Geology and Soils

Carbonates are added to a water system if the water passes through soil and rock that contain carbonate minerals, such as calcite (CaCO3).

Where limestone and sedimentary rocks and carbonate-rich soils are predominant, (such as the eastern part of the Boulder Creek watershed) waters will often have high alkalinity.

Where igneous rocks (such as granite) and carbonate-poor soils are predominant (such as the western part of the Boulder Creek watershed) waters will have low alkalinity.

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Changes in pH

Because alkalinity and pH are so closely related, changes in pH can also affect alkalinity, especially in a poorly buffered stream.

See the section on pH for more information on factors affecting pH.

Sewage Outflow

The effluent from Wastewater Treatment Plants (WWTPs) can add alkalinity to a stream.

The wastewater from our houses contains carbonate and bicarbonate from the cleaning agents and food residue that we put down our drains.
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Water Quality Standards and Other Criteria Regarding Alkalinity

Because alkalinity varies greatly due to differences in geology, there aren’t general standards for alkalinity.

Levels of 20-200 mg/L are typical of fresh water.

A total alkalinity level of 100-200 mg/L will stabilize the pH level in a stream.

Levels below 10 mg/L indicate that the system is poorly buffered, and is very susceptible to changes in pH from natural and human-caused sources.

Other Information about Alkalinity

Above pH 8.3, alkalinity is mostly in the form of carbonate (CO32-); below 8.3, alkalinity is present mostly as bicarbonate (HCO3-).

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