lkalinity is often defined as the ability of an aquatic ecosystem to buffer acids. Today, alkalinity refers to the ability to neutralize strong acids (i.e., HCl, H
4, and HNO
3) and is used interchangeably with Acid Neutralizing Capacity (ANC). It includes the quantities and types of compounds that combine to increase the pH towards the alkaline side of neutrality. These compounds are predominantly carbonate, bicarbonate, and hydroxide, but may include small amounts of other compounds. The buffering capacity of aquatic ecosystems is very important in the maintenance of life.
Alkalinity of natural waters fluctuates readily in aquatic systems, generally resulting from influxes of carbon dioxide and rainwater. Carbon dioxide (CO
2) dissolves in water, forming carbonic acid, which lowers the pH of the system. However, this weak carbonic acid solution increases the system's capacity to dissolve minerals as it flows through the watershed, which in turn increases the alkalinity of the water. This cyclic movement keeps the environment in balance.
Traditionally, hardness was defined as the ability of water to precipitate soap. Hardness has not been of particular interest to limnologists in the past, but was of significant importance to aquatic chemists and engineers working with water quality in water treatment plants. Hardness can be readily observed as a residual precipitate left on containers following evaporation of hard water. Hard water is of concern to the homeowner as deposits form on plumbing fixtures and appliances. Home water treatment systems are often installed to treat hard water. Hard water is not harmful to humans, and contains many of the minerals needed for normal plant and animal growth, but may affect the tolerance of fishes to toxic metals. Hardness is expressed as milligrams of calcium carbonate per liter (mg/L CaCO
4-2) is widely distributed in nature and may be present in natural waters in concentrations ranging from a few to several thousand milligrams per liter. Mine drainage waste may contribute large amounts of sulfates through pyrite oxidation.
Chloride, in the form of chloride (Cl
-) ion, is one of the major inorganic anions in water and wastewater. In potable water, the salty taste produced by chloride concentrations is variable and dependent on the chemical composition of water. Some waters containing 250 mg Cl
- may have a detectable salty taste if the cation is sodium. On the other hand, the typical salty taste may be absent in waters containing as much as 1000 mg/ L when the predominant cations are calcium and magnesium.
Many of the cations are analyzed by atomic absorption spectrometry. In flame atomic absorption spectrometry, a sample is aspirated into a flame and atomized. A light beam is directed through the flame, into a monochromator, and onto a detector that measures the amount of light absorbed by the atomized element in the flame. Because each metal has its own characteristic absorption wavelengths, a source lamp composed of that element is used; this makes the method relatively free from other interferences.
Sodium is a chemical element with the symbol Na. It belongs to a group of chemical elements called alkali metals. Sodium is the sixth most common chemical element in the earth's crust and never occurs as a separate element in nature. It combines with many other elements to form compounds. One of the most common sodium compounds is sodium chloride (table salt). Minerals such as borax and cryolite contain sodium. Plants and animals contain small amounts of sodium. Sodium compounds have many uses:
- Sodium borate (borax) is used in making ceramics, soaps, and water softeners.
- Sodium hydroxide (caustic soda) is used in refining petroleum, making paper, soaps and textiles.
- Sodium carbonate (soda ash or washing soda) is used in the manufacture of sodium bicarbonate (baking soda).
- Sodium nitrate is included in fertilizers.
- Photographers use sodium thiosulfate to fix photos when printing pictures.
Pure sodium can be used as a catalyst in manufacturing some types of synthetic rubber. Some nuclear power plants use sodium in liquid form to cool nuclear reactors. Sodium is used to produce such metals as titanium and zirconium.
Potassium is a chemical element with the symbol K. It belongs to a group of chemical elements called alkali metals. Like sodium, potassium always occurs combined with other elements. It is found in the form of minerals such as carnallite and sylvite. Potassium is an abundant element and makes up nearly 2.5 percent of the earth’s crust. Large deposits of potassium chloride and potassium sulfate are found in Canada and Germany. The Dead Sea is another source of potassium compounds. There are many uses for potassium:
- Sodium-potassium alloys are used in the heat transfer systems of some types of nuclear reactors.
- Potassium carbonate (potash) is used in making glass and soaps.
- Potassium nitrate is used in producing matches and explosives.
Calcium is a chemical element with the symbol Ca. It belongs to a group called the alkaline earth metals. It makes up about 3.5 percent of the earth's crust. In nature, it occurs only as compounds such as calcium carbonate, calcium fluoride and calcium sulfate. There are many uses for calcium compounds:
- Calcium oxide is used in leather tanning and petroleum refining.
- Calcium fluoride and calcium sulfate are used in making cement and plaster.
- Other calcium compounds are used in manufacturing fertilizer, paint and medicines.
Magnesium is a chemical element with the symbol Mg. It does not occur as pure magnesium in nature. Many minerals contain magnesium including magnetite, brucite, dolomite, amphibole, asbestos, olivine, serpentine, talc and come silicate minerals. Magnesium and its alloys have many manufacturing uses:
- Their light weight makes them especially suitable for aircraft and car parts, tools and equipment.
- It is used in steel manufacturing to remove sulfur and other impurities, as well as in fireworks, flares and incendiary bombs.
- Medicine finds uses for magnesium in milk of magnesia and Epsom salts.
Iron is a element with the symbol Fe. It is rarely found is a pure state in nature. Iron is a very useful metal:
- It can be hammered into thin sheets or drawn out into fine wires.
- Iron is easily magnetized and unites easily with nonmetals.
- It also unites easily with oxygen to form iron oxide which is known as rust.
- Iron can be alloyed with carbon to form steel.
Manganese is a brittle, silver-gray metallic element with the symbol Mn. The earth's crust contains much manganese and it occurs in combination with other elements, mainly in the minerals braunite, hausmannite, manganite, psilomelane, pyrolusite and rhodochrosite. All living things require small amounts of manganese.
Industry uses most manganese in the form of alloys and compounds:
- It is used in refining steel, making stainless steel, and making various alloys of aluminum, and copper.
- One of the most widely used compounds of manganese is manganese dioxide, which is used in dry cell batteries, paints, dyes, photographic developer, bricks and glass making.
- Manganese sulfate is used in paint and varnish dryer, and in some fertilizers.
- Potassium permanganate can serve as a disinfectant and deodorizer and is used in water purification.
- Manganous nitrate and manganous oxide act as catalyst in the production of petrochemicals, plastics and synthetic fibers.
Lead is heavy, bluish-gray element with the symbol Pb. Lead ores are produced from underground mines. Many minerals contain lead, but its principle source is galena, a gray metallic ore. Lead is used in the manufacture of lead-acid batteries used to power electrical systems of airplanes, cars and other vehicles. Lead compounds are important in the manufacture of some paints and dyes, explosives, insecticides, ceramics, glass and rubber products.
The most dangerous aspect of lead is the possibility of poisoning. Lead can be taken into the body by inhalation, ingestion or absorption through the skin. The longer a person is exposed to lead, the greater the danger. Lead interferes with the production of red blood cells and may damage the brain, kidneys and other vital organs.