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Phosphates (PO4-3)


Phosphates are chemical compounds made from the elements phosphorous and oxygen. Phosphorous is necessary for plant and animal growth. Phosphorous is usually present in natural waters as phosphate (PO4-3).

Phosphates exist in several forms. Orthophosphates are produced by natural processes and are found in wastewater. Polyphosphates are used for treating water boilers and for making laundry detergents. Organic phosphates are produced by living things and the breakdown of organic pesticides. The sum of these three forms of phosphates is referred to as total phosphates.

Phosphorous is an essential element for life. Animals use phosphorus to conduct metabolic reactions (chemical reactions that occur inside the body that allow the body to stay alive) and to produce bone. Plants use phosphorus in order to grow. Nearly all fertilizers contain phosphates.

Phosphates are usually present in the environment in low concentration which limits plant growth. Phosphates enter the environment from human or animal wastes, fertilizers, soaps, industrial wastes, and the disturbance of land and its vegetation. When too much phosphorus becomes available, plants grow rapidly. Phosphates that enter a stream may cause algae to multiply and grow quickly. This may result in an algae bloom. Algae blooms are thick layers of green slime that cover the surface of ponds or slow moving streams.

Algae blooms are harmful to most aquatic organisms. They cause a decrease in the dissolved oxygen levels of the water. They prevent waves and the surface of the water from coming into contact with the air which provides the main source of oxygen for the water. Their dark color absorbs more heat energy from sunlight causing the water temperature to rise. Warm water holds less oxygen than cold and it causes the metabolic rate of aquatic organisms to increase. In addition, the algae which grow rapidly near the surface block sunlight to plants that live on the bottom causing them to die. Plants which grow fast die fast and sink to the bottom. Dead plant material is decomposed by bacteria increasing the biochemical oxygen demand (BOD). All of these factors combine to cause oxygen levels in the water to decrease rapidly. This can result in a fish kill and the death of many organisms.

Algae blooms are indicators of cultural eutrophication. Eutrophication refers to the aging of a lake or other body of water. Cultural eutrophication occurs when nutrients are added from agricultural runoff, sewage, detergents or other sources such as golf courses or lawns. Advanced stages of cultural eutrophication can be detected by an unmistakable "rotten egg" smell.

HOW TO DO A PHOSPHATE (PO4-3) TEST

Various phosphate types and levels can be tested for in a variety of ways. Most of these tests involve a colormetric change. Some kits measure this change through electronic means. Some use a simple color wheel comparator.

Listed below are the instructions for measuring mid-range (0-5 mg/l) levels of orthophosphates using a Model PO-19 test kit manufactured by Hach Co.

  1. Fill both color viewing tubes to the 5ml mark with the water to be tested.

  2. Open one PhosVer 3 Reagent Powder packet. Add the contents of the packet to one of the sample tubes. Swirl to mix. Allow at least one minute, but no longer than five minutes, for full color development. If phosphate is present, a blue-violet color will develop.

  3. Place the tube containing the prepared sample in the top right opening of the black color comparator box.

  4. Place the tube with the untreated water sample into the top left opening of the black color comparator box.

  5. Hold the comparator box up to a light source and view through the openings in the front. (Use a white sheet of paper for a background.) Rotate the color disk in the comparator box until the two colors match.

  6. Divide the reading in the scale window by 10 to obtain the mg/l phosphate (PO43-). Record the results.

  7. When finished, pour the used chemicals into the waste collection container. Clean the equipment and return it to its proper place.


NOTE - The procedure above measures low range 0-5 mg/l phosphates. To measure low range 0-1 mg/l or high range 0-50 mg/l phosphates follow the instructions that come with your kit designed for the appropriate range.

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