Dissolved Oxygen Lab AP BIO - Dissolved Oxygen Lab Jennie...

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Dissolved Oxygen Lab Jennie Chang 9/22/11 Period 6 Mrs. Burrows AP Biology
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Background Information: Oxygen is a necessary element to all forms of life. Aquatic and terrestrial environments do not possess the same ability to hold oxygen. For equal amounts of air and water, air contains 95% more oxygen than water does. Our atmosphere as an abundance of oxygen, with about 200 milliliters of oxygen for every liter of air whereas in an aquatic environment, there are only about 5 to 10 milliliters of dissolved oxygen in a liter of water. Therefore, dissolved oxygen in aquatic environments is a very important component of water quality. Dissolved oxygen (DO) is measured in parts per million (ppm). When DO is less than 4 ppm, it is considered stressful to most life forms. DO can be short in supply and is often a limiting factor for populations of aquatic organisms. Cold water absorbs more oxygen than warm water does, salinity decreases solubility, and pressure increases solubility. Other factors that affect the DO content of a body of water include temperature, wind, turbulence, and trophic state. As the temperature of water increases, the concentration of DO decreases. Most oxygen enters water as wind blows and mixes oxygen over the surface of water; on windless nights, the lack of oxygen can be so severe sometimes that it kills life in aquatic environments. Turbulence helps mix and agitate the oxygen in the water. Waves, rapids, fallen trees, and waterfalls are some causes of variation in DO concentration. Also, the amount of nutrients in a body of water directly affects and determines how much life can be sustained in the aquatic environment because it determines the amount of oxygen used or released in the water. There are two types of trophic states: eutrophic and oligotrophic. An eutrophic body of water has a fluctuating DO content and is always rich in nutrients whereas an oligotrophic body of water is always rich in oxygen content but poor in plant nutrients. The rest of the oxygen enters the water through the by-product of photosynthesis from primary producers such as microscopic algae. Then, the DO is used up by decomposer bacteria in decaying plants and animals, fish and other aquatic organisms in the process of respiration, and even aquatic plants in times of no light. Lakes and ponds usually have their lowest oxygen levels in the early morning because great quantities of DO are used up by plants and fish all throughout the night where there is no oxygen being produced. It is possible to determine the amount of oxygen in water with the Winkler titrametric method. The procedure involves adding alkaline iodide and manganous sulfate to a water sample. A precipitate, manganous hydroxide is then produced and upon acidification, is then converted to a manganese compound by the oxygen in the water sample. The compound reacts with iodide to release iodine, which turns the water into a dark urine color. The quantity of free iodine is equal to the amount of oxygen in the sample, which we can then find by titrating the iodine with sodium
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Dissolved Oxygen Lab AP BIO - Dissolved Oxygen Lab Jennie...

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