E 5. Dissolved oxygen

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E 5. Dissolved oxygen

• Outline biochemical oxygen demand (BOD) as a
  measure of oxygen- demanding wastes in water.
• Distinguish between aerobic and anaerobic
  decomposition of organic material in water.
• Describe the process of eutrophication and its
  effects.
• Describe the source and effects of thermal
  pollution in water.

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Dissolved oxygen in water

• solubility of oxygen at 1 atm and 293K is
• 9 ppm
• solubility of gases decreases as temperature
  rises
• needed for aerobic respiration of aquatic plants
  and animals, e.g.
• fish need 3 ppm or 3 mg dm-3 (kg of water)
• large biodiversity needs 6 ppm

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Dissolved oxygen

• Aerobic respiration lowers the amount of
  dissolved oxygen in water.
• The more aerobic respiration, the lower the
  amount of dissolved oxygen.
• At times organic waste and other nutrients end up
  in the water of a river or lake.

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Dissolved oxygen in water

• Organic waste faeces, dead organisms, urea,
  untreated sewage, waste from food production.
• Organic waste in water is first broken down by
  aerobic respiration by bacteria (for energy).
• Dissolved oxygen in the water is lowered.
• Products of aerobic respiration water, carbon
  dioxide, inorganic ions like nitrates, nitrites,
  phosphates.

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Dissolved oxygen in water

• BOD or Biochemical Oxygen Demand.
• BOD can be used as an indicator of the amount of
  organic waste (oxygen-demanding waste) and other
  nutrients in a sample of water.
• BOD measured by determining the amount of
  oxygen in a sample of water (in ppm) at the start
  and determining it again after 5 days. The
  difference is the BOD or amount of oxygen used up
  by aerobic bacteria to decompose the organic
  matter in the water sample a fixed over 5 days at
  293K and in the dark.

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Dissolved oxygen in water

• The greater the BOD, the less dissolved oxygen
  there is.
• The greater the BOD, the more organic waste there
  was in the water sample.

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Dissolved oxygen measurement

• Water sample is collected and added to oxygen
  saturated water (O2 (aq) is known).
• Water is left for 5 days at 293K and in the dark.
• New O2 (aq) is measured using an oxygen
  electrode or titration.
• BOD O2 (aq) - O2 (aq)
• Pure water has BOD of less than 1 ppm.

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measuring dissolved oxygen

• Winkler method
• 2Mn2(aq) 4OH-(aq) O2(aq) ? 2 MnO2 (s)
  2H2O (l)
• MnO2 (aq) 2I-(aq) 4H(aq) ? Mn2(aq)
  I2(aq) 2H2O(l)
• 2S2O32- (aq) I2 (aq) ? S4O62- (s) 2I-
  (aq)

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Eutrophication

• Aerobic respiration of large amounts of organic
  matter produce large amounts of inorganic ions
  such as nitrates (fertilizers) and phosphates
  (detergents) in water.
• These ions are nutrients to algae.
• An algal bloom (or red tide) occurs.
• This bloom removes a lot of dissolved oxygen from
  the water to below a level many aerobic organisms
  cannot survive including aerobic bacteria they
  die.
• More organic waste is added reducing oxygen
  further.
• Anaerobic bacteria take over decomposition of
  organic waste.
• Products of anaerobic respiration methane,
  ammonia, amines, hydrogen sulphide
  foul-smelling gases.

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Eutrophication causes

• Untreated sewage (has a lot of organic waste).
• Run-off animal waste from farms.
• Nitrates from fertilizers.
• Phosphates from detergents.

Eutrophication is apparent as increased turbidity
in the northern part of the Caspian Sea, imaged
from orbit. http//en.wikipedia.org/wiki/Eutrophi
cation
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Eutrophication

• A process in which water bodies (rivers, lakes,
  ..) receive excessive amount of nutrients
  (organic waste, nitrates, phosphates) which
  causes excessive plant growth e.g. algae which in
  turn reduces the amount of dissolved oxygen and
  prevents sunlight from entering the water
• http//www.umanitoba.ca/institutes/fisheries/eutro
  .html

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Eutrophication

• Animation
• http//www.mp-docker.demon.co.uk/environmental_che
  mistry/topic_4b/movie.html

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Dissolved oxygen temperature

• Higher water temperatures
• decrease amount of dissolved oxygen in water
• increase rate of metabolism of aquatic life
  requiring them to need more oxygen