Download Dissolved oxygen in water

E 5. Dissolved oxygen
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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.
Dissolved oxygen in water
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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
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Aerobic respiration lowers the amount of
dissolved oxygen in water.
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The more aerobic respiration, the lower the
amount of dissolved oxygen.
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At times organic waste and other nutrients end up
in the water of a river or lake.
Dissolved oxygen in water
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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.
Dissolved oxygen in water
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BOD or Biochemical Oxygen Demand.
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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
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The greater the BOD, the less dissolved oxygen
there is.
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The greater the BOD, the more organic waste
there was in the water sample.
Dissolved oxygen: measurement
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Water sample is collected and added to oxygen
saturated water ([O (aq)] is known).
Water is left for 5 days at 293K and in the dark.
New [O (aq)] is measured using an oxygen
electrode or titration.
BOD = [O (aq)] - [O (aq)]
Pure water has BOD of less than 1 ppm.
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Measuring dissolved oxygen
Winkler method:
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2Mn2+(aq) + 4OH-(aq) + O2(aq)  2 MnO2 (s) + 2H2O (l)
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MnO2 (aq) + 2I-(aq) + 4H+(aq)  Mn2+(aq) + I2(aq) + 2H2O(l)
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2S2O32- (aq) + I2 (aq)  S4O62- (s) + 2I- (aq)
Eutrophication
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Aerobic respiration of large amounts of organic matter in water
produce large amounts of inorganic ions such as nitrates and
phosphates; nitrates and phosphates are also added as a result of use
of fertilizers and 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.
Eutrophication: causes
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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/Eutrophication
Eutrophication
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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
Eutrophication
Animation:
http://www.mp-docker.demon.co.uk/environmental_chemistry/topic_4b/movie.html
Dissolved oxygen: temperature
Higher water temperatures:
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decrease amount of dissolved oxygen in water
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increase rate of metabolism of aquatic life
requiring them to need more oxygen