This document discusses dissolved oxygen levels in water. It outlines biochemical oxygen demand (BOD) as a measure of oxygen-demanding wastes, and distinguishes between aerobic and anaerobic decomposition. Organic wastes lower dissolved oxygen which can lead to eutrophication, an excess of nutrients that causes algal blooms and reduced oxygen levels. Thermal pollution from higher water temperatures also decreases dissolved oxygen. BOD measurements determine the oxygen used by bacteria to break down organic matter over 5 days, indicating waste levels.

1. 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.

2. DDiissssoollvveedd ooxxyyggeenn iinn wwaatteerr
 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

3. 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.

4. DDiissssoollvveedd ooxxyyggeenn iinn wwaatteerr
 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.

5. DDiissssoollvveedd ooxxyyggeenn iinn wwaatteerr
 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.

6. DDiissssoollvveedd ooxxyyggeenn iinn wwaatteerr
 The greater the BOD, the less dissolved
oxygen there is.
 The greater the BOD, the more organic
waste there was in the water sample.

7. DDiissssoollvveedd ooxxyyggeenn:: mmeeaassuurreemmeenntt
 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.

8. 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)
2- (aq) + I2 (aq) ® S4O6
 2S2O3
2- (s) + 2I- (aq)

9. EEuuttrroopphhiiccaattiioonn
 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.

10. 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/Eutrophication

11. EEuuttrroopphhiiccaattiioonn
 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

12. EEuuttrroopphhiiccaattiioonn
Animation:
http://www.mp-docker.demon.co.uk/environmental_chemistry/topic_4b/movie.html

13. DDiissssoollvveedd ooxxyyggeenn:: temperature
Higher water temperatures:
 decrease amount of dissolved oxygen in
water
 increase rate of metabolism of aquatic life
requiring them to need more oxygen