Ho Kit Mun Jasmine

H1 Ruby


IB assessment statements:

  1. Outline biochemical oxygen demand (BOD) as a measure of oxygen-demanding wastes in water.

  2. Distinguish between aerobic and anaerobic decomposition of organic matter in water.

  3. Describe the process of eutrophication and its effects.

  4. Describe the source and effects of thermal pollution in water.

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1. What is dissolved oxygen in water?

• Importance to plants

• Importance to animals

Most aquatic plants and animals require oxygen for aerobic respiration. Hence, they have to rely on oxygen gas that is dissolved in water in order to survive. Fish require a minimum of 3 ppm in order to survive. When levels of dissolved oxygen fall below 3 ppm, aquatic life becomes very tough, or even impossible. To maintain a balanced, healthy and diverse aquatic community, the oxygen content in the water should be maintained at about 5 ppm. Levels of dissolved oxygen in the water provide information on the quality of the water.

Oxygen enters the water body from surrounding air. Photosynthesis is the primary process affecting the relationship between levels of dissolved oxygen and the temperature of the water. Bacteria photosynthesize and release oxygen as a by-product. Strength and duration of sunlight as well as water clarity would then determine the rate of photosynthesis. Another way oxygen enters water is through aeration, which is the flow of water through rapids. Aeration is a process that very rarely occurs in calm waters. It results in the occurrence of various temperatures of the water.

2. Problems with changing levels of dissolved oxygen (DO) in water

DO levels vary with seasons as well as time of day. DO levels are typically at their lowest during summer months where temperatures are high. This is due to the fact that increased temperatures result in the decrease of solubility of gases in water. Due to global warming, fish have been changing their usual habitats, migrating from their homes to look for cooler waters with higher levels of DO. In this case, denser and colder waters are found in deeper parts of the ocean. In cases where fish are unable to resettle in new habitats, fish stocks begin to decline. Rapidly moving water usually contains a large amount of DO, and stagnant water tends to contain less. Stagnant water contains rotting organic matter, which is detrimental for aquatic life. When DO levels decline below the required value for survival, the water is said to be polluted, and aquatic organisms begin to die.

3. Biochemical oxygen demand (BOD)

· What is BOD?

· How is it measured? (which method?)

Organic-demanding wastes account for depleting DO levels as well. Such wastes include bacteria, which require oxygen to decompose animal and plant matter. Hence, aerobic decomposition of organic matter depletes levels of dissolved oxygen in the water. The biological oxygen demand (BOD) is a measure of the dissolved oxygen required by aerobic decomposers to decompose the organic matter in water at a certain temperature, over a specified time period.

BOD is measured over the span of five days. The lower the level of BOD, the cleaner the water body. Water with a BOD level of 6 ppm and above is considered to be polluted. Clean water has a BOD level of about 1 ppm. The cleaner the water source, the lower the level of BOD. Water with a high BOD level that is unable to replenish the oxygen supply will not be able to sustain aquatic life. Fast flowing water bodies are better able to recover from an excess of oxygen-demanding wastes since rapid moving water can become oxygenated at a faster rate. On the other hand, water bodies with low levels of water flow activity will have lower or no chance of reoxygenation.

The Wrinkler method

A sample of water is first collected in a bottle. There must be no empty space left in the sample bottle since outside air sources will contaminate the water. The sample of water is saturated with oxygen such that the initial concentration of dissolved oxygen is known. A fixed volume of the sample is taken and incubated at a fixed temperature for a period of five days. Microorganisms in the water then oxidize the organic material in this period of incubation. Secondly, the amount of oxygen in the water sample must be fixed by using a manganese sulfate and an alkaline-iodide reagent.

The manganese (II) ions are then oxidized to manganese (IV) oxide.

(Under alkaline conditions)

2Mn2+(aq) + 4OH-(aq) + O2 (aq) to give 2MnO2 (s) + 2H2O (l)

The formation of brown manganese (II) oxide indicates the presence of oxygen in the sample. Concentrated sulfuric acid is added, removing the manganese (II) oxide and fixing the amount of oxygen in the sample.

MnO2 (s) + 2I- (aq) + 4H+ (aq) to give Mn2+(aq) + I2 (aq) + 2H2O (l)

The darker brown the solution is, the higher the concentration of I2(aq) and hence, the higher the amount of dissolved oxygen in the water.

external image Dissolved-oxygen-at-hippo-dam-8May2010-photo-Alistair-Clacherty-300x238.jpg

The picture above shows an example of how the colour of the solution can be compared against an indicator to determine roughly, the level of dissolved oxygen in the water.

Next, a titration is performed in order to quantify the amount of dissolved oxygen present. A standardized solution of sodium thiosulfate and starch is used as an indicator. The moment the blue colour of the iodine-starch complex disappears, the titration has ended.

I2 (aq) + 2S2O3 2- (aq) to give S4O6 2- (aq) + 2I- (aq)

The greater the volume of sodium thiosulfate used, the greater the volume of dissolved oxygen present in the water.

**Testing for BOD is now mostly automated.

The diagram below is a pictorial representation of the Wrinkler method.

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4. Aerobic versus anaerobic decomposition

· What are the conditions required for each process to be carried out?

Aerobic decomposition is the most common decomposition process. When sufficient oxygen is present, organic matter decays aerobically. Oxides or oxyanions are produced as a result. In electrochemistry, elements which decay aerobically lose electrons or are oxidized.

An example of aerobic decay would be:

P4 (aq) + 8O2 (g) + 12e- to give 4PO43- (aq)

· P is oxidized, oxidation number increases from 0 to +5.

· P loses electrons.

Not all decomposition processes are aerobic. Anaerobic decay involves organisms which do not require oxygen to carry out decomposition. Elements gain electrons or are reduced. The products of anaerobic decay are in reduced forms, and are often foul-smelling and toxic since anaerobic decomposition usually occurs in areas like marshlands.

An example of anaerobic decay would be :

S8 (S) + 8H2 (g) to give 8H2S (g)

· S is reduced, oxidation number reduces from 0 to -2

· S gains electrons

**Dihydrogen sulfide, H2S, is a foul smelling gas.

Aerobic and anaerobic decomposition:


Aerobic decay product

Anaerobic decay product



CH4 (marsh gas)



NH3 and amines



CH4, NH3, H2S, H2O



H2S (“rotten eggs” gas)



PH3 (phosphine)

5. Eutrophication

· Reasons behind eutrophication

· Effects on the environment and other living things

external image eutrophicationriver.jpg

Eutrophication is the process when water bodies become highly enriched with nutrients. These nutrients, mainly nitrogen and phosphorous, cause extensive degradation of water quality. Excessive fertilizer run-off from agricultural fields causes a large amount of nitrogen-containing compounds to leak into the water body. Phosphates can be found in fertilizers, as well as untreated household waste, which contains large traces of detergents.

The accumulation of nitrates and phosphates cause excessive growth of plants and algae. The excessive plant growth reduces the amount of dissolved oxygen in the water body. This is due to the fact that plants and algae form oxygen-demanding wastes when they decompose. There is hence, insufficient oxygen to support aerobic decomposition.

Anaerobic decomposition occurs when there is insufficient oxygen for aerobic decomposition. Hydrides produced as a result of anaerobic decomposition smell foul, and most importantly, poison the water body. Aquatic life is destroyed due to lack of dissolved oxygen in water.

6.Thermal pollution

· What is thermal pollution?
· Causes of thermal pollution?

The level of dissolved oxygen in water is temperature dependent. The solubility of oxygen decreases with an increase in temperature. As the temperature of water increases, metabolic rates of fish and other aquatic organisms also increase, hence leading to an increase in the demand for oxygen. Also, the fish have to consume more food, which may result in a food shortage. Small changes in temperature have large impacts on the ecosystem. The increase in temperature can be caused by human activities, hence the term “thermal pollution”.

Thermal pollution occurs when industries use water as a coolant. Heated water from the industrial processes is discharged into the water bodies, increasing the temperature of the water bodies. Also, the release of unnaturally cold water into water bodies causes thermal pollution as well. This results in the death of a large amount of fish stocks. Lastly, thermal pollution occurs as a result of logging activities. When trees are cut down, they can no longer provide shade. This allows a greater and more direct amount of sunlight to reach the water’s surface, increasing the temperature of the water body.


What measures can governments, industries and people in general, take to prevent eutrophication?





USGS Water Science for Schools (2011, Feb 8). Water properties: dissolved oxygen. Retrieved from http://ga.water.usgs.gov/edu/dissolvedoxygen.html

The Encyclopedia of Earth (2007, December 18). Eutrophication. Retrieved from http://www.eoearth.org/article/Eutrophication

Geoffrey Neuss (2007). IB Diploma Programme Chemistry Course Companion. New York: OXFORD University Press.

Geoff Neuss (2001). Chemistry for the IB Diploma, standard and higher level. Great Britain: OXFORD University Press.