The Level Of Dissolved Oxygen In Water Samples Biology Essay

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1. Calculate the Dissolved Oxygen (DO) concentration in the three measurements listed in Table 1. Show one example of a worked calculation. Express your results in ppm and compare these results with the Oxygen Solubility Data (Table 1). Try to explain any differences between your results and the saturation values given in Table 2. (Calculations plus approximately one page of discussion) (7 marks)

To determine the level of dissolved oxygen in water using the Winkler method. This shall lead us in the determination of water quality of the sample following the level of dissolved water that we obtain (Alcock, 1995). From the measurements, we can determine the oxygen demand within a given period of time. In this method a number of oxidation- reduction reactions represented as follows (Ibanez et al, 2007):

Reaction 1: O2 + 4MnII(OH)2 + 2H2O ƒ 4MnIII(OH)3

Reaction 2: MnIII(OH)3 + 3H+ ƒ  Mn3+ + 3H2O

Reaction 3: 2Mn3+ + 2I- ƒ  2Mn2+ + I2

Reaction 4: 2I2 + 2I- ƒ  2I3-

Reaction 5: 2I3- + 2S2O32- ƒ  3I- + 2S4O62-

The overall reaction equation will be as follows (Ondrus, nd):

O2 + 4S2O32- + H2SO4 2S4O62- + 2SO42- + 2H2O

This reaction equation appears that molecule of oxygen (O2) requires four molecules of Thiosulfate ions for a reaction to take place (Ondrus, nd).

From the first run in table1the volume of S2O3 is 4.10 ml .

Molarity of S2O32- is 0.025 mol/L (from table)

The number of mole for S2O32-= 0.25*4.10/1000=0.0001025 mole

From the equation the number of mole for dissolved oxygen = 0.0001025/4 =0.000025625

Thus, the molarity of O2 = 0.000025*1000/200 = 0.000128 mol/L

Mg/L= ppm =M*M.W *1000 =4.1

So DO content = 4.1 ppm

Using the same procedure and formulas, the dissolved oxygen in run 2 will be 4.2 ppm and 4.05ppm in run 3; so the average = 4.11 ppm

Comparing with the reference table, there is a difference around 4 to 5 ppm since for the same temperature, the ppm for dissolved oxygen as given in table 2 is between 9.17 to 8.22. This can be explained by the need of oxygen in the environment of the water in the lake. Due to the activities that go on in the river there is a high rate of oxygen change between the water and the environment.

Figure 3: The influence of temperature on Dissolved oxygen.

Retrieved from on 27th February 2011

As seen from the graph in figure 3 above, we deduce that an increase in temperature leads to a decrease of the dissolved oxygen capacity. The standard table shows the ppm at different temperatures of ideal water. The water at Lake Eufaula is not ideal since it contains aquatic animals such fish and have activities such as hiking and hunting. Also, the aquatic life might move or even die since that serious lack of DO (United States Enviromentalprotection Agency, 2011). Furthermore the lake is artificial and thus does not have the real natural lake environment ideal for oxygen solubility.

2. Discuss the results in Table 3 with reference to water quality. Discuss any possible effects on animal and plant organisms living in the water. Note any implications for people or wildlife using the water. (1 - 2 pages) (6 marks)

From this, we can know the quality of the oxygen index which has a general structure as figure 3. The oxygen quality increases as the saturation increases until there is optimum at a saturation of 100.

Figure 3: Oxygen quality index.

Aquatic animals and mammals usually require little levels of oxygen. As from the results of table shown above, there is a possibility that they cannot survive well in this environment. This might be an explanation as to why the lake does not have variety of water plants. Only available in the water is a small population of fish of specific type that can survive in high amount of oxygen. The results are also directly affected by the depth of the water which is too shallow. This makes the lake have a high temperature which affects the rate at which oxygen is exchanged between the water and the environment (Gakii, 2010).

The activities of the people who live around the lake affect the quality of the water too much. It is imperative to note that the water bodies are artificial. This makes so many people to live around the lake are and have their activities especially recreation activities in the water. The effect of such activities is clearly seen in the table as compared to the regulatory table. It does not matter what activities they undertake, it matters what intense the activities are (Gakii, 2010). The more intense the activities are, the low the water quality.

The fact that there is urbanization around the lakes, it is clear that we all understand the effects of urbanization and especially to water bodies. There is much of pollution that is discharged into the waters. This makes the rate at which oxygen is dissolved in the water to increase slightly higher than the normal level. The more the water quality goes down, the less the marine animals are found in the water and the more the people have to avoid using the water. It has negative effects on them. It has to be treated before it is used (Gakii, 2010).

3. Relate the 16/6/02 reported event to the measurements reported in Table 3. Explain the possible impact, if any, of the turbidity measurements. (up to half page) (3 marks)

16 June 2002

Canal beside Woolcock St and Paradise Lake, Townsville 4810

100 dead fish (3-20 cm in size) including bream tarpon barramundi mangrove jack mullet and herring.

This event corresponded with an extremely low tide. There was no evidence of contaminants in fish that were analysed. [1] 

The event reported on 16/6/02 is an event that can be related to the dissolved oxygen in the water. Looking at the figures of the 13th and the 17th of the same month, we can clearly the differences in the figures of the table. There was an abrupt change in the water properties. The value of Dissolved oxygen reduced from 3.11 to 0.6 ppm. The fish that died could have been caused by the tremendous reduction in the dissolved oxygen in the water. This kind of reduction does not contaminate the water but makes fish lack enough respiration for oxygen. Low supply of oxygen in for the water makes them suffocate and die apart from a few species that can survive in low oxygen concentration. The death of fish increased turbidity of the water since the fish remained in the water after being killed. The dead bodies of the fish started using more oxygen from the water and releasing their body compounds to the water.

Assignment 1B: A small observational experiment looking at treatment of turbidity


4. Write a brief description of your observations (provide images if possible). Investigate why water authorities need to monitor their use of aluminium salts very carefully, to ensure excess aluminium salts are not present in domestic water supplies. (Observations, images, and approximately 1 page of discussion) (5 marks)


The two glass containers that contained the turbid water were observed and it was found out that the container that had been added nothing remained muddy with the muddy solution that was brown. This means that it took the color of the soil that was mixed with the water (Downs, 1993). On the other hand, it was noticed that the substance in the container in which the aluminum salt and the lime water was added became cloudy over with a gelatinous precipitate. After this was formed, the color of the muddy water started becoming clear (Downs, 1993). The particles of soil were been immersed and placed at the bottom of the container. Eventually, the water in the container was clean and clear colorless with soil particles collected at the bottom of the container (Downs, 1993).

Figure 4: Aluminum sulfate water container

Discussion and Conclusion

Since the container without reagents remained with muddy solution after at the same period of time as the other container, It leads us to questions as to why the solution of the water becomes clear with the reagents (Downs, 1993). The lime used is for coagulation and flocculation. This makes the particles to clump together and thus settle at the bottom of the container (National Lime Association, 1995). In addition, impurities such as manganese and iron that might be found in water are easily removed by lime. They can also be used in the treatment of acidified water (National Lime Association, 1995). This is because lime is basic. Due to the basic nature of lime, an excessive use of it may make water to become basic so that using such water leads to negative effects like corroding of the teeth (Downs, 1993).

The reason as to why the government should make sure that aluminum salts are eliminated from water is that they react with phosphates which makes the phosphates lack in water for use by the aquatic organisms. Also, the uptake of such water by humans has serious negative effectives. Some of these effects include Dementia, listleness, loss of memory, severe trembling and the damage to the central nervous system.

Part C: Thermal pollution and its effect on water quality

5. Thermal power plants, whether powered by fossil fuel or nuclear energy, discharge large quantities of cooling water to a lake or river. Discuss the meaning of thermal pollution in the context of this assignment (i.e. dissolved gases and consequent effects). (Up to 1 page) (4 marks)

Thermal pollution can be described as the degradation of the quality of water by any kind of process which changes the ambient temperature of the water. From study conducted before, the use of coolant especially by industrial manufacturers and power plants decreases the supply of oxygen which in turn affects the ecosystem around and within the water bodies. The reduction of oxygen means a subsequent reduction in the dissolved oxygen of the water body.

Due to the decrease of oxygen in the water that is supplied with the coolant water, there are organisms in the water body and are adapted to certain quantities of oxygen. The reduction can therefore have a direct impact on the organisms that are not adaptive to tremendous decrease in temperature. In the case seen on date 6 of February 2002, it has been shown that about 100 fish died but without being contaminated. Because there are a number of industries around the lakes, there is a possibility that one of them released coolant water to the lake which made the abrupt change in the water temperature and PH. This caused the death of the fish that could not adapt to the new environment with lesser oxygen levels (Sitti, 2008).

The general decrease in dissolved oxygen reduces the quality of water in a given aquatic environment. The organisms including plants, animals need just enough oxygen for them to survive in such an environment. A fluctuation of the quantities leads to the need of the aquatic organisms' adaptation. This is not usually the case for most of them - they end up date. The death of the animals increases the turbidity of the water and also the PH (Sitti, 2008).