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The aim of this experiment is to measure the BOD and DO of water.

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Experiment #14

December 17/2002

DO and BOD Winkler titration


The aim of this experiment is to measure the BOD and DO of water.


The quality of water depends on several factors including oxygen-demanding wastes, disease-causing pathogens, microorganisms affecting health, plant nutrients, suspended solids and dissolved minerals. Other pollutants may be excess acidity due to acid rain, thermal pollution and substances such as benzene, chromium and mercury that are all toxic to aquatic life.

As humans and land animals obtain oxygen from respiration from the air, for plants and animals to survive in aquatic systems, water must contain a minimum amount of dissolved oxygen. The dissolved oxygen (DO) content of a body of water is an important indicator of its quality. At 200C, DO content of 8 to 9 ppm O2 at sea level is considered to be water of good quality.

The Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen consumed by the biodegradable organic wastes and ammonia in a given amount of water over a time period, normally 5 days at 200C. The greater the oxygen demanding wastes, the higher the BOD.  

The BOD of a sample of water can be determined by the Winkler method. The sample of the water is saturated with oxygen so the initial concentration of dissolved oxygen is known.

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The chemical reactions involved in the Winkler Titration method are:

According to this method, an excess of manganese salt is added to the sample water. Since the manganese (II) ions from the salt – in this case MnSO4- are oxidised manganese (IV) oxide according to the first reaction, we expect the color of the solution to change. That is because manganese is a transition metal that its color changes according to the oxidation state that it is found. Thus, the initial color of the solution is pale yellow and transparent, unlike the final color that is brown. Indeed, the color of the solution becomes much darker and we may observe a precipitate forming, which is the solid MnO2. In the solution NaOH is added because under alkaline conditions the manganese (II) ions will oxidise to manganese (IV) oxide. Potassium iodide is then added which is oxidised by the manganese (IV) oxide in acidic solution to form iodine, making the solution dark green. So we add H2SO4 to create the acidic condition needed. The iodine released is then titrated with standard sodium thiosulfate solution according to the third reaction. The solution becomes blue when titrated and at the presence of starch – that acts as an indicator- the solution decolorizes. Sodium and potassium in this case are just spectator ions not taking part in the chemical reaction.

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-4 g of O2

In 1000ml of the sample there are     x              

x =  0.4g of O2


In 3.5ml of the sample there are 8 x 10-4g of O2

In 1000ml of the sample there are   x        

x = 0.23g of O2


DOinitial – DOfinal= 0.4 – 0.23 = 0.17g = 170mg/ppm of O2


The BOD value for the water sample under investigation was 170ppm, indicating that is comes from untreated sewage (which can range from 100 to 400ppm). Because of the high value, it can be deduced that the sample contains very high oxygen demanding wastes. The initial DO value is higher than that of the final. This decrease is because the biodegradable organic wastes consume the existing oxygen in the 5-days incubating period.

Experimental errors may have arisen during the procedure, involving the titration. The extreme value of 11 was excluded, however, had it been a correct measurement, the resulting BOD would have differed from the one used in the calculations.

If there is an increase in the temperature, the DO level will increase as well. That is in accordance to the collision theory. The rate of the chemical reactions will increase because of the higher kinetic energy that they will have, because of the higher temperature. The temperature was more or less stable (room temperature). However, it might have decreased a few oC during the 5-day period at nights. That would not affect though the DO levels.

Thus, since the BOD value was found 170mg/l, the sample is of unacceptable purity, most probably coming from untreated sewage.

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