• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

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

Extracts from this document...


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.

...read more.



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.

...read more.


-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.

...read more.

This student written piece of work is one of many that can be found in our AS and A Level Probability & Statistics section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Probability & Statistics essays

  1. How Can Samples Describe Populations?

    The ensuing media focus was an embarrassment to Smith's cause. Whilst this shows that it is possible to identify such (unethical) practise, what this really illustrates is the importance of the information quality in terms of integrity and the connotations of biased sampling.

  2. Identifying Relationships -Introduction to Statistical Inference.

    or the measured variable of interest is approximately normally distributed * the more closely the sample represents the population from which it was drawn, the more reliable conclusions about the population ( based on the sample ) * some inaccuracy is inevitable because it relies on substituting the sample mean

  1. I am going to design and then carry out an experiment to test people's ...

    12.5 20 11.5 7 7 8 17 17.5 9 7 8 7 10 9 12 7 9 23.5 30 30 21 7 10 16 12 20 22.5 7 11 11 12 8 21.5 7 12 21.5 14 22 21 7 13 11 12 15.5 9 7 14 19.5 19 21

  2. Data Analysis of American House Price

    In township 5, all the houses (100%) have a pool; followed by township 4 with a 94,4% of the houses. On the other extreme there is township 1 with only 13,33% (2 out of 15 houses) have a pool, followed by township 2 with 22,22% (6 out of 27 houses).

  1. This is an investigation to identify some compounds containing oxygen AimThis experiment is to ...

    add a few drops of sulphuric acid. 3 Add 2-3cm3 of the ammoniacal silver nitrate to each test tube 4 Wait until one of the test tubes forms a silver "mirror" on the edge of the tube. 5 label the bottle which the sample came from 'Propanal' * Identifying the

  2. Differences in wealth and life expectancy of the countries of the world

    23,200 72.51 77.60 74.99 West Bank 800 71.14 74.72 72.88 Mean 8,621 68.45 72.46 69.30 Data Tables Africa Countries GDP - per capita ($) Male Life Expectancy Female Life Expectancy Population Life Expectancy (years) Burundi 600 42.73 44.00 43.36 Cape Verde 1,400 66.83 73.54 70.14 Cote d'Ivoire 1,400 40.27 44.76

  1. Investigating the Relationship Between the Amount of Money a Football Club Receives and its ...

    Another method I will use is to split the sampled teams into groups according to how much money they receive and find the standard deviation of the league position, goal difference and stadium capacity within each group. This is useful as one would expect that clubs that receive similar amounts

  2. The aim of this investigation was to look at the reliability and validity of ...

    The Ancient Greeks for example believed that there were four types of personality: Sanguine, Choleric, Melancholic and Phlegmatic. Since then there have been numerous theorists attempting to analyse the personality. William Sheldon (1940) classified people on the basis of body build and put them into three categories; Endomorphs who would

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work