F.6/7 Chemistry Practical: Dissolved Oxygen in Water Samples

A full report is required for this experiment.

Objective: To determine DO in two water samples by Winkler’s method.

Introduction:

Water pollution means the deterioration in water quality; it may be caused by livestock waste, oil spillage, residues of pesticides, detergents in sewage and industrial effluents. To measure the quality of water there are many parameters, for example, pH value, turbidity, temperature, frothiness, content of ammonia, content of phosphate and concentration of heavy metals such as cadmium, chromium, copper, lead, mercury, nickel and zinc.

In this practical, Winkler’s method is employed to determine the value of DO in two water samples. Sample A is obtained by bubbling air into de-ionized water for 3 days. Sample B is obtained by putting mud into tap-water for 10 days.

In this practical, Winkler’s method is employed to determine the concentration of dissolved oxygen in water samples.Sodium hydroxide solution is added to the water sample. In such an alkaline solution, dissolved oxygen will oxidize manganese(II) to the trivalent state. Potassium iodide is then added and iodide is oxidized to iodine by manganese(III) ion. The analysis is completed by titrating the iodine produced [This is also called iodometry]with sodium thiosulphate.

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Theory: Give principle/reason to explain how do you determine 'dissolved oxygen' in a sample of water in this experiment. The following questions should be useful to you.

  1. What is ‘Dissolved Oxygen’?
  2. If the maximum concentration of dissolved oxygen of water at 16oC is 10 mg dm-3, calculate the concentration of oxygen of a water sample of DO=6 at the same temperature.
  3. What is the importance of the parameter ‘DO’?
  4. Give the relation between pollution by living stocks and the value of DO.
  5. What will happen if DO value of water is very low?
  6. Give an equation for the reaction between oxygen and manganese(II) ion in an alkaline solution. Calculate the e.m.f. for this reaction.
  7. Give an equation for the reaction between manganese(III) ion and potassium iodide in an alkaline solution. Calculate the e.m.f. for this reaction.
  8. Give an equation for the reaction between iodine and thiosulphate in acidic solution. Calculate the e.m.f. for this reaction.
  9. Work out a formula (or an equation) to relate the mean titre you obtained in this practical and the value of DO of the water samples.

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Chemicals:

2 cm3 of manganese(II) sulphate solution -- prepared by dissolving 48 g of MnSO4.4H2O
in water to give 100 cm3 solution; 6 cm3 of concentrate sulphuric (VI) acid; 150 cm3 of standard sodium thiosulphate solution (about 0.013 M); fresh-prepared starch solution; two samples of water (each 250 cm3)
2 cm3 of alkaline potassium iodide solution prepared by dissolving 15 g of KI in about 25 cm3 of water, adding 66 cm3 of 50% NaOH, and diluting to 100 cm3 (6 cc)

Additional Materials:

magnetic stirrer, 1 cm3pipette,0-100oC thermometer.

Safety precaution

Concentrated sulphuric(VI) acid and alkaline potassium iodide solution are corrosive. All other chemicals are considered to be harmful.

Procedure

  1. Measure temperature of the water samples.
  2. Use a 250 cm3 volumetric flask to collect a water sample. Fill the flask completely with water without trapping any air bubbles.
  3. Add 1 cm3 of manganese(II) sulphate solution to the sample using a pipette. Discharge the solution well below the surface (some overflow will occur).
  4. Similarly introduce 1 cm3 of alkaline potassium iodide solution. Be sure that no air becomes entrapped. Invert the bottle to distribute the precipitate uniformly. (Hazard Warning: Care should be taken to avoid exposure to any overflow, as the solution is quite alkaline.)
  5. When the precipitate has settled at least 3 cm below the stopper, introduce 1 cm3 of concentrated sulphuric (VI) acid well below the surface. Replace the stopper and carefully mix until the precipitate disappears. A magnetic stirrer is helpful here.
  6. Allow the mixture to stand for 5 minutes and then withdraw 100 cm3 of the acidified sample into a 250 cm3 conical flask.
  7. Titrate with the standard sodium thiosulphate solution until the iodine colour becomes faint. Then add 1 cm3 of starch solution, and continue adding the thiosulphate solution until the blue colour disappears. Repeat the titration once.
  8. Record the volume of thiosulphate solution used and calculate the dissolved oxygen content in the sample in mg dm-3.
  9. Repeat the experiment for another water sample.

Remark

If the water sample has a low DO value, it is recommended to withdraw 200 cm3 of the acidified sample into a 500 cm3 flask for the titration described in step 5.

Data and Results

Temperature of sample 1 = ______, of sample 2 = ______

Concentration of standard sodium thiosulphate solution = ______

Colour change of the indicator =______

Titration Data

Sample A: / 1 / 2
Final burette reading/cm3
Initial burette reading/cm3
Volume of iron(II) sulphate(VI) used/cm3

Mean titre =

Sample B: / 1 / 2
Final burette reading/cm3
Initial burette reading/cm3
Volume of iron(II) sulphate(VI) used/cm3

Mean titre =

Calculation

Calculate concentration of dissolved oxygen in your samples.

Ask your teacher to get the maximum concentration of dissolved oxygen in water so that you can calculate DO for your water samples.

Discussion (Write this section with reference to the following questions)

  1. Give error sources and estimate percentage error in 'dissolved oxygen'.
  2. If Winkler's method is employed to determine DO of tap water, what kind of interfere is present?
  3. The value of DO cannot be determined by the direct titration of oxygen with sodium thiosulphate. Explain.
  4. Air bubbles should bot be entrapped (step 1 and 3). Explain.
  5. Reagents in step 2 should be discharged in the bulb of the flask (i.e. well below the surface) instead of on the surface of the liquid. Explain.
  6. What are observed in step 3 and 4. Explain these observations.
  7. How do you determine 'BOD' of a water sample?

Hand in your FULL reportbefore 8:40 a.m. on the next day.

Suggested answers for theory section

What is 'dissolved oxygen'?

Dissolved oxygen (DO) is used as an indicator of oxygen content in water. It is measured in mg of oxygen per dm3 of water or percentage saturation of dissolved oxygen. The maximum concentration of dissolved oxygen in water at 16oC is 10 mg dm-3. If a water sample contains 6 mg dm-3 of dissolved oxygen at 16oC, the percentage saturation of DO is then:

The sample is said to be 60% saturated with oxygen.

Oxygen dissolved in water is necessary for aquatic life. Fishes normally require 4-6 mg dm-3 of oxygen for survival. In general the lower the content of dissolved oxygen in a water sample, the more serious is the degree of pollution as this indicates that a large amount of decaying materials is present which consumes dissolved oxygen. If dissolved oxygen content is very low, anaerobic conditions may be found. A minimal DO is also needed for reducing odour problems due to anaerobic degradation of organic matter. Depletion of oxygen may be caused by biological degradation of waste matter. The following table lists levels of DO for various uses:

Water use / Dissolved oxygen
Minimum allowable (mg dm-3)
Public water supply / 4.0
Water contact recreation / 4.0-5.0
Fish propagation and wildlife / 4.0-6.0 (depending on fish species)
Industrial water supply / 3.0-5.0 (depending on use)
Agricultural water supply / 3.0-5.0 (based on application)
Shellfish harvesting / 4.0-6.0 (depending on local conditions)

Success of the method is critically dependent upon the manner in which the sample manipulated. At all stages, every method must be made to assure that oxygen is neither introduced to nor lost from the sample. Furthermore, the sample must be free of any solutes that will oxidize or reduce iodine.

If the analysis for DO is repeated with the water sample that has beenstored in the dark for five days, then the difference between the twomeasurements gives the five-day biochemical oxygen demand, measured inmg dm-3.

F6/7 Chem Prac/Va07_DO/p.1