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.
-----------------------------------------------------------------------------------------------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.
-----------------------------------------------------------------------------------------------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 cm3 pipette,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).
F6/7 Chem Prac/Va07_DO/p.1
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
3
Final burette reading/cm
Initial burette reading/cm3
Volume of iron(II) sulphate(VI) used/cm3
Mean titre =
Sample B:
1
2
3
Final burette reading/cm
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 report before 8:40 a.m. on the next day.
F6/7 Chem Prac/Va07_DO/p.2