Mandatory Experiment 9.4
Estimation of dissolved oxygen (D.O.) by redox titration
Student material
Theory
Organic matter discharged into a watercourse serves as a food source for the bacteria
present there. The bacteria will break down this matter to produce less complex organic
substances, and, eventually, carbon dioxide and water. The bacteria will multiply, using
up the available dissolved oxygen as they do so. If the bacterial uptake of oxygen is faster
than the rate at which dissolved oxygen is replaced from the atmosphere and from the
action of photosynthesis, the water will become depleted in oxygen. In these anaerobic
conditions, bacteria will produce offensive products such as hydrogen sulphide and
ammonia. The depletion of dissolved oxygen may result in other undesirable effects such
as fish kills. The level of dissolved oxygen in a water sample is therefore an indicator of
the quality of the sample.
An iodine/thiosulfate titration can be used to measure the dissolved oxygen present in a
water sample. Because the dissolved oxygen does not directly react with the redox
reagent, an indirect procedure was developed by Winkler.
Under alkaline conditions manganese(II) sulfate produces a white precipitate of
manganese(II) hydroxide:
1. Mn2+(aq) + 2OH-(aq)  Mn(OH)2(s)
This reacts with the dissolved oxygen to produce a brown precipitate.
2. 2Mn(OH)2(s) + O2 (aq)  2MnO(OH)2(s)
Addition of concentrated H2SO4 enables the Mn(IV) compound to release free iodine
from KI.
3. MnO(OH)2(s) + 4H+(aq) + 2I- (aq)  Mn2+(aq) + I2(aq) + 3H2O(l)
The free iodine is then titrated with standard sodium thiosulfate in the usual way.
4. I2(aq) + 2S2O32-(aq)  2I-(aq) + S4O62- (aq)
Overall:
1 mole O2  2 moles MnO(OH)2  4 moles S2O321
Chemicals and Apparatus
Manganese(II) sulfate solution
n
Alkaline potassium iodide solution
Concentrated sulfuric acid
0.005 M sodium thiosulfate solution
Starch indicator solution
Deionised (or distilled) water
Water sample
Burette (50 cm3)
Pipette (25 cm3)
Pipette filler
Droppers
Filter funnel
Beaker (250 cm3)
Conical flask (250 cm3)
Two reagent bottles (250 cm3) with stoppers
Basin
Graduated cylinder (10 cm3)
Retort stand
Boss-head
Clamp
Wash bottle
White tile
White card
Safety glasses
Procedure
NB: Wear your safety glasses.
1. Rinse a 250 cm3 reagent bottle with deionised water, shaking vigorously to wet
the inside and so avoid trapped air bubbles.
2. Completely fill the bottle under water with the sample, making sure that there are
no trapped air bubbles.
3. Using a dropper placed well below the surface of the water, add 1 cm3
(approximately) each of manganese(II) sulfate solution and of alkaline potassium
iodide solution to the bottle.
4. Stopper the bottle so that no air is trapped - a few cm3 of solution will overflow at
this point.
2
5. Invert the bottle repeatedly for about a minute, and then allow the brown
precipitate to settle out.
6. In order to dissolve the precipitate, carefully add 1 cm3 of concentrated sulfuric
acid to the bottle, by running the acid down the side of the bottle.
7. Restopper the bottle - avoiding trapping any air. Invert repeatedly to redissolve
the precipitate. If all the precipitate has not dissolved at this point add 0.5 cm3 of
acid, invert repeatedly, and allow to stand for one minute. Continue repeating the
process of inverting repeatedly followed by addition of acid until all of the
precipitate has dissolved. Iodine should now be released resulting in a golden
brown solution.
8. Wash the pipette, burette and conical flask with deionised water. Rinse the pipette
with the iodine solution and the burette with the sodium thiosulfate solution.
9. The free iodine can now be estimated by means of the thiosulfate titration.
Measure out 50 cm3 samples into clean conical flasks and titrate each of these
with 0.005 M thiosulfate solution in the usual way.
3
10. Add about 1 cm3 starch indicator as the end point approaches (when the solution
becomes pale yellow in colour). Titrate until the blue colour has just disappeared.
11. Record the results in the usual way taking the average of two accurate titration
results, i.e. two titres within 0.1 cm3 of each other.
12. Calculate the results in (i) moles of oxygen per litre (ii) grams of oxygen per litre
(iii) dissolved oxygen in p.p.m.
Table of results
Copy this table into your practical report book.
Rough titre
Second titre
Third titre
Average of accurate titres
Volume of water sample
Molarity of thiosulfate solution
=
=
=
=
=
=
Questions relating to the experiment
1. Why is the reagent MnSO4 used?
2. Why is concentrated H2SO4 used?
3. Why must the bottles be shaken vigorously in step 5 of the procedure?
4. Why are the bottles completely filled?
5. If the white precipitate remains on addition of manganese(II) sulfate solution and
alkaline potassium iodide solution, what does this indicate about the water
sample?
6. State and explain what the letters B.O.D. mean.
7. Why are the bottles used during B.O.D. measurements stored in the dark?
8. In preparing the pipette for the titration it was rinsed with deionised water and
then the water was removed by rinsing the pipette with the iodine solution to be
transferred. Explain why it is important to remove the water. Why was the conical
flask not rinsed with the iodine solution?
4
Teacher material

The mixture of concentrated solutions of manganese(II) sulfate and alkaline
potassium iodide used in this experiment is called Winkler’s reagent.

The exact composition of the brown precipitate (referred to above as MnO(OH)2)
that is produced when Mn(OH)2 reacts with dissolved oxygen is not clear, and
may vary. Other formulas given for this precipitate include Mn(OH)3, Mn2O3 and
MnO2.H2O. This does not matter, however, as the brown precipitate reverts to
Mn2+ in reaction 3, and equations for reactions 1-3 involving any of the four
formulas lead to the same conclusion, i.e. that one mole of O2 produces two moles
of I2.

The solubility of oxygen in water decreases with increasing temperature. Thus,
water that is saturated with oxygen contains 10.8 p.p.m. D.O.at 12 0C, 10.0 p.p.m.
D.O.at 16 0C, and 9.2 p.p.m. D.O.at 20 0C.

The five day Biological Oxygen Demand (B.O.D.) of a water sample is the
amount of dissolved oxygen taken up by bacteria in degrading oxidisable matter,
measured after 5 days incubation in the dark at 20 0C. The B.O.D. is simply the
amount by which the D.O. level has dropped during the incubation period. It can
be measured by carrying out a D.O. titration on samples of the water under test at
the start and at the finish of the incubation period.

Starch solution has a short shelf life, but if 0.5 g of salicylic acid is added to 50
cm3 of the solution immediately after it is made up, the solution will last for a few
months.

A fuller description of titration procedure is to be found in the Student Material
relating to Mandatory Experiment 4.2.
Extension Work
1. The students could compare the dissolved oxygen levels of water samples from
different sources, e.g. tap water, river water, sea water, pond water, bottled water,
water that has been previously boiled etc. Note however that if tap water is used, a
titration figure higher than the true value will be obtained. This is because
chlorine in the tap water liberates iodine from potassium iodide.
2. The B.O.D. of a particular water sample could be determined.
Preparation of Reagents
Manganese(II) sulfate solution: Dissolve 120 g MnSO4.4H2O in 250 cm3 of deionised
water.
5
Alkaline potassium iodide solution: Dissolve with warming 125 g NaOH and 37.5 g KI
in 250 cm3 of deionised water.
0.005 M sodium thiosulfate standard solution: Dilute 50 cm3 0.1 M sodium
thiosulfate standard solution (see below) with deionised water to 1 litre using a
volumetric flask
0.1 M sodium thiosulfate standard solution :
Prepare anhydrous sodium thiosulfate by refluxing about 30 g sodium thiosulfate
pentahydrate crystals in methanol for about 30 minutes, and then filter. Dry in an oven at
70 oC. Dissolve 15.8 g anhydrous sodium thiosulfate in 500 cm3 deionised water in a
beaker and make the solution up to 1 litre using a volumetric flask. To increase the
stability of the solution, add 0.1 g of sodium carbonate.
Starch indicator solution: Pour with stirring a paste containing 1 g starch and a little
cold water into 50 cm3 of boiling water. Boil for two minutes, and allow to cool.
Quantities needed per working group
500 cm3 of the water sample
100 cm3 of sodium thiosulfate solution
2 cm3 of alkaline potassium iodide solution
2 cm3 of manganese(II) sulfate solution
2 cm3 of concentrated sulfuric acid
10 cm3 of starch indicator solution
Safety considerations




The usual precautions when handling glassware for a titration should be observed.
Pipette fillers should be used.
Safety glasses and gloves must be worn.
Eye wash solution should be available, in case of accidental splashing.
Chemical hazard notes
n
Manganese(II) sulfate is harmful if swallowed, inhaled or absorbed through
skin. Skin contact with it should be avoided. Causes eye and skin irritation.
Inhalation of dust may cause manganese poisoning.
Sodium thiosulfate is an irritant to eyes.
Concentrated sulfuric acid is corrosive and causes severe burns to eyes and
skin.
6
Sodium hydroxide is corrosive and can cause severe burns to eyes and
skin. Always wear eye protection.
Disposal of wastes
Dilute with excess water and flush to foul water drain.
Specimen Results
Rough titre
Second titre
Third titre
Average of accurate titres
Volume of water sample
Molarity of thiosulfate solution
= 13.3 cm3
= 13.0 cm3
= 13.0 cm3
= 13.0 cm3
= 50.0 cm3
= 0.005 M
Specimen Calculations
(a) First principles method
Volume of thiosulfate solution used
Moles of thiosulfate used
Ratio of dissolved oxygen to thiosulfate
Moles of dissolved oxygen in water sample
Volume of water sample
Moles/cm3 of dissolved oxygen
Moles/litre of dissolved oxygen
Concentration of dissolved oxygen
= 13.0 cm3
= 13.0 x 0.005 / 1000
= 0.000065 moles
= 1: 4
= 0.000065 / 4
= 0.00001625 moles
= 50.0 cm3
= 0.00001625 / 50.0
= 0.000000325
= 0.000325
= 0.000325 M
= 0.00325 x 32 g/l
= 0.0104 g/l
= 10.4 p.p.m.
(b) Formula method
VA x MA x nB = VB x MB x nA
50.0 x MA x 4 = 13.0 x 0.005 x 1
MA = 13.0 x 0.005 x 1 / (50.0 x 4)
7
= 0.000325 M
Concentration of dissolved oxygen
= 0.000325 M
= 0.00325 x 32 g/l
= 0.0104 g/l
= 10.4 p.p.m.
Suggested Solutions to Student Questions:
1. Why is the reagent MnSO4 used?
Dissolved oxygen will not react completely in its absence. Iodide is
susceptible to air oxidation in an acidic medium:
4I- + 4H+ + O2 → 2I2 + 2H2O
but this is a relatively slow reaction. The use of MnSO4
results in the formation of Mn(OH)2, which reacts completely with dissolved
oxygen.
2. Why is concentrated H2SO4 used?
To enable the Mn(IV) species to release the free iodine needed for the redox
reaction.
3. Why must the bottles be shaken vigorously in step 5 of the procedure?
To help the dissolved oxygen to react.
4. Why are the bottles completely filled?
To prevent additional oxygen from the air dissolving in the water.
5. If the white precipitate remains on addition of manganese(II) sulfate solution
and alkaline potassium iodide solution, what does this indicate about the water
sample?
There is no dissolved oxygen present. The sample appears to be heavily
polluted.
6. State and explain what the letters B.O.D. mean.
Biochemical Oxygen Demand. The five day Biochemical Oxygen Demand of
a water sample is the amount of dissolved oxygen taken up by bacteria in
degrading oxidisable matter, measured after 5 days incubation in the dark at
20 0C.
8
7. Why are the bottles used during B.O.D. measurements stored in the dark?
To prevent oxygen production by photosynthesis.
8. In preparing the pipette for the titration it was rinsed with deionised water and
then the water was removed by rinsing the pipette with the iodine solution to
be transferred. Explain why it is important to remove the water. Why was the
conical flask not rinsed with the iodine solution?
It is important to remove the water because it would dilute the iodine solution.
The conical flask was not rinsed with the iodine solution because if it was,
then traces of the solution would remain, and there would not be a precisely
known amount of the iodine solution in the flask.
9