Estimation of Dissolved Oxygen (D.O.) by Redox Titration

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:

2+ - 1. Mn (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.

+ - 2+ 3. MnO(OH)2(s) + 4H (aq) + 2I (aq)  Mn (aq) + I2(aq) + 3H2O(l)

The free iodine is then titrated with standard sodium thiosulfate in the usual way.

2- - 2- 4. I2(aq) + 2S2O3 (aq)  2I (aq) + S4O6 (aq)

Overall:

2- 1 mole O2  2 moles MnO(OH)2  4 moles S2O3

1 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

3 Manganese(II) sulfate solution: Dissolve 120 g MnSO4.4H2O in 250 cm 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

6 skin.

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 = 13.3 cm3 Second titre = 13.0 cm3 Third titre = 13.0 cm3 Average of accurate titres = 13.0 cm3 Volume of water sample = 50.0 cm3 Molarity of thiosulfate solution = 0.005 M

Specimen Calculations

(a) First principles method

Volume of thiosulfate solution used = 13.0 cm3 Moles of thiosulfate used = 13.0 x 0.005 / 1000 = 0.000065 moles Ratio of dissolved oxygen to thiosulfate = 1: 4

Moles of dissolved oxygen in water sample = 0.000065 / 4 = 0.00001625 moles Volume of water sample = 50.0 cm3 Moles/cm3 of dissolved oxygen = 0.00001625 / 50.0 = 0.000000325 Moles/litre of dissolved oxygen = 0.000325 Concentration of dissolved oxygen = 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

7 MA = 13.0 x 0.005 x 1 / (50.0 x 4) = 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

8 degrading oxidisable matter, measured after 5 days incubation in the dark at 20 0C.

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.