Introduction

Dissolved oxygen (DO) is defined as the amount of oxygen dissolved in a unit volume of

water. DO is crucial in maintaining the survival of aquatic life as well as water maintainence.

There are several factors that affect the solubility of oxygen in water such as temperature,

salinity and turbulence.

In brief according to Pratt, G. H., & Stabler, H. (1906) , Winkler titration method involves

several methods. The first method is the addition of manganese sulphate, MnSO4 potassium

iodide in sodium hydrate into the water sample. Sulphuric acid was added into the solution

containing precipitate to liberate iodine in water since the liberation of iodine is proportional

to the oxygen content in water sample. Lastly, titration of the sample with standardized

sodium thiosulphate. This method is considered accurate and precise in determining the

amount of oxygen dissolved through a series of oxidation-reduction reaction. The overall

reaction could be simplified in the equations below :

Mn2+ + 2OH- + ½ O2 → MnO2 (s) + H2O (1)

MnO2 (s) + 2I- + 4H+ → Mn2+ + I2 + 2H2O (2)

I2 + 2S2O3 → 2I- + S4O62- (3)

According to (Manahan, 2000), to find the concentration of dissolved oxygen in the water, a

back calculation from the volume of thiosulfate used will determine the value of dissolved

oxygen in water sample.

The objectives of the experiment are :

1. To evaluate the amount of dissolved oxygen in tap water and aqua water samples

individually by using Winkler dissolved oxygen method.

2. To understand the entire chemical reactions occurred in Winkler dissolved oxygen

method.

Materials :

Four Erlenmeyer flasks

25 mL burette

20.00 mL pipette

Pasteur pipettes

Three 1.00 mL pipettes

Manganese sulphate stock solution

Alkali-iodide-azide reagent

Concentrated H2SO4

Starch solutiom

Sodium thiosulphate titrant solution

Procedure :

Part A : Titration of Water Samples : Tap Water

1. Tap water was filled into 250 mL-300 mL sample bottle before the addition of 1 mL

MnSO4 solution and 1 mL of alkali-iodide-azide reagent respectively into the same

water sample.

2. The bottle was stoppered tightly to ensure no bubbles and mixed by inverting the

bottle rapidly in a few times. The bottle was inverted again as the precipitate settled to

half of the volume of bottle and allowed to settle before proceeding to the next step.

3. Next, the mixture was added with 1.0 mL of concentrated H2SO4 and re-stoppered.

The mixing process was repeated by inverting the bottle rapidly until a clear yellow

solution formed.

4. 200 mL of the sample was transferred into a conical flask and ready to be titrated with

the standardized thiosulphate solution. As the solution appeared to be pale straw

colour, starch indicator was added immediately and titrated to the clear end point.

5. The titration was repeated twice to obtain the average volume of standardized

thiosulphate solution.

Part B : Titration of Aqua Water.

1. Aqua water was filled into 250 mL-300 mL sample bottle before the addition of 1 mL

MnSO4 solution and 1 mL of alkali-iodide-azide reagent respectively into the same

water sample.

2. The bottle was stoppered tightly to ensure no bubbles and mixed by inverting the

bottle rapidly in a few times. The bottle was inverted again as the precipitate settled to

half of the volume of bottle and allowed to settle before proceeding to the next step.

3. Next, the mixture was added with 1.0 mL of concentrated H2SO4 and re-stoppered.

The mixing process was repeated by inverting the bottle rapidly until a clear yellow

solution formed.

4. 200 mL of the sample was transferred into a conical flask and ready to be titrated with

the standardized thiosulphate solution. As the solution appeared to be pale straw

colour, starch indicator was added immediately and titrated to the clear end point.

5. The titration was repeated twice to obtain the average volume of standardized

thiosulphate solution.

Results & Discussion.

In this experiment, two type of water samples were used which are tap water and aqua water.

The experiment began with the addition of MnSO4 and alkali iodide reagent. Brown

precipitate was formed slowly inside the water sample and the bottle was stoppered and

inverted rapidly twice to ensure all the reactions completed. During this process, alkaline

iodide reagent was added to create an alkaline surrounding which promotes the oxidation of

Mn2+ to Mn4+ if any of the dissolved oxygen presents forming brown hydrated oxide- brown

precipitate (Leo M.L. Nollet, 2014) . The precipitate settled at the bottom of the bottle. The

reaction was shown below :

Mn2+ + 2OH- + ½ O2 → MnO2 (s) + H2O (4)

Immediately, concentrated sulphuric acid was added into the water sample. This step will

acidify the mixture of water sample and dissolved the brown hydrated oxide. The acidic

environment will push the equilibrium to the left side and increase the liberation of iodine. At

this state, the Mn4+ was reduced to Mn2+ and liberated the iodine. As a result, intense yellow

brownish solution was formed. The intensity of the colour given by the liberation of iodine is

directly proportional to O2 content in the water sample.

MnO2 (s) + 2I- + 4H+ → Mn2+ + I2 + 2H2O (5)

To determine the oxygen content, the titration of water sample with thiosulfate was

conducted. As the thiosulfate titrant added into the water sample, the intensity of yellow

colour of solution was decreasing. The titration was paused when the solution change to pale

straw solution and the reading was recorded. Then, starch indicator was added into the

solution and the colour changed to blue due to the presence of iodine in the solution. The end

point was reached when the solution turned to a clear solution and the titration was stopped.

The blue colour faded because all of the iodine dissociates into iodide ion.

I2 + 2S2O32-

→ 2I- + S4O62- (6)

The volume of thiosulfate used was recorded. in Table 1 and Table 2 for tap water and aqua

water respectively.

Table 1 : Volume of thiosulfate used in titration of tap water.

Sample

no

Volume of

tap water

(mL)

Volume of thiosulfate

used in changing yellow

to pale straw (ml)

Volume of thiosulfate

used after addition of

starch (mL)

Total Volume (mL)

Initial Final Initial Final

1 200 0.00 3.40 3.40 10.90 10.90

2 200 11.00 15.30 15.30 22.50 11.50

3 200 22.50 27.30 27.30 34.50 12.00

Average volume (mL) 11.47

Table 2 : Volume of thiosulfate used in titration of aqua water.

Sample

no

Volume of

Aqua water

(mL)

Volume of thiosulphate

used in changing yellow

to pale straw (ml)

Volume of thiosulphate

used after addition of

starch (mL)

Total volume (mL)

Initial Final Initial Final

1 200 21.00 24.60 24.60 31.50 10.50

2 200 31.50 35.30 35.30 42.30 10.80

Average volume (mL) 10.65

Table 1 and Table 2 displayed the volume of thiosulfate used during Winkler titration

method with tap water and aqua water respectively. The titration method was conducted three

times for tap water to get the average volume of thiosulfate which was 11.47 mL. Meanwhile

the average volume of thiosulfate used in titration of aqua water was 10.65 mL. To calculate

the concentration of dissolved oxygen content in both water samples, the formula below was

used :

mg O2/L = L Sample (7)

Determining the dissolved oxygen content in tap water :

mg O2/L = (0.01147mL ) (0.0250 M )

0.2 L

= 11.47 mg O2/L

Determining the oxygen content in aqua water :

mg O2/L = (0.01065 mL) (0.0250 M )

0.2 L

= 10.65 mg O2/L

From the series of calculation above, the concentration of dissolved oxygen inside the tap

water and aqua water are 11.47 mg O2/L and 10.65 mg O2/L respectively.

Aqua water- water that contains aquatic life and aquatic plant on the water surface usually

contains a lot of dissolved oxygen because the aquatic plant fixes the carbon dioxide through

photosynthesis and liberate oxygen as the by product. The oxygen content in aqua water is

relatively higher than tap water. Theoretically, aqua water should have high amount of

dissolved oxygen compared to tap water. From the experiment, it was found that tap water

has higher amount of dissolved oxygen compared to the aqua water. This result might due to

some errors when conducting the experiment. Firstly, when filling in the tap water, the water

flows rapidly inside the bottle and this caused more oxygen to be dissolved in water. As a

precaution, the water flow should be moderately to minimize the intake of oxygen. Another

error identified is there was presence of bubble inside the bottle. The bubble presence might

reduce the dissolved oxygen in water and might cause error in titration. As a precaution, the

bottle should be stoppered and re-inverted rapidly to remove bubbles.

Conclusion

All in all, Winkler titration is one of the method used to determine the concentration of

dissolved oxygen in water samples due to its accuracy. Winkler’s method applies the concept

of oxidation and reduction, and using back calculation to determine the value of dissolved

oxygen. To find the concentration of dissolved oxygen in the water, a back calculation from

the volume of thiosulfate used will determine the value of dissolved oxygen in water sample

From the experiment, the value of dissolved oxygen in inside the tap water and aqua water

are 11.47 mg O2/L and 10.65 mg O2/L respectively. Overall, the Winkler titration method

was successfully applied in determining the concentration of dissolved oxygen in water

sample.

References

Leo M.L. Nollet, L. S. (2014). Handbook of Water Analysis. New York, Washington: CRC

PRESS.

Manahan, S. E. (2000). Environmental Science, Technology and Chemistry. New York,

Washington: CRC PRESS LLC.

Pratt, G., & Stabler, H. (1906). Dissolved Oxygen in Water: Some Preliminary Work on a

Colorimetric Process. Public Health Papers and Reports, 146-166.

Questions

1. What range of DO would you expect for natural water samples?

The expected range of DO for natural water samples is from 0 – 18 mg/L

2. Table 2 is given for a dry atmosphere. How would the values given in this table

change if you had an atmosphere with high humidity?

There will be decrease in solubility of dissolved oxygen as humidity of surrounding increases

3. Review the reagents used to fix the oxygen. Which reagents are critical ( must be

added in quantitative manner), and which are not critical?

The critical reagent used to fix the oxygen is thiosulfate because the addition of thiosulfate

will convert iodine into iodide ion. The amount of iodine conversion indicates how much

oxygen dissolved in the water sample. The non-critical reagent is alkaline-iodide azide

reagent because it only provide alkaline medium and prevent the conversion of iodide to

iodine back due to nitrite interference.

4. You titrate 200 mL of sample with 0.0250 M of thiosulfate and the titration takes 8.65

mL of thiosulfate to reach end point. What is the DO content of the sample?

mg O2/L = L Sample

= 0.02

= 8.65 mg O2/L