Determination of the Amount of Dissolved Oxygen in a Water Sample by Iodometry

Abstract

The objective of this experiment was to determine the amount of dissolved oxygen (DO) in a water sample using the iodometry-based Winkler's method. This method involves a series of chemical reactions to quantify the concentration of DO. The average concentration of DO in the tap water sample was found to be 7.3 mg/L.

Introduction

Dissolved oxygen is a critical parameter in assessing water quality and its suitability for aquatic life. In this experiment, we employed the Winkler's method, which is widely used for the precise determination of DO levels in water samples.

The method involves a series of chemical reactions that culminate in the titration of iodine, allowing us to quantify the amount of dissolved oxygen.

Materials and Methods

Apparatus:

• Volumetric pipette
• 3 conical flasks
• Burette
• Burette clamp
• Pasteur pipette
• Reagent bottle
• Conical flask stopper
• Retort stand
• White tile

Materials:

• 2 mL manganese sulfate solution
• 2 mL alkaline-iodine solution
• 0.025M sodium thiosulphate solution
• 2 mL concentrated sulfuric acid
• Starch solution

Experimental Procedure

1. Collect a representative water sample in a clean bottle, ensuring it is filled to overflowing and tightly sealed without introducing air.
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2. If the water is from a tap, let it run for at least 5 minutes before sampling.
3. Add 2 mL of manganese sulfate solution to the water sample.
4. Add 2 mL of alkaline-iodine solution.
5. Seal the bottle, mix thoroughly by inversion and rotation, allowing manganese hydroxide to settle.
6. Add 2 mL of concentrated sulfuric acid and mix until the precipitate dissolves completely.
7. Pipette 200 mL of the acidified sample into a 500 mL conical flask and titrate with 0.
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   025 M sodium thiosulphate until the solution turns pale yellow.

8. Add a few drops of starch indicator and continue the titration until the blue color disappears. Repeat the titration twice.

Results

Calculation

Moles of S₂O₃^2- used = 0.0001875 mol

Moles of S₂O₃^2- = 0.000046875 mol

Mass of O₂ = [0.000046875 × 2 × 15.999] = 1.4999 mg

1000 mL of tap water contains 7.5 mg O₂

Concentration of DO in water sample = 7.3 mg/L

Discussion

The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved oxygen (D.O.) is widely used in water quality studies and routine operation of water reclamation facilities. An excess of manganese (II) salt, iodide (I–) and hydroxide (OH–) ions is added to a water sample causing a white precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the water sample into a brown manganese precipitate. In the next step, a strong acid is added to acidify the solution. The brown precipitate then converts the iodide ion (I–) to iodine. The amount of dissolved oxygen is directly proportional to the titration of iodine with a thiosulfate solution. Iodemetry reacts directly, fast and quantitively with many organic and inorganic substances.

Because of its relatively low, pH independent redox potential and reversibility of the iodide/iodine reaction, iodometry can be used both to determine amount of reducing agents and of oxidizing agent. In my first titration, initial reading is 10.00 and final reading is 18.00. The titrants used is 8.00. In my second titration, initial reading is18.00 and final reading is 25.50. The titrants used is 7.50. In my third titration, my initial reading is25.50 and my final reading is 18.50. The titrant used is7.00. Adequate dissolved oxygen is necessary for good water quality.

Oxygen is a necessary element to all forms of life. Natural stream purification processes require adequate oxygen level in order to provide for aerobic life form. As dissolved oxygen levels in water drop below 5.0mg/L, aquatic life is put under stress. The lower the concentration, the greater the stress. Oxygen levels that remain below 1-2mg/L for a few hours can result in large fish kills. Total dissolved gas concentration in water should not exceed 110 percent. Concentration above this levels can be harmful to aquatic life. Conclusion: The concentration of DO in tap water is 7.5ppm.

Conclusion

The concentration of dissolved oxygen (DO) in the tap water sample was determined to be 7.3 mg/L using the iodometry-based Winkler's method. This value indicates that the water sample contains an adequate level of DO, which is essential for supporting aquatic life and maintaining good water quality.

Recommendations

Continuous monitoring of dissolved oxygen levels in natural water bodies and wastewater is recommended to ensure the health of aquatic ecosystems. Additionally, further studies and analyses should be conducted to assess the overall water quality and identify any potential pollutants that may affect DO levels.