Measurement of Dissolved Oxygen

  

Measuring Dissolved  Oxygen in Water and Wastewater: A Laboratory Guide

Dissolved Oxygen (DO) is one of the most important indicators of water quality. Whether it’s drinking water, surface water, or wastewater, DO levels tell us a lot about biological activity, pollution load, and overall ecosystem health. In this blog, we’ll walk through the laboratory procedure for determining Dissolved Oxygen using the classic iodometric (Winkler) titration method—explained clearly and practically.

                                   

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Purpose of the Test

The aim of this procedure is to accurately determine the amount of dissolved oxygen present in water and wastewater samples using a standardized laboratory method.

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Scope of Application

This method is applicable in environmental and water-testing laboratories where routine analysis of:

• Drinking water
• Surface water
• Groundwater
• Wastewater

is required for Dissolved Oxygen measurement.

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Roles and Responsibilities

• Laboratory Chemist: Performs the DO analysis as per the procedure.
• Technical Manager: Reviews the analytical process and results.
• Quality Manager: Ensures implementation of the SOP and compliance with quality standards.

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Principle of the Method

The determination of dissolved oxygen is based on a redox reaction involving manganese salts and iodide ions:

1. Oxygen present in the sample oxidizes divalent manganese (Mn²⁺) to a higher oxidation state under alkaline conditions.
2. This forms a brown precipitate of manganese hydroxide.
3. Upon acidification, the manganese compound releases iodine from potassium iodide.
4. The amount of iodine liberated is directly proportional to the dissolved oxygen in the sample.
5. The released iodine is titrated with standardized sodium thiosulfate using starch as an indicator.

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Instruments and Equipment Required

• BOD bottle (300 mL capacity)
• Burette (25 mL)
• Conical flask
• Pipettes and standard laboratory glassware

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Reagents Used

1. Manganese Sulfate Solution

Prepared by dissolving manganese sulfate in distilled water and making up to one liter. The solution must be free from any oxidizing impurities.

2. Alkali–Iodide–Azide Reagent

A mixture of sodium hydroxide, potassium iodide, and sodium azide. Sodium azide suppresses nitrite interference during the analysis.

3. Concentrated Sulfuric Acid

Used to acidify the sample and dissolve the precipitate.

4. Starch Indicator

Freshly prepared starch solution gives a sharp blue endpoint during titration.

5. Sodium Thiosulfate Solution

• 0.1 N solution: Prepared and standardized using potassium dichromate.
• 0.025 N solution: Obtained by diluting the 0.1 N solution and re-standardizing.

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Step-by-Step Test Method

1. Collect the water or wastewater sample carefully in a 300 mL BOD bottle, ensuring no air bubbles are trapped.
2. Add 2 mL of manganese sulfate solution, followed by 2 mL of alkali–iodide–azide reagent. The pipette tip should remain below the liquid surface during addition.
3. Stopper the bottle immediately and mix by gently inverting it 3–4 times.
4. Allow the brown precipitate to settle completely.
5. Add 2 mL of concentrated sulfuric acid, restopper, and mix until the precipitate dissolves, resulting in a clear yellow-brown solution.
6. Transfer 203 mL of the treated sample into a conical flask.
7. Titrate with 0.025 N sodium thiosulfate until the solution becomes pale yellow.
8. Add a few drops of starch indicator; the solution turns blue.
9. Continue titration until the blue color just disappears—this is the endpoint.

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Why 203 mL Is Used for Titration

During reagent addition, 4 mL of reagents displace the original sample volume. To compensate:

• Actual sample required: 200 mL
• Adjusted volume taken for titration:

200x{300}/{(300 - 4)} = 203 mL

This correction ensures accurate DO calculation.

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Calculation of Dissolved Oxygen

• 1 mL of 0.025 N sodium thiosulfate corresponds to 0.2 mg of oxygen.
• Since the effective sample volume is 200 mL:

DO (mg/L= 0.2x1000=200

Therefore:

1 mL of 0.025 N sodium thiosulfate = 1 mg/L of Dissolved Oxygen

Multiply the burette reading by 1 to obtain DO in mg/L.

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Final Thoughts

The Winkler titration method remains a reliable and widely accepted technique for Dissolved Oxygen analysis when performed carefully. Proper sample collection, fresh reagents, and precise titration are key to obtaining accurate and reproducible results.

Understanding each step—not just following it—helps ensure data quality and regulatory compliance in water and wastewater testing.

Happy testing! 🧪💧