Determination of Dissolved Oxygen in Wastewater Treatment

Molecular oxygen dissolved in water is termed dissolved oxygen (DO). The saturation level of dissolved oxygen is closely related to the partial pressure of oxygen in air, atmospheric pressure, and water temperature. Clean surface water typically has DO levels near saturation, while wastewater generally contains lower DO.

1. Method Selection

Common methods for determining dissolved oxygen include the ‌iodometric method‌ (and its modifications) and ‌dissolved oxygen meters‌. Oxidizing substances can release iodine from iodides, causing positive interference; reducing substances can reduce iodine to iodides, causing negative interference. Modified iodometric methods or electrode methods are recommended for most polluted surface waters and industrial wastewater.

2. Sample Collection and Preservation

Samples should be collected in ‌DO bottles‌ without introducing bubbles. Fill the bottle by gently pouring water along the inner wall until it overflows (1/3–1/2 of the bottle volume). After collection, fix the sample immediately on-site and store in the dark.

Iodometric Method

1. Principle

Dissolved oxygen oxidizes divalent manganese (Mn²⁺) to tetravalent manganese (Mn⁴⁺) in the presence of manganese sulfate and alkaline iodide, forming a tetravalent manganese hydroxide precipitate. Acidification dissolves the precipitate, releasing iodine via reaction with iodide. The iodine is titrated with sodium thiosulfate using starch as an indicator, allowing DO calculation.

2. Apparatus

250–300 mL DO bottles

. Reagents

‌(1) Manganese Sulfate Solution

Dissolve 240 g MnSO₄·4H₂O (or 182 g MnSO₄·H₂O) in water and dilute to 500 mL.₄-₂-₄-₂-

(2) Alkaline Iodide Solution

Dissolve 250 g NaOH in 200 mL water and 75 g KI in 100 mL water. After cooling the NaOH solution, mix the two solutions and dilute to 500 mL.

If precipitates form, let stand overnight, decant the supernatant, store in a brown bottle with a rubber stopper, and protect from light.

‌(3) 1:5 Sulfuric Acid Solution‌ Mix 1 part concentrated H₂SO₄ with 5 parts water.

‌(4) 1% Starch Solution

Mix 1 g soluble starch with a small amount of water to form a paste, then dilute with freshly boiled water to 100 mL. After cooling, add 0.4 g ZnCl₂ as a preservative.

(5) 0.02500 mol/L (1/6 K₂Cr₂O₇ Standard Solution)₂Cr₂O₇)

Dissolve 1.2258 g K₂Cr₂O₇ (dried at 105–110°C for 2 h and cooled) in water, transfer to a 1000 mL volumetric flask, and dilute to the mark.

‌(6) Sodium Thiosulfate Solution

Dissolve 6.2 g Na₂S₂O₃·5H₂O in boiled and cooled water, dilute to 1000 mL, and store in a brown bottle. ‌Standardize before use‌ as follows:₂-₂O₃-₂-

-

Add 100 mL water, 1 g KI, 10.00 mL 0.02500 mol/L K₂Cr₂O₇, and 5 mL 1:5 H₂SO₄ to a 250 mL iodine flask. Seal, mix, and let stand in the dark for 5 min. Titrate with Na₂S₂O₃ until pale yellow, add 1 mL starch solution, and continue titration until blue disappears. Record the volume used.

‌(7) Concentrated Sulfuric Acid

3. Procedure

‌(1) Fixation of Dissolved Oxygen (Fixed On-Site During Sampling)

① Insert a pipette below the liquid surface and add 1 mL MnSO₄ solution + 2 mL alkaline iodide solution.

② Seal the bottle and invert several times to mix. Let stand.

③ After the precipitate settles to half the bottle height, invert again. Wait until the precipitate settles completely.

‌(2) Iodine Release

Carefully open the bottle, insert a pipette below the liquid surface, add 2.0 mL concentrated H₂SO₄, reseal, and mix until the precipitate dissolves. Store in the dark for 5 min.

‌(3) Titration

Transfer 100.0 mL of the solution to a 250 mL conical flask. Titrate with Na₂S₂O₃ until pale yellow, add 1 mL starch solution, and continue titration until blue disappears. Record the volume used.

Sodium Azide Modification (Iodometric Method)

1. Overview

Nitrites in water interfere with the iodometric method. Sodium azide (NaN₃) is added to decompose nitrites and eliminate interference. For samples with 100–200 mg/L Fe³⁺, add 1 mL 40% KF solution to suppress interference (if no other redox substances are present).³⁺ .

2. Reagents

(1) Alkaline Iodide-Azide Solution‌: Dissolve 250 g NaOH in 200 mL water, 75 g KI in 100 mL water, and 5 g NaN₃ in 20 mL water. Mix the three solutions after cooling the NaOH solution, dilute to 500 mL, store in a brown bottle with a rubber stopper, and protect from light.

‌(2) 40% (m/v) Potassium Fluoride (KF)‌: Dissolve 40 g KF·2H₂O in water and dilute to 100 mL. Store in a polyethylene bottle.₂-

‌(3) Procedure‌: Same as the standard iodometric method, but replace alkaline iodide with ‌alkaline iodide-azide solution‌. For Fe³⁺ interference, add 1 mL 40% KF solution below the liquid surface first.³⁺.

Notes

(1)Sodium azide is ‌highly toxic and explosive‌. Do not acidify alkaline iodide-azide solution directly, as it releases toxic fumes.

(2)Adjust strongly acidic/alkaline samples to neutral pH with NaOH or H₂SO₄.

(3)‌Avoid introducing bubbles‌ during operations.

(4)Sodium thiosulfate concentration varies easily; standardize before each use