Experiment 3: Determination of Dissolved Oxygen in Water by Winkler’s Method

• BOD bottle (300 mL capacity)

• Burette (50 mL)

• Pipette (10 mL and 1 mL)

• Conical flask (100 or 250 mL)

• Measuring cylinder

• Volumetric flask

• Funnel

• Glass rod

• White tile (for color observation)

• Starch indicator dropper bottle

• Wash bottle with distilled water

• Titration stand and clamp

• Manganese(II) sulfate solution (MnSO₄)

• Alkaline iodide-azide reagent (KOH + KI + NaN₃)

• Concentrated sulfuric acid (H₂SO₄)

• Standard sodium thiosulphate solution (Na₂S₂O₃)

• Starch indicator solution

• Distilled water

Background

Dissolved oxygen (DO) refers to the level of free, non-compound oxygen present in water or other liquids. It is a critical parameter for assessing water quality because it is essential for the survival of aerobic aquatic organisms, such as fish and invertebrates. The decomposition of organic matter by aerobic bacteria also consumes DO.

Definitions and Principles

Dissolved Oxygen (DO): The amount of gaseous oxygen (O₂) dissolved in water. Its concentration is influenced by temperature (solubility decreases as temperature increases), atmospheric pressure, and salinity.

Significance: A low DO level is often an indicator of pollution from organic matter (e.g., sewage), which triggers high bacterial consumption of oxygen. Healthy water bodies generally have high DO levels (> 5 mg/L).

Principle of Winkler’s Method: Winkler’s method is a titrimetric technique used to determine the concentration of dissolved oxygen (DO) in water. It involves a series of redox reactions where the dissolved oxygen is chemically fixed, converted into iodine, and then titrated using sodium thiosulphate.

Stepwise Reactions and Explanation:

1. Initial Reaction (Oxygen Fixation):

Water is treated with manganese(II) sulfate (MnSO₄) and an alkaline potassium iodide (KOH + KI) solution. In the presence of dissolved oxygen, manganese(II) hydroxide [Mn(OH)₂] is formed, which gets oxidized to manganese(IV) oxide-hydroxide [MnO(OH)₂], forming a brown precipitate.

2. Formation of Brown Precipitate:

The Mn(OH)₂ is oxidized in the presence of dissolved oxygen to form a brown precipitate of MnO(OH)₂ or MnO₂·H₂O.

3. Liberation of Iodine:

Upon acidification (usually using concentrated H₂SO₄), the brown precipitate dissolves, and the manganese(IV) oxidizes iodide (I⁻) into iodine (I₂).

4. Titration with Sodium Thiosulphate:

The liberated iodine (I₂) is titrated with a standard sodium thiosulphate (Na₂S₂O₃) solution. Starch is used as an indicator that forms a blue-black complex with iodine, which disappears once all iodine is reduced.

5. Calculation of Dissolved Oxygen (DO):

(Note: The factor “8” is based on the stoichiometry and molar mass of oxygen.)

1. A 300 ml glass-stoppered BOD (Biochemical Oxygen Demand) bottle was carefully filled to the brim with the water sample, ensuring no air bubbles were trapped inside.

2. 1 ml of manganese sulfate solution was added, followed immediately by 1 ml of alkali-iodide-azide reagent. The pipette tips were placed just below the water surface to avoid introducing air.

3. The bottle was carefully stoppered and inverted several times to mix the contents thoroughly. A brown precipitate was allowed to form and settle.

4. 1 ml of concentrated sulfuric acid was added by allowing the acid to run down the neck of the bottle.

5. The bottle was re-stoppered and mixed gently by inversion until the precipitate was completely dissolved and a uniform brownish-yellow color was obtained.

6. 50 ml of this solution was measured and transferred to a clean conical flask.

7. The solution was titrated with a standard N/40 sodium thiosulphate solution from a burette until the solution color faded to a pale yellow.

8. 1-2 ml of starch indicator was added, which turned the solution a blue-black color.

9. The titration was continued dropwise with constant swirling until the blue color disappeared for the first time. This was the endpoint.

10. The final burette reading was recorded, and the procedure was repeated to get concordant readings.

Observation Table:

• Concordant volume of Na₂S₂O₃ (hypo) used: 0.1 ml

• Volume of water sample taken for titration: 50 ml

• Normality of standard Na₂S₂O₃ solution (N): 1/40 N (or 0.025 N)

• Normality Factor (f): 1.009

• Equivalent weight of Oxygen: 8

Calculation:

Result: The concentration of dissolved oxygen in the given water sample was found to be 0.40 mg/L.

Discussion: The measured Dissolved Oxygen (DO) level of 0.40 mg/L is a critical indicator of severe water quality degradation. This value is extremely low and points to a highly polluted, unhealthy aquatic environment. 📉

For a healthy aquatic ecosystem capable of supporting fish and other organisms, a DO level above 5 mg/L is necessary. Furthermore, the guideline for drinking water in Nepal specifies a minimum of 6 mg/L.

Our result of 0.40 mg/L falls drastically below these essential thresholds. This indicates the water body is under extreme stress and is hypoxic (experiencing very low oxygen) or nearly anoxic (devoid of oxygen). A DO level this low is lethal to most aquatic species and strongly suggests significant organic pollution, likely from sources like raw sewage or heavy agricultural runoff. The high rate of oxygen consumption by decomposer bacteria has depleted the available oxygen. This water is entirely unfit for drinking and cannot sustain a healthy aquatic ecosystem.

Health Impact Caused by Low DO (Dissolved Oxygen) Level

• Affects oxygen transport

• Metabolism disruption

• Increased toxicity

• Digestive and cardiovascular issues

• Long-term health risks

The experiment successfully determined the dissolved oxygen concentration to be 0.40 mg/L using the Winkler method. This critically low value is far below the minimum levels required for healthy aquatic ecosystems and safe drinking water, indicating severe water quality impairment. The water body must be considered highly polluted and incapable of supporting most forms of aquatic life.

Given the critically low DO level of 0.40 mg/L, immediate and aggressive intervention is required to prevent a complete ecological collapse. The following strategies should be employed with urgency:

1. Pollution Control: Immediately identify and eliminate all sources of organic pollution. This requires an urgent investigation into untreated sewage discharges, industrial effluent, or agricultural runoff. Strict enforcement and rapid remediation are paramount.

2. Intensive Aeration: The water body must be artificially aerated to raise oxygen levels. Deploying high-capacity mechanical aerators or bubble diffusers is an emergency measure to re-oxygenate the water. Constructing cascades or weirs can serve as a long-term aeration solution.

3. Temperature Management: As a long-term supporting strategy, planting trees along the banks can provide shade, helping to lower water temperature and slightly increase oxygen’s natural solubility. 🌳 However, this is secondary to addressing the immediate pollution crisis.

• Sample collection must be done carefully to prevent the introduction of atmospheric oxygen, which would lead to a falsely high result.

• The stopper of the BOD bottle must be inserted without trapping any air bubbles.

• All chemical reagents should be of high quality and added below the surface of the sample.

• The titration should be performed soon after the acidification step to prevent loss of iodine through volatilization.

1. Discuss the environmental significance of dissolved oxygen.

Dissolved Oxygen (DO) is arguably the most critical indicator of a water body’s health and its ability to support life. Its significance is multifaceted:

• Respiration: Nearly all aquatic organisms, from fish and insects to aerobic bacteria, need oxygen to breathe (respire). If DO levels fall below a certain threshold (generally 5 mg/L), it puts stress on these organisms. Critically low levels, like the 0.40 mg/L in your sample, lead to “dead zones” where most aquatic life cannot survive.

• Indicator of Pollution: A healthy, clean water body has high DO levels. A sudden drop in DO is a clear sign of pollution, typically from organic wastes like sewage, agricultural runoff, or industrial discharge. Bacteria decompose this waste, consuming oxygen in the process and causing DO levels to plummet.

• Decomposition: DO determines how organic waste is broken down.

• Aerobic decomposition (with oxygen) is efficient and produces non-toxic byproducts like carbon dioxide and water.

• Anaerobic decomposition (without oxygen) is slow, smelly, and produces harmful byproducts like hydrogen sulfide and methane, further degrading the ecosystem.

• Nutrient Cycling: The presence of oxygen affects the chemical form of many nutrients. For example, in oxygen-rich water, phosphorus binds to sediment. In anoxic (no oxygen) conditions, this phosphorus is released back into the water, which can fuel massive algal blooms.

2. Most of the critical conditions related to dissolved oxygen deficiency occur during summer months. Why?

This happens for two main reasons that work together to create a “perfect storm” for low oxygen:

• Physical Reason (Solubility): The solubility of any gas in a liquid is inversely proportional to temperature. Put simply, warmer water cannot hold as much dissolved oxygen as cold water. So, as temperatures rise in the summer, the water’s maximum oxygen-holding capacity naturally decreases.

• Biological Reason (Metabolism): Summer’s warmer temperatures increase the metabolic rate of all aquatic life, including the decomposer bacteria that consume oxygen. These bacteria become more active, decomposing organic matter at a much faster rate and consuming oxygen more quickly.

Therefore, in summer, you have less oxygen available in the water to begin with, and it’s being used up much faster, leading to critical DO deficiencies.

3. Why do we use 0.025 N sodium thiosulphate solution for the titration?

The choice of 0.025 N sodium thiosulphate (Na₂S₂O₃) is for practicality and precision in the Winkler titration method.

In this titration, the volume of titrant used is directly related to the amount of DO in the sample. The 0.025 N concentration is a “Goldilocks” strength that is ideal for the typical range of DO found in environmental water samples (e.g., 2-10 mg/L).

• If the solution were too concentrated (e.g., 1.0 N), only a tiny drop would be needed for the titration. This makes it impossible to measure the volume accurately with a standard burette, leading to massive errors.

• If the solution were too dilute (e.g., 0.001 N), a very large volume would be required, possibly more than the burette can hold. This is inconvenient and less precise.

Therefore, 0.025 N provides a titrant volume that is large enough to be measured accurately (typically a few milliliters) but small enough to be practical for a single titration.

4. The turbulence of water should be encouraged. Why?

Turbulence is highly beneficial for a water body’s health because it promotes reaeration. 💨

Atmospheric oxygen enters the water primarily at its surface. In calm, still water, this process is very slow, and the oxygen tends to stay in the top layer.

Turbulence, such as from wind, rapids, or waterfalls, aggressively mixes the water. This action:

1. Increases Surface Area: It creates waves and splashes, dramatically increasing the surface area of water that is in direct contact with the air.

2. Promotes Mixing: It mixes the oxygen-rich surface water with deeper, oxygen-poor water.

Both effects significantly speed up the rate at which oxygen from the atmosphere dissolves into the water and gets distributed throughout the entire water column, naturally increasing the DO level and helping the water body cleanse itself.

5. Draw the oxygen saturation curve.

As a text-based AI, I cannot draw a visual graph. However, I can describe the oxygen saturation curve in detail. It’s a fundamental graph in water quality analysis.

Imagine a graph:

• The Y-axis (vertical) represents the Dissolved Oxygen saturation concentration in milligrams per liter (mg/L).

• The X-axis (horizontal) represents the Water Temperature in degrees Celsius (°C).

The curve itself shows the maximum amount of oxygen that water can hold at a given temperature. The key features are:

• It’s a downward-sloping curve. It starts high on the left (at cold temperatures) and goes down as it moves to the right (toward warmer temperatures).

• The relationship is inverse: As temperature increases, the water’s capacity to hold dissolved oxygen decreases.

• The curve is steepest at colder temperatures and becomes slightly flatter at higher temperatures.

Example Data Points on the Curve (at sea level):

• At 0°C, water can hold about 14.6 mg/L of oxygen.

• At 15°C, it can hold about 10.2 mg/L.

• At 25°C, it can hold only about 8.2 mg/L.

This curve is crucial for interpreting DO measurements. By measuring the water’s temperature and its actual DO, you can use this curve to calculate the “% saturation,” which tells you how healthy the water is relative to its maximum potential.