The Winkler titration is a classic method used to measure dissolved oxygen (DO) in water, involving a series of chemical reactions and a final titration step. Here's a breakdown of the process:
1. Sample Collection and Preservation
- Sample Collection: Collect the water sample carefully to avoid introducing atmospheric oxygen. Fill a BOD (Biochemical Oxygen Demand) bottle completely, ensuring no air bubbles are trapped.
- Preservation (Fixing): Immediately after collection, "fix" the oxygen in the sample. This prevents further oxygen consumption by biological activity.
2. The Winkler Titration Procedure
The Winkler method relies on a series of reactions:
| Step | Reagent(s) Added | Chemical Reaction and Observation |
|---|---|---|
| Oxygen Fixation | Manganous Sulfate (MnSO₄) and Alkaline Iodide-Azide Reagent (NaOH/KI/NaN₃) | Dissolved oxygen oxidizes Mn²⁺ to Mn⁴⁺, forming a brown precipitate of MnO(OH)₂. The azide removes nitrite interference. |
| Acidification | Concentrated Sulfuric Acid (H₂SO₄) | MnO(OH)₂ reacts with the iodide (I⁻) in the solution to release free iodine (I₂). The amount of iodine released is stoichiometrically equivalent to the dissolved oxygen. |
| Titration | Sodium Thiosulfate (Na₂S₂O₃) | The iodine (I₂) is titrated with a standardized solution of sodium thiosulfate (Na₂S₂O₃). Starch indicator is added near the endpoint to sharpen the color change. |
| Endpoint Determination | Starch Indicator | The solution turns from a dark blue/purple (iodine-starch complex) to colorless, indicating the endpoint of the titration. |
3. Chemical Reactions in Detail
-
Manganese(II) oxidation:
2Mn2+(aq) + 4OH-(aq) + O2(aq) → 2MnO(OH)2(s)
-
Iodide oxidation:
MnO(OH)2(s) + 2I-(aq) + 4H+(aq) → Mn2+(aq) + I2(aq) + 3H2O(l)
-
Titration with thiosulfate:
I2(aq) + 2S2O32-(aq) → 2I-(aq) + S4O62-(aq)
4. Calculations
The amount of dissolved oxygen is directly proportional to the amount of sodium thiosulfate used in the titration. The following formula is used to calculate the dissolved oxygen concentration:
DO (mg/L) = (V N 8000) / P
Where:
- V = Volume of sodium thiosulfate used (mL)
- N = Normality of sodium thiosulfate
- P = Volume of water sample titrated (mL)
- 8000 is a conversion factor
5. Significance of Each Reagent
- Manganous Sulfate (MnSO₄): Provides Mn2+ ions that react with dissolved oxygen.
- Alkaline Iodide-Azide Reagent (NaOH/KI/NaN₃): Provides the alkaline environment for the reaction, iodide ions (I⁻) that are later oxidized, and azide (N₃⁻) to remove nitrite interference.
- Sulfuric Acid (H₂SO₄): Acidifies the solution to release iodine from the manganese precipitate.
- Sodium Thiosulfate (Na₂S₂O₃): Used as the titrant to react with the liberated iodine.
- Starch Indicator: Forms a dark blue complex with iodine, making the endpoint of the titration easily visible.
6. Potential Errors and Considerations
- Air bubbles: Trapped air bubbles in the sample bottle can lead to inaccurate results.
- Interferences: Certain substances in the water sample can interfere with the reactions. Nitrites, for instance, can react with iodide and release iodine, leading to falsely high DO readings. The azide in the alkaline iodide-azide reagent eliminates nitrite interference.
- Temperature: The solubility of oxygen in water is temperature-dependent. Record the water temperature during sampling.
- Standardization: Accurate standardization of the sodium thiosulfate solution is crucial for precise results.
By carefully following the Winkler titration procedure and addressing potential sources of error, you can accurately measure the dissolved oxygen concentration in water samples.
