Dissolved Oxygen Analysis

Dissolved Oxygen: Theory, Procedure, and Calculation

Dissolved Oxygen is a critical parameter in assessing water quality, reflecting the physical, chemical, and biological processes occurring in the water body. BOD is defined as the quantity of dissolved oxygen required by aerobic bacteria for the oxidation of organic matter under aerobic conditions. The most common method for determination is the Winkler Method (Iodometric Titration).

Theory

The Winkler test is based on the oxidation of manganous sulfate ($MnSO_4$) in an alkaline medium. The process follows these chemical stages:

• Fixing: Manganous ions react with dissolved oxygen to form a brown precipitate of manganic hydroxide.
• Acidification: Upon adding acid, the precipitate dissolves, and the manganic ions oxidize iodide ($I^-$) to free iodine ($I_2$).
• Titration: The amount of liberated iodine is equivalent to the original dissolved oxygen and is measured by titration with sodium thiosulfate ($Na_2S_2O_3$).

Procedure

1. Sampling: Collect the water sample in a 300 mL BOD bottle, ensuring no air bubbles are trapped.
2. Fixing the Sample: Add 2 mL of Manganous Sulfate and 2 mL of Alkali-Iodide-Azide reagent well below the surface. Stopper and mix by inverting. A brown precipitate indicates the presence of oxygen.
3. Dissolution: Add 2 mL of concentrated sulfuric acid ($H_2SO_4$). Stopper and mix until the precipitate disappears and the solution turns a clear golden-brown.
4. Titration: Take 201 mL of the prepared sample in a flask. Titrate with 0.025N Sodium Thiosulfate until the straw-yellow color fades.
5. End Point: Add 2 mL of starch indicator (turns blue). Continue titrating drop-wise until the blue color disappears.

Calculation

Since 1 mL of 0.025N sodium thiosulfate is equivalent to 0.2 mg of oxygen, and we use a specific sample volume to simplify the math, the formula is:

$$DO \text{ (mg/L)} = \frac{V_1 \times N \times 8000}{V_2}$$

Where:

• $V_1$: Volume of titrant used (mL)
• $N$: Normality of sodium thiosulfate (0.025N)
• $V_2$: Volume of sample titrated (mL)
• 8000: Constant to convert to mg/L

Simplified: If you titrate 200 mL of sample with 0.025N titrant, then:
$DO \text{ (mg/L)} = \text{Volume of Titrant (mL)}$

Related Topic Analysis of Biochemical Oxygen Demand