The Winkler Method – Biological Oxygen Demand BOD

The Winkler method is a widely used analytical technique for determining the amount of dissolved oxygen in water samples. Dissolved oxygen is essential for aquatic life and is an important indicator of water quality. The Winkler method involves the addition of reagents to a water sample that react with dissolved oxygen to produce a measurable change in color. By measuring the extent of this color change, the concentration of dissolved oxygen can be calculated. The Winkler method is widely used in environmental monitoring, water treatment, and scientific research. It is a simple and relatively inexpensive method, but requires careful attention to detail to ensure accurate results.

Safety and Environmental Impacts:

• Wear protective gear like gloves and goggles when handling chemicals.
• Avoid contact with skin, eyes, or inhalation of chemical fumes.
• Properly dispose of chemical waste according to local regulations.
• Work in a well-ventilated area to minimize exposure to chemical fumes.

Materials Required:

• Water sample
• Manganous sulfate solution (8.5 g in 500 mL of deionized water)
• Alkaline-iodide-azide reagent (10 g potassium iodide, 5 g sodium hydroxide, and 2 g sodium azide in 500 mL of deionized water)
• Sodium thiosulfate solution (0.025 M)
• Starch solution (1% solution)

Method

• Collect a water sample in a clean bottle, avoiding any contamination or exposure to air.
• Add 1 mL of manganous sulfate solution to the sample and mix well.
• Add 1 mL of alkaline-iodide-azide reagent to the sample and mix well.
• Stopper the bottle and allow the reaction to proceed for 10-15 minutes in the dark.
• Add 1 mL of sodium thiosulfate solution to the sample to remove excess iodine.
• Add 1 mL of starch solution to the sample as an indicator and titrate with sodium thiosulfate solution until the blue color disappears.
• Repeat the titration until two concordant readings are obtained.
• Record the volume of sodium thiosulfate used to calculate the dissolved oxygen concentration in the water sample.

Solution Preparation

• Manganous Sulfate Solution: Weigh 8.5 g of manganous sulfate and dissolve it in 500 mL of deionized water.
• Alkaline-iodide-azide Reagent: Weigh 10 g of potassium iodide, 5 g of sodium hydroxide, and 2 g of sodium azide and dissolve it in 500 mL of deionized water.
• Sodium Thiosulfate Solution: Dissolve 6.2 g of sodium thiosulfate in 1 liter of deionized water to obtain a 0.025 M solution.
• Starch Solution: Dissolve 1 g of starch in 100 mL of deionized water and heat the solution until it becomes clear.

Here is how to carry out the calculation from the Winkler Method:

1. Calculate the number of moles of thiosulfate used:

Number of moles of thiosulfate used = Volume of thiosulfate used (in liters) x concentration of thiosulfate solution

2. Calculate the number of moles of oxygen produced:

Number of moles of oxygen produced = 2 x Number of moles of thiosulfate used

Note: Two moles of thiosulfate react with one mole of oxygen.

3. Calculate the dissolved oxygen concentration:

Dissolved oxygen concentration (in mg/L) = Number of moles of oxygen produced x 31.9988

Note: 31.9988 is the molar mass of oxygen.

For example, let’s assume that 25 mL of 0.025 M sodium thiosulfate solution was used to titrate the water sample, and the volume of water sample used was 100 mL.

1. Number of moles of thiosulfate used = (25 mL / 1000 mL) x 0.025 N = 0.000625 moles
2. Number of moles of oxygen produced = 2 x 0.000625 = 0.00125 moles
3. Dissolved oxygen concentration = 0.00125 x 31.9988 = 0.039975 mg/L

Therefore, the dissolved oxygen concentration in the water sample is 0.039975 mg/L.

In conclusion, the Winkler Method calculation involves determining the number of moles of thiosulfate used, the number of moles of oxygen produced, and the dissolved oxygen concentration. It is essential to perform the calculation accurately to obtain reliable results.