9.1 The Winkler method (SL)

Summary

The Winkler method is a technique used to determine the concentration of dissolved oxygen in water, which is crucial for assessing water quality and calculating biochemical oxygen demand (BOD). The method involves three steps: fixing oxygen with manganese sulfate, oxidizing iodide ions to iodine, and titrating the iodine with sodium thiosulfate. A real-world example demonstrates how to calculate dissolved oxygen and BOD. The video concludes by explaining how different BOD values correlate with water quality, from very good to polluted, offering valuable insight into environmental monitoring and water treatment processes.

Takeaways

• 😀 The Winkler Method is used to measure the concentration of dissolved oxygen (DO) in a water sample.
• 😀 The method consists of three main steps: fixing dissolved oxygen, oxidizing iodide ions, and titrating iodine with sodium thiosulfate.
• 😀 In the first step, manganese sulfate is added to water, which reacts with dissolved oxygen, creating manganese hydroxide.
• 😀 The second step involves oxidizing iodide ions in an acidic solution to produce iodine.
• 😀 The iodine produced is then titrated with sodium thiosulfate in the final step.
• 😀 To calculate biochemical oxygen demand (BOD), two water samples are taken: one is tested immediately, and the other is stored for five days.
• 😀 BOD is calculated by subtracting the final concentration of dissolved oxygen (after 5 days) from the initial concentration.
• 😀 The BOD is typically expressed in milligrams per decimeter cubed (mg/dm³) or parts per million (ppm).
• 😀 The ratio of oxygen to sodium thiosulfate during the Winkler Method is 1:4, meaning one mole of oxygen reacts with four moles of sodium thiosulfate.
• 😀 A practical example showed a BOD calculation, resulting in a BOD of 3.44 mg/dm³ or 3.44 ppm, indicating moderate water quality.
• 😀 BOD values help classify water quality: 1-2 ppm for very good, 3-5 ppm for moderate, 6-9 ppm for somewhat polluted, and >10 ppm for polluted water.

Q & A

What is the Winkler method used for?

The Winkler method is used to determine the concentration of dissolved oxygen in a water sample and is commonly used to measure the biochemical oxygen demand (BOD) of water.

What are the three main steps in the Winkler method?

The three main steps in the Winkler method are: 1) Fixing the dissolved oxygen in the water sample by adding manganese sulfate in an alkaline solution. 2) Oxidizing iodide ions in an acidic solution to form iodine. 3) Titrating the iodine produced with aqueous sodium thiosulfate.

What is the role of manganese sulfate in the first step of the Winkler method?

In the first step of the Winkler method, manganese sulfate is added in an alkaline solution to fix the dissolved oxygen in the water. The manganese ions are oxidized while the oxygen is reduced.

How are iodide ions involved in the Winkler method?

In the second step of the Winkler method, iodide ions are oxidized in an acidic solution to form iodine. This oxidation process reduces the manganese ions and produces iodine.

What is the final step in the Winkler method and why is it important?

The final step in the Winkler method involves titrating the iodine produced with aqueous sodium thiosulfate. This is important because it helps determine the amount of dissolved oxygen in the sample based on the iodine content.

How is the biochemical oxygen demand (BOD) of a water sample calculated?

The BOD of a water sample is calculated by measuring the dissolved oxygen at two time points: immediately and after 5 days. The BOD is the difference between the initial and final dissolved oxygen concentrations.

What units are typically used to express BOD?

BOD is usually expressed in milligrams per decimeter cubed (mg/dm³) or parts per million (ppm).

What is the molar ratio of oxygen to sodium thiosulfate in the Winkler method?

In the Winkler method, the molar ratio of oxygen to sodium thiosulfate is 1:4, meaning one mole of oxygen reacts with four moles of sodium thiosulfate.

How do you calculate the amount of dissolved oxygen in the water sample based on sodium thiosulfate used?

To calculate the amount of dissolved oxygen, the number of moles of sodium thiosulfate used in the titration is first calculated using the formula N = C × V. Then, this value is divided by 4 (since the ratio of oxygen to sodium thiosulfate is 1:4) to find the moles of oxygen.

What does the BOD of a water sample indicate about its water quality?

The BOD indicates the water quality based on the amount of oxygen consumed by microorganisms. A BOD between 1-2 ppm indicates very good water quality, 3-5 ppm indicates moderate cleanliness, 6-9 ppm suggests somewhat polluted water, and greater than 10 ppm indicates polluted water.