The Winkler Method

Summary

In this video, Candace Berkeley, a development coordinator at Carolina Biological Supply Company, demonstrates the Winkler method for measuring dissolved oxygen in water. The process involves two key steps: fixing the oxygen content in a sample to prevent contamination and performing a titration to measure the oxygen concentration. The demonstration emphasizes the importance of dissolved oxygen for aquatic life and system health. Using the Lamont's dissolved oxygen kit, the video provides a hands-on approach to understanding the chemical reactions involved in this vital water quality test, suitable for classroom and field applications.

Takeaways

• 😀 The Winkler method is used to measure dissolved oxygen in water samples, crucial for assessing water quality.
• 😀 Dissolved oxygen content indicates the health of aquatic environments and their ability to support life.
• 😀 The Winkler method involves two main steps: oxygen fixation and titration to measure oxygen levels.
• 😀 Oxygen fixation prevents additional oxygen from affecting the results by stabilizing the sample.
• 😀 Proper safety equipment, such as gloves and goggles, is recommended during the process.
• 😀 Water samples are collected by submerging a DO bottle into the water to avoid air bubbles.
• 😀 Manganous sulfate and alkaline potassium iodide are added to the sample to create manganous hydroxide, a precipitate.
• 😀 Sulfuric acid is then added to dissolve the precipitate and fix the oxygen in the sample.
• 😀 The titration step uses sodium thiosulfate to react with free iodine, indicating the dissolved oxygen concentration.
• 😀 The color change from blue to clear in the titration indicates the conversion of free iodine to sodium iodide.
• 😀 The amount of sodium thiosulfate used in titration directly correlates with the dissolved oxygen concentration, with each 0.1 mL indicating 1 ppm of dissolved oxygen.

Q & A

What is the Winkler method used for?

The Winkler method is a technique used to measure the concentration of dissolved oxygen in freshwater environments, which is critical for the health of aquatic systems.

Why is dissolved oxygen important in aquatic systems?

Dissolved oxygen content in water is essential for supporting aquatic life and is also an indicator of the overall health of the ecosystem.

What are the two main parts of the Winkler method protocol?

The first part involves fixing the oxygen in the water sample to prevent it from being affected by additional oxygen, and the second part is a titration to measure the dissolved oxygen concentration in the sample.

What safety precautions should be taken when performing the Winkler method?

It is recommended to use proper safety equipment, such as gloves and goggles, during the procedure to ensure safety.

How should the DO (dissolved oxygen) bottle be filled with a water sample?

To fill the DO bottle, tilt it at a 45° angle and slowly submerge it into the freshwater system. The bottle should be capped while submerged to ensure no air bubbles are trapped inside.

What happens when manganous sulfate and alkaline potassium iodide are added to the sample?

When these chemicals are added to the sample, they react to form manganous hydroxide, which appears as a white precipitate, and the reaction is crucial for fixing the oxygen in the sample.

Why is sulfuric acid added to the DO bottle?

Sulfuric acid is added to dissolve the precipitate formed by manganous sulfate and alkaline potassium iodide, and it also neutralizes other compounds, preventing interference from additional oxygen.

How is the titration process conducted in the Winkler method?

The titration involves adding sodium thiosulfate drop by drop to the fixed solution, with swirling after each drop, until the sample turns light yellow. Then, starch indicator is added, and the titration continues until the blue color disappears, indicating the amount of free iodine has been neutralized.

What does the disappearance of the blue color in the titration indicate?

The disappearance of the blue color indicates that all the free iodine has been converted to sodium iodide, meaning the titration is complete.

How is the dissolved oxygen concentration determined from the titration?

The amount of sodium thiosulfate used in the titration is proportional to the initial dissolved oxygen level. Each 0.1 milliliter of sodium thiosulfate used indicates 1 part per million (ppm) of dissolved oxygen in the sample.

How can the Winkler method be useful in a classroom or field setting?

The Winkler method provides students with hands-on experience in measuring dissolved oxygen, allowing them to better understand the process of oxygen fixation and titration, which are key concepts in water quality testing.