18. Dissolved Oxygen (Winkler)

Summary

This video demonstrates the Winkler method for measuring dissolved oxygen in water using Redux titration. It involves collecting a water sample, adding manganese sulfate and alkaline potassium iodide, and observing the formation of a brown precipitate. The precipitate is then treated with sulfuric acid to release iodine, which is titrated against sodium thiosulfate to determine the oxygen content. The experiment concludes with calculating the dissolved oxygen in parts per million.

Takeaways

• 🔬 The experiment measures dissolved oxygen in water using the Winkler method.
• 💧 A water sample is collected in a bottle, ensuring no air is present to avoid inaccurate results.
• 🌊 The bottle is filled to the top to prevent air bubbles and ensure an oxygen-free environment.
• 💧 Manganese II sulfate solution is added to the water sample to initiate the reaction.
• 🌀 Alkaline potassium iodide solution is then added, leading to the formation of a brown precipitate.
• ⚗️ Manganese(II) hydroxide reacts with dissolved oxygen to form Manganese(III) hydroxide, a brown precipitate.
• 🌐 The precipitate is redissolved by adding concentrated sulfuric acid, which liberates iodine.
• 🔍 The liberated iodine is titrated against a standard sodium thiosulfate solution to determine its concentration.
• 📊 Two titrations are performed to ensure accuracy, as a third titration is not feasible due to bottle capacity.
• 📈 The amount of dissolved oxygen is calculated from the iodine concentration, expressed in parts per million.

Q & A

What is the purpose of the experiment described in the transcript?

The purpose of the experiment is to measure the amount of dissolved oxygen in a sample of water using a method known as the Winkler method.

How is the water sample collected for the experiment?

The water sample is collected by submerging a bottle completely in the water to be analyzed and filling it right to the top, ensuring no air is present in the bottle.

Why is it important to avoid air in the bottle when collecting the water sample?

Air in the bottle would raise the oxygen level and give an inaccurate result, as the experiment aims to measure the dissolved oxygen in the water sample.

What are the two solutions added to the water sample and their respective purposes?

Manganese II sulfate solution and alkaline potassium iodide solution are added. Manganese II sulfate reacts with OH- ions to form Manganese II hydroxide, which then reacts with dissolved oxygen to form a brown precipitate of Manganese III hydroxide. Alkaline potassium iodide provides iodide ions that react with Manganese III hydroxide to liberate iodine.

What is the significance of the brown precipitate formed in the bottle?

The brown precipitate is Manganese III hydroxide, which indicates the presence of dissolved oxygen in the water sample as it forms as a result of the reaction between Manganese II hydroxide and dissolved oxygen.

How is the amount of iodine liberated determined in the experiment?

The amount of iodine liberated is determined by titrating it against a standard sodium thiosulfate solution until a pale straw color is obtained, then adding a few drops of starch solution and continuing titration until the blue-black color disappears.

Why is concentrated sulfuric acid added to the bottle?

Concentrated sulfuric acid is added to the bottle to cause the iodide ions from the alkaline potassium iodide to react with Manganese III hydroxide, liberating iodine.

What is the role of sodium thiosulfate solution in the titration process?

Sodium thiosulfate solution is used as a titrant to determine the concentration of iodine in the water sample by reacting with the liberated iodine until the color change indicates the endpoint of the titration.

How many titrations are performed in the experiment, and why?

Two titrations are performed to determine the concentration of iodine in the water sample. The second titration figure must be as accurate as possible since a third titration is not possible due to the capacity of the bottle.

How are the results of the experiment expressed?

The results of the experiment are expressed in parts per million (ppm), which indicates the concentration of dissolved oxygen in the water sample.

What is the final step to calculate the amount of dissolved oxygen in the water?

The final step is to calculate the amount of dissolved oxygen using the amount of iodine liberated and the method of calculation provided in the textbook.