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.

Outlines

00:00

🧪 Dissolved Oxygen Measurement Using Winkler Method

This paragraph describes an experiment to measure the amount of dissolved oxygen in water using the Winkler method. The process begins with collecting a water sample in a bottle and ensuring no air is present. Manganese II sulfate and alkaline potassium iodide solutions are added to react with the dissolved oxygen, forming a brown precipitate of manganese dioxide hydroxide. The experiment involves shaking the bottle and adding concentrated sulfuric acid to dissolve the precipitate and liberate iodine. The amount of iodine is then determined by titration against a standard sodium thiosulfate solution, which is crucial for calculating the dissolved oxygen content.

05:02

🔬 Titration to Determine Iodine Concentration

The second paragraph details the titration process to ascertain the concentration of iodine in the water sample. A 100 cm³ pipette is used to transfer the sample into a conical flask, and sodium thiosulfate solution is added to titrate the iodine. The endpoint is indicated by a color change from pale straw to blue-black, which signifies the complete reaction of iodine. The procedure is repeated with a second sample to ensure accuracy, as a third titration is not feasible due to the bottle's capacity. This step is essential for calculating the dissolved oxygen levels in the water.

10:06

📊 Calculating Dissolved Oxygen in Water

The final paragraph concludes the experiment by emphasizing the calculation of dissolved oxygen in water, expressed in parts per million (ppm). The method for this calculation is provided in the accompanying textbook. The paragraph highlights the importance of accurate titration results for determining the dissolved oxygen levels. The experiment wraps up with a reminder that the results are always presented in ppm, and the process outlined is crucial for understanding the oxygen content in water samples.

Keywords

💡Dissolved Oxygen

Dissolved oxygen refers to the amount of oxygen that is dissolved in a liquid, such as water. In the context of the video, dissolved oxygen is the primary subject of the experiment, where its concentration in water is measured using the Winkler method. It is crucial for aquatic life, and understanding its levels can indicate the health of a water body.

💡Redux Titration

Redux titration, also known as redox titration, is a laboratory procedure used to determine the concentration of a substance that undergoes a reduction-oxidation reaction. In the video, this method is employed to measure dissolved oxygen by titrating the iodine liberated from a reaction against a standard solution.

💡Winkler Method

The Winkler method is a specific technique for measuring dissolved oxygen levels in water. It involves a series of chemical reactions that result in the formation of a brown precipitate, which is then used in a titration to determine the oxygen content. The video script describes the step-by-step process of this method.

💡Manganese II Sulfate

Manganese II sulfate (MnSO4) is a chemical compound used in the Winkler method. In the video, it is added to the water sample to react with the dissolved oxygen, forming a white precipitate of manganese(II) hydroxide, which then reacts further to form a brown precipitate of manganese(III) hydroxide.

💡Alkaline Potassium Iodide

Alkaline potassium iodide is a mixture of sodium hydroxide (NaOH) and potassium iodide (KI) used in the Winkler method. It is added to the water sample to react with manganese ions, leading to the formation of iodine, which is then titrated to determine the amount of dissolved oxygen.

💡Precipitate

A precipitate is a solid that forms from a solution during a chemical reaction. In the video, the formation of a white precipitate of manganese(II) hydroxide and a brown precipitate of manganese(III) hydroxide are key steps in the Winkler method, indicating the presence of dissolved oxygen.

💡Sodium Thiosulfate

Sodium thiosulfate is a chemical compound used as a titrant in the redox titration process described in the video. It is used to titrate the iodine produced in the Winkler method, allowing for the calculation of dissolved oxygen levels.

💡Concentrated Sulfuric Acid

Concentrated sulfuric acid is a strong acid used in the Winkler method to dissolve the brown precipitate and to facilitate the reaction that liberates iodine. The video describes adding it carefully to the sample to avoid dissolving oxygen from the air.

💡Titration

Titration is a common laboratory procedure used to determine the concentration of an unknown solution by reacting it with a solution of known concentration. In the video, titration is used to measure the amount of iodine, which correlates to the amount of dissolved oxygen in the water sample.

💡Parts Per Million (ppm)

Parts per million (ppm) is a measure of concentration for a solute in a solution. In the context of the video, ppm is used to express the results of dissolved oxygen levels in water, which is a standard unit for such measurements in environmental science.

Highlights

Introduction to the Winkler method for measuring dissolved oxygen in water.

Procedure for collecting a water sample without air bubbles to ensure accuracy.

Rinsing the bottle with water to remove impurities and prevent trapped air.

Filling the bottle completely to the top to avoid air and ensure accurate oxygen measurement.

Adding Manganese II Sulfate solution to the water sample to initiate the reaction.

Introduction of Alkaline Potassium Iodide solution and its reaction with Manganese II Sulfate.

Formation of a brown precipitate indicating the presence of dissolved oxygen.

Explanation of the chemical reaction between Manganese II hydroxide and dissolved oxygen.

Adding concentrated Sulfuric Acid to the bottle and its role in the experiment.

Dissolving the precipitate and the importance of shaking the bottle to ensure complete reaction.

Liberation of iodine due to the reaction with Sulfuric Acid and its significance.

Performing titrations to determine the concentration of iodine in the water sample.

Use of standard Sodium Thiosulfate solution for titration against liberated iodine.

Achieving a pale straw color as an endpoint in the titration process.

Adding starch solution to indicate the endpoint of the titration.

Repeating the titration for accuracy and the importance of the second titration result.

Calculating the amount of dissolved oxygen in water using the results of the titration.

Expressing the results in parts per million (ppm) as a standard measure.

Conclusion of the experiment and summary of the method's effectiveness.

Transcripts

[Music]

experiment to measure the amount of

dissolved oxygen in a sample of Water by

means of Redux

titration in this experiment we will

measure the amount of dissolved oxygen

in a sample of water using a method

known as the Winkler

method this experiment involves

collecting a sample of water in a

bottle pour some of the water being

analyzed into the bottle

shake the bottle a few

times and empty out the

water rinsing out the bottle with water

removes any impurities wets the inside

of the bottle and helps avoid trapped

air

bubbles submerge the bottle completely

in the water to be analyzed and fill the

bottle right to the

top the bottle is then

stopper when collecting the water it is

important that no air is present in the

bottle as this would raise the oxygen

level and give an inaccurate result

therefore the bottle must be completely

filled to the

top remove the stopper from the

bottle then using a graduated dropper

insert the end of the dropper under the

surface of the water in the bottle and

add 1 cm cubed of Manganese 2 sulfate

solution using another graduated dropper

insert the end of the dropper under the

surface of the water and add 1 cm cubed

of the alkaline potassium iodide

solution since both Solutions are quite

dense note that these Solutions SN to

the bottom of the

bottle note the brown precipitate being

formed in the bottle what is happening

can be explained as follows when the

manganese 2 sulfate solution mnso4 and

alkaline potassium iodide solution a

mixture of Na and Ki are added to the

water the mn2+ ions and the oh minus

ions from the alkal react together to

form a white precipitate of Manganese 2

hydroxide as shown in the equation on

screen this white precipitate of

Manganese 2 hydroxide then reacts with

the dissolved oxygen in the water to

form a brown precipitate of Manganese

tree hydroxide the equation for this

reaction is shown on the screen

stopper the

bottle there will be a slight overflow

of the

water shake the bottle vigorously for

about half a

minute allow the brown precipitate to

settle for a few minutes until there is

at least a 5 cm depth of clear liquid

below the stopper

using graduated dropper carefully add 1

cm cubed concentrated sulfuric acid to

the bottle allowing the acid to flow

down the inside of the

glass rester the bottle carefully to

avoid dissolving any oxygen from the

air shake the bottle to redissolve the

precipitate if the precipitate is

difficult to dissolve we may have to add

a few more drops of concentrated

sulfuric

acid sulfuric acid that we added to the

bottle causes the iodide ions from the

alkaline Ki to react with the manganese

tree

hydroxide to liberate

iodine the equation for this reaction is

shown on the

screen at this stage we can see the red

brown color due to the liberated

iodine the amount of iodine is

determined by titrating it against

standard sodium tyo sufate

solution we will now perform two

titrations to determine the

concentration of iodine in the water

sample using a pet 100 cm cubed were

placed from the bottle into a conical

flask note that we are using a lar

sample so the pet was filled a number of

times

the buet was filled to the zero Mark

with sodium thyr sulfate

solution titrate in the usual manner

until a pale straw color is

obtained

for

for

for for

add a few drops of starred solution and

continue titrating until the blue black

color

disappears

for

for for

repeat the procedure with a second

sample from the

bottle since it will not be possible to

carry out a third titration due to the

capacity of the bottle the second

titration figure obtained must be as

accurate as

possible knowing the amount of iodine

liberated it is quite easy to calculate

the amount of dissolves oxygen in the

water the results are always expressed

in parts per million the method of

calculating this is shown in your

textbook that concludes the experiment

[Music]