18. Dissolved Oxygen (Winkler)
Summary
TLDRThis 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
- đŹ The experiment measures dissolved oxygen in water using the Winkler method.
- đ§ A water sample is collected in a bottle, ensuring no air is present to avoid inaccurate results.
- đ The bottle is filled to the top to prevent air bubbles and ensure an oxygen-free environment.
- đ§ Manganese II sulfate solution is added to the water sample to initiate the reaction.
- đ Alkaline potassium iodide solution is then added, leading to the formation of a brown precipitate.
- âïž Manganese(II) hydroxide reacts with dissolved oxygen to form Manganese(III) hydroxide, a brown precipitate.
- đ The precipitate is redissolved by adding concentrated sulfuric acid, which liberates iodine.
- đ The liberated iodine is titrated against a standard sodium thiosulfate solution to determine its concentration.
- đ Two titrations are performed to ensure accuracy, as a third titration is not feasible due to bottle capacity.
- đ 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
đ§Ș 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.
đŹ Titration to Determine Iodine Concentration
The second paragraph details the titration process to ascertain the concentration of iodine in the water sample. A 100 cmÂł 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.
đ 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.
Mindmap
Keywords
đĄDissolved Oxygen
đĄRedux Titration
đĄWinkler Method
đĄManganese II Sulfate
đĄAlkaline Potassium Iodide
đĄPrecipitate
đĄSodium Thiosulfate
đĄConcentrated Sulfuric Acid
đĄTitration
đĄParts Per Million (ppm)
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]
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