Carrying out the titration: Measure the amount of dissolved oxygen in a sample of water by redox
Key Terms
name and justify a suitable indicator for this titration Measure the amount of dissolved oxygen in a sample of water by redox
Starch solution. Iodine turns blue-black in the presence of starch.
state the colour change that occours during the titration Measure the amount of dissolved oxygen in a sample of water by redox
Red-brown to yellow to pale yellow. NOW ADD STARCH - a blue-black colour forms. Blue-black to colourless.
explain the colour change that occours during the titration Measure the amount of dissolved oxygen in a sample of water by redox
As the sodium thiosulfate is added and reacts with the iodine, the iodine is being used up, causing a less intense colour and changes from brown to...
Why is the starch added only when a pale yellow colour forms
| Measure the amount of dissolved oxygen in a sample of water by redox
Waiting until a pale yellow colour forms in the conical flask tells us that the end point is very near. The sodium thiosulfate can be added slowly ...
What is meant by BOD | Measure the amount of dissolved oxygen in a sample of water by redox
Biochemical oxygen demand. It is the amount of oxygen consumed by biological action when a sample of water is kept at 20°C for 5 days in the dark.<...
Related Flashcard Decks
Study Tips
- Press F to enter focus mode for distraction-free studying
- Review cards regularly to improve retention
- Try to recall the answer before flipping the card
- Share this deck with friends to study together
| Term | Definition |
|---|---|
Carrying out the titration: Measure the amount of dissolved oxygen in a sample of water by redox | The sodium thiosulfate in the burette. The iodine is in the conical flask continuously swirling and washing down the sides of the conical flask with deionized water. A yellow colour is observed. As the iodine is being used up a pale yellow colour is observed. Starch solution indicator is added. The solution in the conical flask turns blue-black. At the end point, the blue-black colour decolourises. |
name and justify a suitable indicator for this titration Measure the amount of dissolved oxygen in a sample of water by redox | Starch solution. Iodine turns blue-black in the presence of starch. |
state the colour change that occours during the titration Measure the amount of dissolved oxygen in a sample of water by redox | Red-brown to yellow to pale yellow. NOW ADD STARCH - a blue-black colour forms. Blue-black to colourless. |
explain the colour change that occours during the titration Measure the amount of dissolved oxygen in a sample of water by redox | As the sodium thiosulfate is added and reacts with the iodine, the iodine is being used up, causing a less intense colour and changes from brown to pale yellow. At the endpoint, starch is added and a blue-black colour forms due to the small amount of iodine left. The sodium thiosulfate is now slowly added in drops and as soon as iodine has been completely used up, the blue-black colour decolourises. |
Why is the starch added only when a pale yellow colour forms | Measure the amount of dissolved oxygen in a sample of water by redox | Waiting until a pale yellow colour forms in the conical flask tells us that the end point is very near. The sodium thiosulfate can be added slowly in drops resulting in a very inaccurate end point. Iodine adsorbs onto starch preventing it reacting with the sodium thiosulfate, resulting in an inaccurate endpoint would be obtained. |
What is meant by BOD | Measure the amount of dissolved oxygen in a sample of water by redox | Biochemical oxygen demand. It is the amount of oxygen consumed by biological action when a sample of water is kept at 20°C for 5 days in the dark. |
Why should a sample of water be analysed for dissolved oxygen immediatly? | Measure the amount of dissolved oxygen in a sample of water by redox | Due to biochemical oxygen demand. Living organisms in the water use up oxygen for respiration - a lower result for dissolved oxygen would not be obtained if not analysed immediately. |
What is the purpose of adding Manganese sulfate to the reaction bottle | Measure the amount of dissolved oxygen in a sample of water by redox | To provide a source of Mn2+ ions necessary for the formation of Mn(OH)2. |
what is the purpose of adding alkaline conditions/ OH-/KOH to the reaction bottle | Measure the amount of dissolved oxygen in a sample of water by redox | Provides alkaline conditions for the formation of Mn(OH)2. |
why did a brown percipate form in the bottle | Measure the amount of dissolved oxygen in a sample of water by redox | Due to Mn(OH)3 being formed from the reaction of Mn(OH)2 with dissolved O2. |
what does it mean if a white percipate forms/no brown percipate formed | Measure the amount of dissolved oxygen in a sample of water by redox | There is no dissolved oxygen in the water sample being dissolved. The white precipitate is due to the formation of Mn(OH)2. |
why was potassium iodide added to the reaction bottle | Measure the amount of dissolved oxygen in a sample of water by redox | Provides the I- ions necessary to produce I2. |
Why was exess potassium iodide added to the reaction bottle NB | Measure the amount of dissolved oxygen in a sample of water by redox | To ensure all of the oxygen in the water is used and detected and makes oxygen the limiting reagent. The I2 that is formed will react with excess I- forming the triiodide ion I3 which is a soluble version of iodide, it keeps iodine in solution. |
Why is concentrated sulfuric acid added to the percipate NB | Measure the amount of dissolved oxygen in a sample of water by redox | In an acidic environment Mn(OH)3 will react with I- ions to liberate iodine. |
what is the colour observed on the additon of concentrated sulfuric acid | Measure the amount of dissolved oxygen in a sample of water by redox | A red-brown colour forms. |
why are all the solutions added to the sample of water concentrated | Measure the amount of dissolved oxygen in a sample of water by redox | Minimises the volume of water displaced hence minimising the dissolved oxygen lost. Ensure the other reagents are in excess despite being in small volume, meaning all dissolved oxygen reacts. |
How was the manganese sulfate solution, the alkaline potassium iodide and the sulfuric acid added to the bottle containing the water sample and why were they added in this manner | Measure the amount of dissolved oxygen in a sample of water by redox | Using a dropper and ensuring it is under the level of water before adding the solution to avoid bubbling any air into the water. |
What percaution should be taken when stoppering and shaking the bottle after each addition of each solution | Measure the amount of dissolved oxygen in a sample of water by redox | Taking care not to bubble any air into the water as this would increase the amount of dissolved oxygen in the water. |
What name is givento calculating the concentration of dissolved oxygen by liberating iodine from the oxygen in this way | Measure the amount of dissolved oxygen in a sample of water by redox | Winkler method. |
explain why the winkler method is unsuitable for chloronated water | Measure the amount of dissolved oxygen in a sample of water by redox | Chlorine in the water would react with the iodide ions to form iodine. The titration value would be too large, causing an inaccurate end point. |