Chapter 15 - Redox Volumetric Analysis Flashcards
Is potassium permanganate a primary standard?
No - it’s not available in pure state and its solubility is low
Why must the ammonium iron (II) sulfate be dissolved in water that has been acidified?
Dissolved in sulfuric acid as the acid prevents the Fe²⁺ ions from being oxidised to Fe³⁺ ions by oxygen in the air
Why is dilute sulfuric acid added in excess to the conical flask containing the ammonium iron (II) sulfate solution?
To ensure a sufficient supply of H⁺ ions are present so that Mn⁺⁷ fully reduces to Mn⁺², not Mn⁺⁴
If sufficient acid was not added, a brown precipitate of MnO₂ would form
What is the colour change observed in the titration of ammonium iron (II) sulfate and potassium permanganate?
colourless → faint permanent pink
Why can’t HCl or nitric acid be used in the titration of ammonium iron (II) sulfate and potassium permanganate?
The Cl⁻ ions would be oxidised to Cl₂ gas
Nitric acid is an oxidising agent
Why are the iron tablets crushed and dissolved in dilute sulfuric acid and water?
Dissolved in sulfuric acid as the acid prevents the Fe²⁺ ions from being oxidised to Fe³⁺ ions by oxygen in the air
Why is dilute sulfuric acid added in excess to the conical flask containing the iron tablet solution?
To ensure a sufficient supply of H⁺ ions are present so that Mn⁺⁷ fully reduces to Mn⁺², not Mn⁺⁴
If sufficient acid was not added, a brown precipitate of MnO₂ would form
What is the colour change observed in the titration of iron tablets and potassium permanganate?
colourless → faint permanent pink
What is sodium thiosulfate used for?
To develop photos
Is sodium thiosulfate a primary standard?
No - it is not available in a pure state and its crystals are efflorescent (spontaneous loss of water of crystalisation to the air)
Is iodine a primary standard?
No - it sublimes (changes directly from a solid to vapour) even at room temperature
Which is the reducing agent and which is the oxidising agent: sodium thiosulfate and iodine?
sodium thiosulfate: reducing agent
iodine: oxidising agent
How do you create I₂ ions in the titration of sodium thiosulfate and iodine experiment?
By reacting a standard solution of acidified potassium permanganate with excess potassium iodide
In the titration of sodium thiosulfate and iodine, why must deionised water be used?
Tap water contains small amounts of chlorine that may interfere with the result by increasing the amount of iodine (chlorine displaced iodine from iodine solutions)
Why is dilute sulfuric acid added in excess to the conical flask containing potassium permanganate solution in the titration of sodium thiosulfate and iodine experiment?
To supply sufficient H⁺ ions for the complete reduction of Mn⁺⁷ to Mn⁺², not Mn⁺⁴
Why is potassium iodide solution added in excess to the conical flask containing potassium permanganate solution in the titration of sodium thiosulfate and iodine experiment? What is observed?
The solution turns red/brown as the iodine has been formed
The iodine is insoluble in water and so the KI helps to keep it in solution; and also to ensure all the KMnO₄ is reacted (ie max amount of iodine produced)
State the colour changes observed in the titration of sodium thiosulfate and iodine experiment
purple -(add potassium iodide)→ reddish/brown -(titrate)→ straw/pale yellow -(add starch indc.)→ blue/black -(titrate)→ colourless
Why is the indicator only added in half way through the titration of sodium thiosulfate and iodine experiment?
This is when most of the iodine has disappeared. If added earlier, the iodine present would become strongly adsorbed on to the starch, which would make the final reading less accurate as the iodine would be desorbed slowly so react with the sodium thiosulfate
What is the oxidising agent in the bleach experiment?
The hypochlorite ion is the oxidising agent that converts iodide ions to iodine
Why is the bleach diluted in for the titration?
As, otherwise, a very large volume of sodium thiosulfate would be required to react with the large amount of I₂ liberated
Why is dilute sulfuric acid added in excess to the conical containing the bleach solution in that titration?
To supple sufficient H⁺ ions so that all of the hypochlorite ions (ClO⁻) are completely reduced to chloride ions (Cl⁻)
Why is potassium iodide solution added in excess to the conical flask containing the bleach solution in the bleach titration? What is observed?
The solution turns red/brown as the iodine has been formed
The iodine is insoluble in water and so the KI helps to keep it in solution; and also to ensure all the KMnO₄ is reacted (ie max amount of iodine produced)
Why is the indicator only added in half way through the bleach titration?
This is when most of the iodine has disappeared. If added earlier, the iodine present would become strongly adsorbed on to the starch, which would make the final reading less accurate as the iodine would be desorbed slowly so react with the sodium thiosulfate
State the colour changes observed in the bleach titration
colourless -(add potassium iodide)→ reddish/brown -(titrate)→ straw/pale yellow -(add starch indc.)→ blue/black -(titrate)→ colourless
