Sodium thiosulphate and iodine titrations Flashcards
What will be oxidised in the reaction?
Sodium thiosulphate
What is the equation for sodium thiosulphate?
Na2S2O3.5H20
What is sodium thiosulphate?
A reducing agent
What does Na2S2O3.5H20 react with?
Iodine that has been liberated from solutions containing an excess of potassium iodide, KI
Is sodium thiosulphate a primary standard?
No
Why is sodium thiosulphate not a primary standard?
As it is not available in a pure state.
Where does the sodium thiosulphate go?
Into the burette.
Is iodine soluble or insoluble in water?
Insoluble
Why is iodine insoluble in water?
As it is non-polar and water is a polar solvent.
What does iodine undergo at room temperature?
Sublimation
What standard is iodine?
Secondary standard
Why is iodine a secondary standard?
As it is not stable in air.
How is a standard solution of iodine obtained?
By reacting a standard solution of KMno4 with excess potassium iodine.
What is the oxidising agent in the titration?
KMnO4
Why is it important that the potassium iodide is in excess?
- so that the maximum amount of iodine is released due to the amount of oxidising agent used.
- To keep iodine in solution
What is the limiting reactant in this reaction?
The oxidising agent = iodine
What is the amount of iodine determined by?
By the amount of KMnO4 used (limiting reactant).
What / How much does 2 KMnO4 produce
5, Iodine2 molecules
What happens when the I2 is liberated?
When liberated it reacts with the I- ions in solution (from KI) to form the tri-iodide ion I3-.
Why may we regard the solution formed as a solution of I2?
As the I3- ion is similar in its behaviour to I2.
What is the first step in the titration?
The sodium thiosulphate is placed in the burette, observing the usual precautions.
What happens after the sodium thiosulphate is placed into the burette?
The iodine solution is placed in the conical flask.
What colour is the iodine when it is first placed in the conical flask?
Red/brown
Why is iodine red/brown when first placed into the conical flask?
Due to the liberated iodine.
What happens after the iodine is placed in the conical flask?
The sodium thiosulphate is added to the conical flask until the solution in the conical flask becomes straw-yellow colour.
What happens after the sodium thiosulphate is added and the solution in the conical flask becomes straw-yellow colour?
A few drops of starch indicator is added.
What colour did the solution turn after the starch indicator was added? (before & after)
Straw-yellow –> blue/black
What would happen if the starch was added before this stage (stage added)?
The blue/black complex that is formed is too concentrated and too stable to decompose fast enough to give an accurate end-point.
What happens after the starch indicator is added?
More sodium thiosulphate is added until the blue-black colour becomes colourless.
What indicates the end point?
When the solution becomes colourless.
What are the four colours in the conical flask?
Red/brown - straw/yellow - blue/black - colourless
Sodium thiosulphate is not a primary standard? Explain fully a primary standard. (L.C)
Pure from which solutions of known concentration can be made.
Describe how the crystalline thiosulfate was dissolved, and how the solution was transferred to the volumetric flask and made up exactly 500cm³. (L.C)
- Rinse from clock glass into beaker containing deionised water
- Stir
- Dissolve
- Pour through funnel into volumetric flask
- Add rinsings of beaker
- Add deionised water until bottom of meniscus on mark
Pure iodine is almost completely insoluble in water. What must be added to bring iodine into aqueous solution? (L.C)
Source of iodide
What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached? (L.C)
Red / brown → Straw coloured →- Blue-black → colourless
Explain how iodine, a non-polar substance of very low water solubility, is brought into aqueous solution. (L.C)
Reaction with potassium iodide
The iodine solution was made up in a 500cm³ volumetric flask. Describe the procedure for measuring 25.0cm³ of this solution into a conical. (L.C)
- Pour some iodine into a clean, dry beaker
- Use pipette
- Previously rinsed with deionised water and iodine solution
- Fill using pipette filler until bottom of meniscus is on mark
- Read at eye-level4
Name a suitable indicator for this titration. (L.C)
Starch
At what stage is the indicator added? (L.C)
When colour is in conical flask is straw coloured / when close to end point
State the colour change at the end point in the presence of the indicator. (L.C)
Blue-black → colourless
Explain why the use of distilled water instead of deionised water throughout this experiment would be likely to ensure a more accurate result. (L.C)
Deionised water could contain non-ionic substances that could be oxidised or reduced.
