EXPERIMENT
A redox titration
In the next experiment, you perform a titration but this time you titrate a solution of an oxidant into a solution of a reductant. This type of titration is called a redox titration.
Aim
The purpose of this experiment is to balance the equation for the reaction between sodium thiosulphate and iodine.
a Na2S2O3(aq)
+ b I2(aq) Products
Introduction
You are to determine the ratio of a to b and so determine the stoichiometry of the reaction. You do this by taking a known amount of iodine and titrating it with standard sodium thiosulphate solution.
The indicator you use in this titration is starch solution, which is deep blue in the presence of iodine; it is added near the end of the titration when the solution is straw-coloured. If you add starch too soon, you may get a blue-black precipitate, which does not dissolve again easily even though there is an excess of thiosulphate. The end-point in this titration is the point at which the addition of one drop of sodium thiosulphate causes the disappearance of the deep-blue colour.
Requirements
safety spectacles
filter funnel
burette, 50 cm3 and stand
2 beakers, 100 cm3
sodium thiosulphate solution, standardised
pipette, 10 cm3
pipette filler
iodine solution, standardised
2 conical flasks, 250 cm3
starch indicator solution
white tile
wash bottle of distilled water
Procedure
1. Using the funnel, rinse the burette and tip with the sodium thiosulphate solution. Fill it with the same solution. Don't forget to fill the tip.
Record the initial burette reading in Results Table below.
2. Rinse the pipette with some of the iodine solution and carefully transfer
10.0 cm3 of the solution to one of the conical flasks.
3. Titrate this solution until the colour of the iodine has almost gone (as
indicated by a pale straw colour).
4. Add 1-2 cm3 of starch solution and continue the titration, adding sodium
thiosulphate dropwise until the end-point. Use the first flask for a trial run. Record the final burette reading.
5. Repeat the titration three more times. Enter your results into the Results Table. These titrations should agree to within 0.10 cm3.
Results Table
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Pipette solution |
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mol dm-3 |
cm3 |
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Burette solution |
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mol dm-3 |
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Indicator |
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Trial |
1 |
2 |
3 |
(4) |
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Burette readings |
Final |
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Initial |
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Volume used (titre)/cm3 |
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Mean titre/cm3 |
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Calculation
1. Use your results to determine the stoichiometric coefficients, a and b, in the equation:
a Na2S2O3(aq)
+ b I2(aq) Products
2. All the iodine forms sodium iodide NaI. There is one other product work out its formula.