ES - Iodine-sodium thiosulfate titrations Flashcards
What do iodine-sodium thiosulfate titrations do?
Find out the concentration of an oxidising agent.
Describe how you would carry out an iodine-sodium thiosulfate titration to find out the concentration of a solution of the oxidising agent potassium iodate(V).
Stage 1 - Use a sample of oxidising agent to oxidise as much iodide as possible.
1) Measure out a certain volume of potassium iodate(V).
2) Add this to an excess of acidic potassium iodide solution. The iodate(V) ions in the potassium iodate(V) solution will oxidise some of the iodide ions to iodine.
Stage 2 - Find out how many moles of iodine have been produced.
This is done by titrating the resulting solution with sodium thiosulfate.
I2(aq) + 2S2O3 2-(aq) –> 2I-(aq) + S4O6 2-(aq)
Titration
- Put solution from stage 1 in a flask.
- From the burette, add sodium thiosulfate solution to the solution in the flask.
- When the iodine colour turns straw yellow, add 2cm3 of starch solution as an indicator (to detect presence of iodine).
- Add sodium thiosulfate 1 drop at a time until the blue colour disappears.
- This means all of the iodine has just reacted.
- Now calculate the number of moles of iodine in the solution.
Stage 3 - Calculate the concentration of the oxidising agent in moldm-3.
What would mean that results produced in a iodine-sodium thiosulfate titration aren’t accurate?
Contaminated apparatus - make sure it is clean and rinsed out with sodium thiosulfate before you start as water will dilute the solution. Wash flask out between repeat experiments of get a new, clean one.
Reading burette incorrectly - read it carefully from the bottom of the meniscus with eyes at the level of the liquid.
Random errors - Repeat the experiment until you get 3 results within 0.1cm3 of each other then take an average.
What are some problems with iodine-sodium thiosulfate titrations?
The solutions used will react (very slowly) with oxygen in the air, so they should be made up as freshly as possible.
If you add starch too soon during the titration, the iodine will ‘stick’ to the starch and won’t react as expected with the thiosulfate, making the result unreliable. Only add starch when the solution is pale yellow.
When do you add the starch in an iodine-sodium thiosulfate titration?
When the solution is pale yellow and NOT BEFORE!!!
Where does the sodium thiosulfate go in an iodine-sodium thiosulfate titration?
In the burette.
