Iodine Thiosulphate Titrations

Question 1

What is an iodine-thiosulphate titration?

Answer 1

A titration which you can use to find out the concentration of a solution of oxidising agent.

Question 2

What is the procedure for finding the concentration of ClO- in a bleach solution?

Answer 2

1. Use a Pipette to accurately measure out a fixed volume (25cm3) of the bleach containing ClO- into a conical flask.
2. Add a solution containing iodide ions in excess, then sulphuric acid solution in excess.
   the solution would now be brown due to I2 being brown in aqueous solution
3. Add sodium thiosulphate of a known concentration from a burette
   the solution will go from brown to straw yellow
4. Once straw yellow, add a few drops of starch solution until the solution goes blue-black. Continue to titre until dropwise until the solution goes clear.
5. Once clear record the titre and repeat until concordant results.
6. Use the average titre in the calculation

Question 3

How do you calculate the concentration of ClO- in bleach using your average titre?

Answer 3

1. Calculate the number of moles of thiosulphate (n=C x V)
2. Use the provided equations, and use a ratio of S2O3:I2:ClO-
3. Calculate the concentration of ClO- (C= n/V)