Iodine-Thiosulphate titrations - ES

Question 1

What is iodine-thiosulphate titration used for

Answer 1

it is used to find the concentration of a solution of an oxidising agent (like chlorate (I) ions in bleach)

Question 2

Method for finding conc of ClO⁻ in a bleach solution

Answer 2

use a pipette to accurately measure out a fixed volume of the bleach into a conical flask (25cm^3)
Add a solution containing iodide ions making sure to add an excess of iodide ions. Also add sulphuric acid in excess. All the ClO⁻ ions in the bleach will react, some of the I⁻ will but some will be left over (from colourless to brown as I₂)
Add a sodium thiosuplhate solution of known conc from a burette, the thiosuplhate ions will react with the I₂ produced (from brown to colourless as I₂ used up)
When the colour of the solution in the conical flask has faded to yellow add a few drops of starch solution. While I₂ is still present this gives a blue-black colour, which will go colourless when all the I₂ is gone. Add the thiosulfate from the burette dropwise with swirling until the mixture goes colourless. Record
Repeat until 2 results are with 0.1 cm^3