Iodine-Thiosulfate Titrations

Question 1

which type of reaction is involved in an iodine-thiosulfate titration?

Answer 1

redox

Question 2

what is the 6 step process for finding the concentration of ClO- in a bleach solution?

Answer 2

1. use a pipette to accurately measure out a fixed volume of the ClO- containing bleach solution into a conical flask
2. add a solution containing iodine ions making sure to add excess of iodide ions. add also sulphuric acid solution in excess. all the ClO- ions in the bleach will react and some of the I- ions will be left over
3. add a sodium thiosulfate solution of known concentration from a burette. the thiosulfate ions will react with the I2 produced
4. when the solution in the conical flask has faded to straw yellow, add a few drops of starch solution which will turn the solution blue-black. this will turn colourless at the end point
5. add the thiosulfate dropwise from the burette, swirling until mixture goes colourless. record titre
6. repeat until you have at least 2 results within 0.1 cm^3
7. use the average titre to calcuate moles of thiosulfate used, moles of I2 is half of this. mols I2 = moles ClO- in volume pipetted at the start. from that you can calculate the concentration of the ClO- solution

Question 3

why add excess I- and sulphuric acid solution?

Answer 3

iodide ions: ensures all the ClO- reacts
sulph acid: provides the H+ ions needed in the equation

Question 4

why is starch added to the straw yellow solution?

Answer 4

it turns blue-black when iodine is present