[xiankai]

i had an experiment in which the concentration of hydrogen peroxide was measured. we added a test-tube full of sulfuric acid and a test-tube full of aq. potassium iodide to a conical flask containing 25.0cm3 of hydrogen peroxide. the solution then turned orange. when we had titrated enough hydrogen peroxide such that the solution turned pale yellow, we were instructed to add a few drops of starch indicator (sodium thiosulfate). this turned the solution dark blue. then we had to continue titrating until the solution had turned colorless. now i have a bunch of questions for curiousity's sake:

1) to speed up the process after the 1st 2 titrations, i titrated roughly 3/4 of the previous value at one go before waiting for it to settle. to my surprise, it turned colorless. after quite a long time, the solution turned back to yellow. what is taking place?

2) after adding the starch indicator, i slowly added hydrogen peroxide and waited for it to decolorise. after a while it did, completely, but within a few seconds the color returned. so i added more hydrogen peroxide, and it did turn colourless, but this time it turned back to the dark blue colour in ten seconds. this time i was really mystified, so i added much more hydrogen peroxide. i thought i had finally decolorised the solution, but NO, it turned dark blue after a few minutes. what is the puzzling color change involving?

[Dude]

Look up "Iodine Clock Reaction" on Google.

[xiankai]

thank you!