Sciencemadness Discussion Board

pot. iodide in tap water

Sophism - 24-3-2008 at 23:04

Dissolving KI in tap H2O give a yellow solution which upon adding Na2S2O3 it disappear, but the solution give no colour with starch !!!!!! so what is the source of the yellow colour

jizmaster - 25-3-2008 at 08:27

No idea! Try doing it with distilled water and a little bleach to oxidise the iodide maybe? To check there's nothing wrong with one of your chems.

jokull - 25-3-2008 at 08:40

Yellow color in tap water comes from oxidizers, e.g. ozone, peroxides, chlorine, chlorine dioxide. With exception of peroxides, the remaining substances are used as disinfectants by municipal water suppliers.

MagicJigPipe - 25-3-2008 at 10:14

Yes, this is how chloroform is generated in tap water from very small amounts of acetone and chlorine. VERY SMALL amounts though so nothing to be worried about, of course.

I am actually surprised that there was enough oxidant in the water to impart a yellow color to the iodide solution. Perhaps you could boil out the chlorine and if your water treatment plant doesn't use ClO2 then the yellowness should be reduced significantly.

jizmaster - 25-3-2008 at 10:20

But don't even very dilute iodine solutions become much darker on addition of starch? That's what confused me.

woelen - 25-3-2008 at 23:45

Free iodine indeed forms a very dark complex with starch.

But just to be sure, make a test solution as well.
Dissolve some KI in distilled water.
Acidify the water slightly (add 1 drop of vinegar)
Add a single drop of diluted bleach (e.g. a 0.5% solution)

This should give you a yellow solution. To this add some of your starch. If the liquid does not become blue, then your starch is not OK.

Another test for testing your KI:
Dissolve some in distilled water
Add some hydrochloric acid, e.g. several drops of a concentrated solution
Add a few drops of concentrated bleach.

This should give a dark brown solution, and maybe even some solid I2. If this does not happen, then most likely your KI is not OK.

garage chemist - 26-3-2008 at 04:24

You need to use a starch solution, made by boiling starch with water for a short time and decanting the clear solution above the undissolved residue after cooling down.

Sophism - 23-4-2008 at 12:32

i am sure of my starch as i used it recently
& thanx for ur responses