Sophism
Harmless
Posts: 6
Registered: 23-12-2007
Member Is Offline
Mood: No Mood
|
|
pot. iodide in tap water
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
Harmless
Posts: 23
Registered: 1-2-2005
Member Is Offline
Mood: No Mood
|
|
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
National Hazard
Posts: 506
Registered: 22-2-2006
Location: Everywhere
Member Is Offline
Mood: Ice glassed
|
|
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
International Hazard
Posts: 1554
Registered: 19-9-2007
Location: USA
Member Is Offline
Mood: Suspicious
|
|
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.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
|
|
|
jizmaster
Harmless
Posts: 23
Registered: 1-2-2005
Member Is Offline
Mood: No Mood
|
|
But don't even very dilute iodine solutions become much darker on addition of starch? That's what confused me.
|
|
|
woelen
Super Administrator
Posts: 7976
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
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
chemical wizard
Posts: 1803
Registered: 16-8-2004
Location: Germany
Member Is Offline
Mood: No Mood
|
|
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
Harmless
Posts: 6
Registered: 23-12-2007
Member Is Offline
Mood: No Mood
|
|
i am sure of my starch as i used it recently
& thanx for ur responses
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
