how does lugol's reagent work?

Question

i know what it does, but why or how does it cause everything to turn blue-black if it detects starch?

Answer 1

Iodine (I2) is not very soluble in water.
In the presence of iodide ions (I-) a soluble complex is formed according to the reaction I2+I(-) <=> I3(-)
So, lugol's reagent is practically KI3 or I3(-) if you want.

Starch is a mixture of 2 polysaccharides: amylose and amylopectin.
Amylose is less branched then amylopectin and has a more coiled structure. I3(-) gets trapped within these coils. (As you can guess from starch only amylose reacts with I3(-) to give the colour). When I3(-) forms a complex with amylose, it also forms a poly-iodide chain which is responsible for the colour.

You can see a picture of it at
http://www.iupac.org/didac/Didac%20Eng/Didac04/Content/BP20.htm

If you want advanced info you can read which species and electron energy levels are responsible for the colour at the following link (I couldn't access the pdf): http://209.85.129.104/search?q=cache:VNP0gaewtGwJ:www.fpl.fs.fed.us/documnts/pdf1996/yu96a.pdf+amylose+iodine+complex&hl=en&ct=clnk&cd=7

Answer 2

Lugols Reagent

Answer 3

This Site Might Help You.

RE:
how does lugol's reagent work?
i know what it does, but why or how does it cause everything to turn blue-black if it detects starch?

Answer 4

IKI will stain starches due to its interaction with the coil structure of the polysaccharide. IKI solution will not detect simple sugars such as glucose or fructose.