how does lugol's reagent work?

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/Con...

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:...

Lugols Reagent

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RE:

how does lugol&#39;s reagent work?

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

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.