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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: kylon on July 17, 2007, 07:48:16 PM

Title: Function of starch solution(1)
Post by: kylon on July 17, 2007, 07:48:16 PM
When having an experiment "rate law of an iodine clock reaction", a starch solution is added. What is the function of starch solution?
Thank for help first...
Title: Re: Function of starch solution(1)
Post by: Yggdrasil on July 17, 2007, 07:58:39 PM
The starch is likely acting as an indicator for the presence of triiodide ion.  Starch and triiodide form a complex which gives the solution a deep blue-black color.

http://en.wikipedia.org/wiki/Iodine_clock_reaction
Title: Re: Function of starch solution(1)
Post by: kylon on July 18, 2007, 05:04:14 AM
Sorry, I miss some important things.
The experiment is using sulfuric asid, potassium iodine, thiosulfate, starch solution and distilled water.

The equations are:

H2O2 + 2I- + 2H3O+
---> I2 + 4H2O

I2 + 2S2O32- ----> S4O62- + 2I-

there are no triiodine ion.


Title: Re: Function of starch solution(1)
Post by: AWK on July 18, 2007, 05:59:27 AM
http://www.elmhurst.edu/~chm/vchembook/548starchiodine.html
Title: Re: Function of starch solution(1)
Post by: Borek on July 18, 2007, 06:59:28 AM
I2 + 2S2O32- ----> S4O62- + 2I-

there are no triiodine ion.

Yes, there is :) It is present in the solution whenever you have both I2 and I-

I2 + I- <--> I3-