Re: unexplained yellow color in acid-iodine-sulfite system-l

From: Farooq (farooq_w_at_hotmail-dot-com.no-spam.invalid)
Date: 07/21/04 

Date: 21 Jul 2004 12:29:01 -0500

> Mark Tarkawrote:
farooq_w@hotmail.com (Mohammed Farooq) wrote in message
news:<66756669.0407090916.232462de@posting.google.com>...
> Recently Wilco posted a problem in a seemingly simple reaction:
> To a solution of potassium iodide acidified by dilute sulfuric acid
or
> HCl is added sodium metabisulfite (NaHSO4) solution, immediately a
> lemon yellow color is developed on mixing.
> I repeated his experiment and found the same and speculated that it
is
> tri-iodide ion which is causing this color:
> The problem:
> The Vis spectrum of this solution shows only a single peak at 346
nm,
> matching very closely to peak of known tri-iodide solution at 347
nm,
> suggesting that it is indeed tri-iodide, but this yellow solution
is
> not decolorized by sodium thiosulfate, no matter how much of it is
> added. I did not take its derivative spectrum to resolve
overlapping
> peaks, if any, which suggests that it not *simple* I3(-).
>
> Searching more about this reaction I came across a reference in
> *Inorganic Chemistry*.
> "Non-Metal Redox Kinetics: Reactions of Iodine and Triiodide with
> Sulfite and Hydrogen Sulfite and the Hydrolysis of Iodosulfate",
Yiin,
> B. S.; Margerum, D. W. Inorg. Chem. 1990, 29, 1559-1564.
>
> which I think might explain Wilco's problem and solve the riddle
which
> so much perplexing me.
>
>From the report cited, p. 1562: "Excess sulfite and I2/I3(-)
solutions were mixed in the stopped-flow spectrophotometer,
and the reaction was monitored from 260 nm to 360 nm at 10-nm
intervals. Little or no signal change with time was observed
except at 300 nm. An excellent first-order absorbance decrease
was observed at 300 nm whereas no signal was seen at 353 nm
(I3(-) adsorption peak)."

Possibly Margerum's equipment wasn't properly calibrated.

  Mark[/quote:5a5cde7db9]

Interestingly Margerum wrote in a private communication to Wilco that
he was unaware of such yellow color. Partly it is my instrument's
fault... the actual absorption maximum ( as reported in some Russian
Journals) is 4000 Angstroms not 346 nm as I earlier posted.
Steve Turner very kindly provided Margerum's paper to me, this paper
did not mention the word yellow!. Read the post "explanation of the
yellow color in iodiDe-sulfite-acid system" if you are interested.

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