Re: unexplained yellow color in acid-iodine-sulfite system-literature help

From: Mark Tarka (mark_tarka_at_yahoo.com)
Date: 07/21/04 

Date: 21 Jul 2004 08:48:03 -0700

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

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