Ask A Scientist Chemistry Archive

name         Jennifer L.
status       educator
age          40s

Question -   Why does iodized salt turn light brown when heated in a clean test tube?
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Hey, I have to try that.

Meanwhile...
Iodide (negative ion of iodine) is not very hard to oxidize back to elemental iodine.
3% Hydrogen peroxide can do it.  Oxygen in air can do it, when encouraged by heat.
Iodine crystal or vapor is brownish even in very small concentrations, even if trapped inside salt crystals.
One little 1mm sized iodine crystal can tint a whole liter of water noticeably.

Iodized salt is    NaCl + 0.01% NaI (sodium iodide)
NaI   is  ions Na+, I-

Oxidation by air:
  short form:   2 I-  +  02 (gas) -->    O2(2-)   + I2 (gas)
  long form:     2 NaI(NaCl) + O2 (gas) -->  Na2O2(NaCl) + I2 (trapped gas or dissolved solid)
      ( instead of Na2O2 it might be Na2O or NaOH.   Details...)

I suppose that "trapped gas" implies that crushing and grinding the salt crystals would allow some of the I2 gas to escape, and the salt would get lighter brown, whiter.  But then, smaller particle size always makes off-white things whiter.
Dissolving in water and re-crystallizing would release trapped iodine even more, but that might eliminate your effect other ways too.
Maybe the same heat for a long time would make your light brown salt crystals turn white again.

I am taking your word for all this... Have you tried the same heating on non-iodized salt, and found that it does not turn brown?

Jim Swenson
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Jennifer,

Perhaps the (potassium) iodide in the salt is being oxidized to form free molecular 
iodine. If so, that would impart a brownish color to the salt.

Regards,
ProfHoff 827
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Iodized salt contains iodine in the form of sodium or potassium iodide.
When heated the iodide is probably oxidized to I2, elemental iodine, which
when dilute has a brown color. That is my guess.

Vince Calder
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