Lab 17-1 - Photochemical Reaction of Iodine and Oxalate

[Kenji Totsuka]

Dear Home Scientist

  I'm doing Lab 17-1 and I have some question.  

I have created the reference standard with different iodine concentration and it looks just like the picture on the manual.

I have mixed the oxalic acid solution and ammonia to create ammonium oxalate solution and the solution is clear.  So far so good (or at least it seems).

However, when I add tincture of iodine to the ammonium oxalate, the solution turns black.  The color of tincture of iodine is brown and I think the color shouldn't change. 

I left the black solution in the dark in a test tube for several hours and the black precipitate was at the bottom of the tube.  Then I swirled the test tube and the solution became black again.  I'm guessing that the black precipitate is Iodine in solid form.  

I left the black solution under sunlight in another test tube for several hours.  The solution was clear for about bottom 60%.  The remaining top 40 % was full of white fuzz.  

Because the solution turns black, I can't use the reference standard to gauge the progress of the reaction.

So here is my question:

  "Do you have any idea why the solution turned black when I added the tincture of iodine into the ammonium oxalate solution?"

Here is the ingredient list of the tincture of iodine (bought at CVS) in case it makes any difference. 

    - 1 oz

    - 47% alcohol (v/v)

    - 2 % Iodine 

    - 2.4 % NaI

    - Inactive ingredient

      - Purified water

For Oxalic acid and ammonia, I'm using ones from Elemental Scientific

Thank you very much!