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Lori Watson, Earlham College
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Joined: 11/17/2007 - 10:58am

Good visual ppt demo?

In gen chem we demonstrate the wonders of a precipitation reaction using the very nice, very visual, and very un-green Lead Nitrate + Potassium Iodide demo. I like it because the reaction is VERY easy even for beginning students to write the equation of, and demonstrates the solubility rules very nicely by making two clear solutions form a spectacular yellow ppt!  I dislike it because I make two large graduated cylinders full of PbI2 (you can't do a demo just once!).  Anyone have a similar ppt reaction with (preferably inexpensive) inorganic salts that makes nice colors and comes with an easily written ionic equation that does not use lead salts (or something worse)?
Joanne Stewart, Hope College
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Joined: 11/17/2007 - 11:05am
I agree with your concern about generating lead waste, but I sometimes think I became a chemist because we made lead iodide in 7th grade. A close runner up was making hydrogen in a test tube and inserting a glowing splint. Oh, I miss those post-Sputnik middle school science classes!
Norman Dean, Indiana University
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Joined: 10/29/2008 - 2:16pm
How do you feel about cobalt? cobalt(II) nitrate, while not clear, makes a nice purple precipitate when mixed with a colorless sodium hydrogen phosphate solution.
Adam R. Johnson, Harvey Mudd College
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Joined: 11/17/2007 - 10:55am

I remembered having a similar consideration when I was trying to figure out a good Ksp demo for frosh chem a few years ago.  It took me a while to remember what it was... the "Halloween Reaction" or the "Old Nassau" reaction (named for a building on Princeton's campus).  The demo is in Shakashiri, or you can find it using the power of the inter-webs.  Unfortunately, while this reaction does 'get the lead out...' it replaces the lead with mercury...

However, all is not lost, as there was an article published in J. Chem. Ed. which updated the reaction to be more environmentally friendly. The color changes are less dramatic, but it uses silver, starch and iodate.  This is not a clock reaction (it only goes once) but does give a dramatic color change.

here's a ref.

J. Chem. Ed., 2004, 81(10) 1474.

http://dlib.jce.divched.org//SPT--FullRecord.php?ResourceId=301

 Adam