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Category: Inorganic
Posted on: January 17, 2008 11:39 PM, by Molecule of the Day
Lead and chromium (VI) - you can't do much better for toxicity.
The lead (II) salt of chromium (VI) oxide gives a vivid yellow. Before organic dye technology became robust, we were stuck with metal salts for color. This means metals. Unfortunately, there are more toxic metals than nontoxic metals, so you were stuck in a tough place there - especially if you wanted to make, say, a yellow baby rattle.
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Comments
Ksp(PbCrO4) = 1.8x10^(-14)
Ksp(BaSO4) = 1.1x10^(-10)
Barium is exceedingly toxic (cardiac hyperexcitablity). BaSO4 is dumped down human gullets for radiological contrast. Purely enormous tonnages are used in drilling mud (heavy spar). Don't sweat the lead chromate.
Posted by: Uncle Al | January 18, 2008 11:29 AM
http://webexhibits.org/pigments/indiv/history/cryellow.html
Very interesting indeed !
Posted by: Moebius | January 21, 2008 11:05 AM
but BaSO4 is insoluble and will pass through the human gullet while releasing next to nill Ba. therefor its more or less a nontoxic form. im geussing lead chromate is a little more willing to give up some of its looser bonded poisons... sweat the lead chromate...
Posted by: cal | January 30, 2008 7:12 PM
The Ksp values Uncle Al cited above are solubility constants, the lower the less soluble the stuff.
So he's saying that lead chromate is even less soluble then barium sulfate, making it rather harmless in practice.
It's probably a good thing too for a dye to be insoluble, wouldn't want the baby rattle to fade if a baby ever sucks on it..
Posted by: Chris | April 19, 2008 2:00 AM
It wasn't so much that we were stuck with metal pigments as much as the metal pigments didn't sun bleach. No one wants to watch their $300,000 astroturf fade a couple of years down the road. Now we have come full circle with the removal of most metal pigments from consumer items. Red sailcloth fades in the first year while the 100 year old barn is just as red as ever. Don't get me wrong, I'm glad the metals are gone but face it, the substitutes are just not as good.
Posted by: BlindSquirrel | April 19, 2008 2:48 AM
Oh ya, just a tech, but nothing is insoluble. Its all a matter of degree. I know you knew.
Posted by: BlindSquirrel | April 19, 2008 2:51 AM