Dear Isogeochemists,

CrO3 was added (by McCrea also) to the witch's brew in order to oxidize any 
organic matter present in the commercially available 85%H3PO4 (and the P2O5 
added).  The H2O2 used in the old recipe was a REDUCING agent to bring down 
the oxidation state of chromium from +6 down to +3 ( thus the green color).

The advantage we have today is that one can purchase 107% H3PO4 that is very 
pure so that one no longer needs to add the CrO3.  So Bjorn, your laboratory 
can now be relatively free of carcinogenic compounds.

Jim O'Neil (from Tuebingen)

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Dr. Torsten W. Vennemann
Institut fuer Geochemie
Wilhelmstr. 56
D-72074 Tuebingen
Germany
phone ++49-(0)7071-294992
Fax ++49-(0)7071-296870
e-mail: [log in to unmask]
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