>Dear list members,
>
>I have to prepare H3PO4 suitable for conventional CO2 extraction on
>carbonates and have 4 questions concerning the preparation procedure as
>described in Coplen et al. (1983). What is the role of CrO3? Is it possible
>to replace it by other Cr-compound like potassium di-chromate, I already
>have in the lab? What is the role of H2O2? Why does it need to be heated so
>long?
>Sorry for these basic questions; I will be grateful for any information.
>
>Best regards
>Marie
>Marie-C. Gerbe


Hi Marie,

The CrO3 is used as an oxidising agent for removing organic material from
the phosphoric acid. The H2O2 is used as a reducing agent to reduce the
oxidation state of the chromium.  This is supposed to prevent exchange of
carbon isotopes between the CO2 evolved and organic material in the acid.
Using a modern rapid reaction carbonate devices,  the CrO3 may be left out
of the brew as the CO2 gas has no time to exchange with organic material
contained in the acid.

If you are doing conventional (overnight) reactions then you should
consider using the oxidiser.

I'm unsure about using the potassium dichromate as a replacement. I'm sure
someone else on the list will be able to advise you.

The acid needs to be heated for a long time to dehydrate the acid.  You
start with 85% orthophosphoric acid and dehydrate to produce an excess of
P2O5 and H3PO4 to produce the so called 105% acid.

Best of luck

Joe Cali



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Joseph A. Cali                    |                               |
Environmental Geochemistry Group  |                               |
Research School of Earth Sciences | Phone :  +61 2 6249 3246      |
Australian National University    |   Fax :  +61 2 6249 3683      |
Canberra, Australia,  0200        | Email :  [log in to unmask]  |
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From 1st Jan  -  1st July, 1998.
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