Hi Bernd:

        Look up the Coplen et al 1983 paper in Nature. He describes
the recipe you are looking for.  We routinely make H3PO4 this way and
always get a sp. gr. of 1.92-1.93. I'm at home right now so this is
from memory - the key is to heat at 220C (?) for 7 hours, then raise
the temp to 240C (?) (check on these temps in the paper) for 4 hours.
Then cool the acid overnight. We blow N2 or Ar over the beaker during
cooling to prevent atmospheric H2O from dissolving into the surface
layer of the acid.  There are a couple of other things that we add
such as 3 ml of H2O2 (30%) and CrO3, but these are probably not
necessary. Good luck.

Howie Spero

>Hello!
>
>I have an intruction for making 103 % phosphoric acid (mix 250 g 97% P2O5 +
>500 ml 85% H3PO4 by constant stirring. Than I would obtain a 103 % H3PO4
>with a density of 1.9 g/cm3). But I can not obtain this density. Only 1.85
>g/cm3!!!!!. What can I do? I need the acid for carbonates analysis. Can I
>do this with the acid with 1.85 density or must I have one with density of
>1.9?
>
>Best regards
>
>Bernd

--

                Prof. Howard Spero
                Department of Geology
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                University of California, Davis
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