No stir bar is necessary.  From Nature, v. 302, p. 236 (1983):
"... slowly dissolving 2,010 g P2O5 in 4300 g 85  %  H3PO4 in a 4-L beaker.
Ten mg CrO3 is added and the solution is covered and heated at 200 deg C
for 7 h.  Three ml H2O2 is added and the solution is covered and heated to
220 deg C for an additional 4.5 h.  After cooling overnight, the acid is
stored in glass bottles with caps containing plastic inserts for tight
sealing.  Because the acid dissolves the black marking on a thermometer,
the thermometer should be inserted in a glass tube sealed at one end before
use ..."


|---------+---------------------------->
|         |           "Georges L.      |
|         |           Paradis"         |
|         |           <[log in to unmask]
|         |           U>               |
|         |           Sent by: Stable  |
|         |           Isotope          |
|         |           Geochemistry     |
|         |           <[log in to unmask]
|         |           UVM.EDU>         |
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|         |           05/17/2004 11:25 |
|         |           AM               |
|         |           Please respond to|
|         |           Stable Isotope   |
|         |           Geochemistry     |
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  |       To:       [log in to unmask]                                                                                  |
  |       cc:                                                                                                                |
  |       Subject:  low tech phosphoric acid question                                                                        |
  >--------------------------------------------------------------------------------------------------------------------------|




From: Georges L. Paradis
To: [log in to unmask]
Sent: Monday, May 17, 2004 8:23 AM
Subject: low tech phosphoric acid question

I'm new to the isotope game and I'm having difficulties preparing 103%
Phosphoric acid.  This may seem like a trivial question, but what is a good
stir bar to use?   I've toasted a teflon stirbar and just this morning I
broke a pyrex stirbar for the second time.  I was using VWR Spinbar Pyrex
Stir Bars.  The setting was 150 rpm, which seems like very gentle stirring.
It may be that the temperature increase is expanding the gas inside the
stirbar and the pressure is causing a breach in the pyrex.

 Any suggestions would be greatly appreciated.

Much Thanks,
Georges


~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Georges L. Paradis
ICPMS Laboratory Manager
Department of Geological Sciences
University of California Santa Barbara
Santa Barbara, CA 93106
(805) 893-7182
[log in to unmask]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~