Dear Marie,
This situation is a typical for the "external heating" device
without Pt-catalisator near graphite surface and without compulsory
oxygen circulation around graphite. The graphite (1) can be placed into
Pt-carrier bag; (2) graphite can be crushed (particles near 0.25-0.5
mm) and placed into quartz bag (half-cylindrical shape) together with
Pt pieces. The circulation of oxygen can be obtained using Toeppler
Hg-pumps or freezing-heating cycles of fingers containing porous
graphite (oxygen freezes at liquid nitrogen temperature on its). But this
method has many lacks, because the outgasing of such line is slow,
and You needs also turn off the heating of graphite-Pt zone to
prevent the CO formation during CO2 removing from U tube.

As a compromiss You can use a device with internal heating (we works
with it many years) - at this case You will not needs in circulation
of oxygen, the system more pure and very fast.

With best wishes
Dr. Elena O. Dubinina   


>Dear all,
>
>We are building a conventional fluorination line, and now testing the
>conversion chamber by introducing an aliquot of pure oxygen tank gas into
>our system, while heating a graphite rod up to 650-700°C. The furnace
>consists of a couple of IR lamps focussed on a graphite rod contained in a
>quartz tube (external heating), and the temperature of the graphite is
>checked by a thermocouple inserted inside the graphite. While heating the
>graphite and cooling down an U tube in a liquid nitrogen trap,  we
>introduce the oxygen in the previously pumped conversion chamber; at that
>moment what we should observe is first an increase of the pressure, as O2
>is introduced,  and then a progressive drop while the CO2 is condensed. We
>just see the O2 introduction and no CO2 seems to be trapped. We can't find
>out what is wrong.Two possible causes come to my mind :
>- we just form CO, but why?
>- the graphite quality we got is not compatible with an oxidation process?
>I am quite puzzled, as I used a similar device, not exactely the same but
>quite similar in principle in NIGL, few years ago, but it was no problem as
>Peter Greenwood and other friends there could attest. The major recognized
>difference about the quality of the graphite is that the one I bought here
>in France is far denser than the one currently used in UK; furthermore the
>shape of the graphite is different as we needed plane surfaces for the IR
>lamps heating....but I can't see any other major differences.
>
>If anybody can be of some help? Should I try another type of graphite?
>Which one? All ideas are really welcome, as we are quite novice in the
>business.
>
>Cheers
>Marie
>
>PS Sorry I mixed to addresses, so I send it again
>
>* * * * * * * * * * * * * * * * * * * * * * * * *
>*       Dr. Marie - Christine  GERBE            *
>* Departement de Geologie- Petrologie           *
>* Université Jean Monnet                        *
>* UMR CNRS 6524 "Magmas et volcans"             *
>* 23 rue du Dr. Paul Michelon                   *
>* F-42023 Saint-Etienne cedex 02 - France       *
>* tel. 04 77 42 18 23 - Fax. 04 77 42 18 08     *
>* From foreign countries:                       *
>* tel. 33 4 77 42 18 23 - Fax 33 4 77 42 18 08  *
>* e-mail : [log in to unmask]             *
>* * * * * * * * * * * * * * * * * * * * * * * * *
>
***********************************************
Dr. Elena O. Dubinina                              
(http://www.iem.ac.ru/staff/elena),            
                                               
Stable isotope geochemistry group,             
Institute of Experimental Mineralogy of RAS    
(http://www.iem.ac.ru/staff/elena/isolab.htm)  
                                               
142432, Moscow distr.,                         
Chernogolovka, IEM RAS, RUSSIA                 
***********************************************