ACCURATE CONTROL OF FH(2) IN COLD-SEAL PRESSURE-VESSELS WITH THE SHAWMEMBRANE TECHNIQUE

An experimental setup is described to use the Shaw membrane technique in cold-seal pressure vessels (CSPV), allowing constant partial pressu res of hydrogen (P-H2) and hence hydrogen and oxygen fugacities (f(H2) , f(O2)) to be maintained in experimental charges for long durations ( up to several weeks). The experimental setup permits the accurate dete rmination of f(H2) prevailing in gas pressurized vessels. In contrast to previous designs (CSPV and internally heated pressure vessels), osm otic equilibrium with this setup can be attained by diffusion of H-2 f rom the membrane toward the pressure vessel (without adding H-2 to the pressure medium before the heating up). P-H2 and thus f(H2), can be f ixed at any desired value with a minimum precision of +/- 1 bar. Osmot ic equilibrium is reached within less than 2 or less than 20 hours, wi th membranes made of Ag23Pd77 and Ag70Pd30 alloys, respectively (at 77 0 degrees C, 1 kbar total pressure and P-H2 up to 50 bars). The techni que described here has been successfully used to calibrate the f(O2) i mposed by the Co-CoO solid buffer assemblage at 735 degrees and 780 de grees C at 1 kbar.