ANALYSIS OF ULTRAHIGH-VACUUM ISOTHERM DATA WITH THE BRUNAUER-EMMETT-TELLER EQUATION

The Brunauer-Emmett-Teller (BET) isotherm equation is probably the mos t widely used analytic equation in the range 0.01 less than or similar to P/P-0 less than or similar to 0.3 where it provides a standard mea sure of microscopic surface area. Here P is the pressure above the ads orbed layer, and P-0 is the vapor pressure of the adsorbate, both at t he temperature of the measurement. Once two measurable constants have been obtained the BET equation permits the calculation of the entire i sotherm from Henry's law at low pressures and coverages to the vapor p ressure at multilayer coverages. A critical comparison between the ent ire equation and experimental data requires data over a very broad ran ge of pressure and coverage. These were essentially unavailable before the advent of ultrahigh vacuum and, even today, are rare. But experim ental data published many years ago by the author for physical adsorpt ion isotherms of argon, krypton, and xenon on an adsorbent of porous s ilver at T = 77.4 K are suitable for a test of the whole BET equation that was not carried out at the time. The present article fulfills tha t omission. It is found that, while the BET equation is an adequate de scription of the data in the range 0.01 less than or similar to P/P-0 less than or similar to 1, serious and increasing divergencies occur a s the pressure decreases further. A review of ultrahigh vacuum isother m data for helium and nitrogen leads to the same conclusion for these gases also. The special case of hydrogen is important in predicting th e base pressures in modern colliders. Here, however, the vapor pressur e of hydrogen at 4.2 K is about 3.2x10(-7) Torr, and magnitude some fo ur orders in pressure below this (required for a decisive test of the BET equation) is already well into the range where quantitative hydrog en measurements are difficult. Hence conclusions about the applicabili ty of the BET equation to hydrogen isotherm data are less cer;ain. An analytic equation based, in part, on the Dubinin-Radushkevich isotherm at low pressures, which was proposed earlier, gives a good descriptio n of the argon, krypton, and xenon data on porous silver over the enti re measured range, some thirteen orders of magnitude in pressure. (C) 1996 American Vacuum Society.