Saturated Krypton

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

The working temperature, 77 K, is well below the triple point of krypton, 116 K, but if the solid is taken as the reference state the isotherm shows an unusually sharp upward turn at the high-pressure end. The usual practice, following Beebe, is therefore to take p° as the saturation vapour pressure of the supereooled liquid (p° = 2-49 Torr at 77-35 K and 27-5 Torr at 90-2 K). 

Our experimental techniques have been described extensively in earlier papers (2, 13). The gamma ray irradiations were carried out in a 50,000-curie source located at the bottom of a pool. The photoionization experiments were carried out by krypton and argon resonance lamps of high purity. The krypton resonance lamp was provided with a CaF2 window which transmits only the 1236 A. (10 e.v.) line while the radiation from the argon resonance lamp passed through a thin ( 0.3 mm.) LiF window. In the latter case, the resonance lines at 1067 and 1048 A. are transmitted. The intensity of 1048-A. line was about 75% of that of the 1067-A. line. The number of ions produced in both the radiolysis and photoionization experiments was determined by measuring the saturation current across two electrodes. In the radiolysis, the outer wall of a cylindrical stainless steel reaction vessel served as a cathode while a centrally located rod was used as anode. The photoionization apparatus was provided with two parallel plate nickel electrodes which were located at equal distances from the window of the resonance lamp. 

Taylor and Jarman [1] observed SL spectra in the range of 280-740 nm from 2 M NaCl solutions saturated with argon, krypton and xenon sonicated at frequencies of 16 and 500 kHz. The spectra showed a continuum background with bands at about 310 nm and a peak of sodium D line, which exhibited appreciable asymmetric broadening, as shown in Fig. 13.2. The bands around 310 nm result from the A2L+ — X2n transition of OH radicals. The OH bands are quenched in salt solutions compared with those in water, which suggests the energy transfer reaction... 

The blue satellite peak associated with resonance line of rubidium (Rb) saturated with a noble gas was closely examined by Lepoint-Mullie et al. [10] They observed SL from RbCl aqueous solution and from a 1-octanol solution of rubidium 1-octanolate saturated with argon or krypton at a frequency of 20 kHz. Figure 13.4 shows the comparison of the SL spectra of the satellite peaks of Rb-Ar and Rb-Kr in water (Fig. 13.4b) and in 1-octanol (Fig. 13.4c) with the gas-phase fluorescence spectra (Fig. 13.4a) associated with the B —> X transition of Rb-Ar and Rb-Kr van der Waals molecules. The positions of the blue satellite peaks obtained in SL experiments, as indicated by arrows, exactly correspond to those obtained in the gas-phase fluorescence experiments. Lepoint-Mullie et al. attributed the blue satellites to B — X transitions of alkali-metal/rare-gas van der Waals species, which suggested that alkali-metal atom emission occurs inside cavitating bubbles. They estimated the intracavity relative density to be 18 from the shift of the resonance line by a similar procedure to that adopted by Sehgal et al. [14],... 

Krypton Sorption. Volumetric adsorption using gases with low saturated vapor pressure has been found to be an effective technique to gain detailed structural information for small quantities of porous materials, especially using krypton (Kr).27 The substitution of nitrogen by Kr reduces significantly the amount of unadsorbed molecules in the dead volume, allows for the characterization of small surface areas, and is thus ideal for mesoporous... 

Laser Flash Photolysis at 248 nm of TDI-PU. MDI-PUE. and Model Compounds. Figures 1 and 2 show the transient absorption spectra of MDI-PUE (5.5 X lO-3 g/dL) and TDI-PU (2.3 X 10 3 g/dL) in THF at a 2.0 ns delay after pulsing with a krypton fluoride excimer laser (Xex=248 nm) in air and nitrogen saturated samples. Both spectra have common peaks in nitrogen saturated solutions (shown by arrows) at 310 nm, 330-360 nm (broad), and above 400 nm (broad, diffuse absorbance).. The MDI-PUE sample has an additional and quite distinctive peak at 370 nm. In the presence of air, the peak at 370 nm for MDI-PUE is completely extinguished, while the sharp peaks at 310 nm for TDI-PU and MDI-PUE and the broad band above 400 nm are only marginally quenched by oxygen. 

If the saturation vapour pressure and molecular area of krypton at this temperature are 19.0 mmHg and 21 x lO-20 m2, respectively, calculate a specific surface area for the solid. 

The observed krypton and xenon concentrations in each water source in the Rift Valley study were converted in this way to percent air saturation... 

Fig. 13.6 Percentage of air saturation of krypton and xenon for the Jordan Rift Valley waters. The values were obtained by dividing the measured amounts by those expected for water equilibrated with air at the temperature at which each sample was collected. All samples, except one, were found to be air supersaturated, indicating that the rare gases were retained under closed system conditions. Differences in duplicate samples are attributed to gas losses to the atmosphere prior to sampling (Mazor, 1975).

Fig. 13.8 Percent air saturation of krypton and xenon for warm springs in Swaziland (same spring numbers as in Fig. 13.7). All samples were oversaturated at the temperature of emergence (35-52 °Q, indicating the systems were closed (Mazor et al., 1974).

Current-voltage saturation curves for krypton resonance lamps were determined in a special cell with two internal rectangular (1.1 X 1.7 cm.) nickel electrodes 1.6 cm. apart. The LiF window extended between the electrodes. For 1.3 torr NO the saturation current was typically about 2.5 fiamp. The lamp intensities were usually about 1015 quanta/sec. Lamp intensities were monitored at about 10 hour intervals of operation. 

Values extracted and in some cases rounded off from those cited in Rabinovich (ed,), Thermo physical Properties of Neon, Argon, Krypton and Xenon, Standards Press, Moscow, 1976, This source contains values for the compressed state for pressures up to 1000 bar, etc, t = triple point. Above the solid line the condensed phase is solid below it, it is liquid. The notation 5,646,-4 signifies 5,646 x lO"", At 83,8 K, the viscosity of the saturated liquid is 2,93 x 10" Pa S = 0.000293 Ns/m. This book was published in English translation by Hemisphere, New York, 1988 (604 pp,),... 

Fischer et al, [122] proposed a model to predict the adsorption isotherm of krypton in porous material at supercritical temperature. In their study, a model pore of infinite length is formed by concentric cylindrical surfaces on which the centers of solid atoms are located. The interaction between an adsorbate and an individual center on the pore wall is described by the LJ 12-6 theory, and the overall potential is the integral of this interaction over the entire pore surface. With thermodynamic relations, Fischer et al. obtained the functional dependence of the saturation adsorption excess and the Henry s law constant on the pore structure. The isotherm was then produced by the interpolation between Henry s law range and saturation range. They tested their theory with the adsorption of krypton on activated carbon. It was shown that, with information on the surface area of the adsorbent and thermodynamic properties of the adso bate, their model gives more than quantitative agreement with experimental data. If a few experimental data such as the Henry s law constant at one temperature are available, the isotherms for all temperatures and pressures can be predicted with good quality. 

Bergman and co-workers studied the reaction of (Cp )Rh(CO)2 with CgH,2 by IR laser flash kinetics in inert gas solvents. Their conclusions are different from those of Lees and co-workers. At -31°C in xenon, irradiation gives a new species that Bergman and co-workers assign as (Cp )Rh(CO) or its solvated form, (Cp )Rh(CO)Xe. A similar but more reactive species is observed in krypton, and when cyclohexane is added to the solution, the (Cp )Rh(CO)Kr undergoes oxidative addition with a rate that shows saturation behavior described by... 

Nitrogen is used most often to measure BET surface, but if the surface area is very low, argon or krypton may be used, as both give a more sensitive measurement because of their lower saturation vapor pressures at liquid nitrogen temperature. 

Most species adsorption on MCM-41 gives rise to type IV isotherms (Fig. 2). Two steps are well observed. The relative extension of the two parts depends on the chemical nature of foe confined molecules [1,11]. It is very high in the case of hydrogen and low in the case of krypton. The first uptake at this very low relative pressure (P/Po S 0.1) corresponds to foe formation of a film of uniform thickness on the pore walls up to two layers in the case of hydrogen, one layer in the case of krypton (Po is the saturated vapor pressure of the bulk... 

---