Adsorption volumetric method

Values for x , the film pressure of the adsorbed film of the vapor (of the liquid whose contact angle is measured), are scarce. Vapor phase adsorption data, required by Eq. X-13, cannot be obtained in this case by the usual volumetric method (see Chapter... 

This method is smiple but experimentally more cumbersome than the volumetric method and involves the use of a vacuum microbalance or beam balance [22], The solid is suspended from one ann of a balance and its increase in weight when adsorption occurs is measured directly. The dead space calculation is thereby avoided entirely but a buoyancy correction is required to obtain accurate data. Nowadays this method is rarely used. 

A number of potential sources of error must be taken into account. In the volumetric method the following items need attention (a) constancy of the level of liquid nitrogen (b) depth of immersion of the sample bulb ( S cm) (c) temperature of sample (monitoring with vapour pressure thermometer close to sample bulb) (d) purity of adsorptive (preferably 99-9 per cent) (e) temperature of gas volumes (doser, dead space), controlled to 01 C. 

Few studies have been made of benzene chemisorption by the volumetric method. Zettlemoyer et al. (8) have examined the adsorption of benzene vapor at 0°C on powders of nickel and of copper. First, the monolayer coverage of argon (vm) A, was measured. The argon was then removed by pumping and the amount of benzene required to form a monolayer, (vmi) Bz, was measured. Weakly adsorbed benzene was then removed by pumping, after which further benzene adsorption provided the value (vm2) Bz. Some results are reproduced in Table I. On the assumption that the same extent of surface is accessible both for argon and for benzene adsorption, it is clear that complete monolayers of benzene were not achieved, that some (Ni) or all (Cu) of the benzene was adsorbed reversibly. It was considered that only the irreversibly adsorbed benzene was chemisorbed, the remainder being physically adsorbed. Thus chemisorption of benzene on copper appeared not to occur. The heat of adsorption of benzene on nickel at zero... 

Hydrogen uptake of reduced catalysts (X) was measured by volumetric method with an AUTOSORB-l-C analyzer (Quantachrome Instruments). Hydrogen adsorption was carried out at 373 K after in situ H2 reduction at 773 K for 6 h in the adsorption cell. The dispersion and particle size of metallic Co were calculated by the following equations, assuming that the stoichiometry for hydrogen adsorption on the metallic site is unity ... 

The water adsorption isotherms were determined by the liquid-volumetric method [21] The rust samples were put in an atmosphere with certain amount of water up to obtaining an established equilibrium, the pressure is measured and the water adsorbed may be calculated. 

A known amount of zeolite was loaded into a 10 mm NMR tube with an attached vacuum valve. The sample was evacuated to about 2xl0 < torr for 3 days at room temperature, then it was heated to 350 C with a heating rate of 0.2 C/min, the sample was allowed to maintain at this temperature for about 30 hours (2x10 torr). After cooling to room temperature, a known amount of xenon gas was introduced into the sample tube and was sealed by the vacuum valve. All the xenon adsorption isotherms were measured by volumetric method at room temperature. 

Because of the dosing technique associated with the vacuum volumetric method there exists a potential source of error which in principle cannot be avoided on samples that are slow to equilibrate. Figure 14.2 represents a plot of the pressure in the sample cell versus time. The pressure up to time tj represents the equilibrium pressure in the sample cell before a new quantity of adsorbate is admitted. At time adsorbate is admitted into the sample cell and is accompanied by a rapid pressure rise. The pressure then gradually decreases to a new equilibrium value at time t2- If the decay to the new equilibrium pressure is slow, it is possible for open or accessible parts of the surface, such as the interior of wide pores, to contain more adsorbate before equilibrium is attained than after equilibrium is established. Less accessible regions, such as the interior of long narrow pores, will adsorb slowly and as the pressure falls because of adsorption in these pores, desorption must occur from the more accessible pores which tend to equilibrate more rapidly. If a porous sample is subjected to... 

The consequences of this type of activated physical adsorption is not only that the quantity adsorbed can lie off the isotherm but also that the measured quantity of adsorption is far less than the equilibrium value. No experiments have been conducted to illustrate whether or not the quantity adsorbed lies within the hysteresis loop. The occasional failure of the vacuum volumetric method to agree with the dynamic method, which is not subject to any pressure overshoot, may in part be attributed to this phenomenon. 

When the initial slope of a chemisorption isotherm is not large so that monolayer coverage is approached at conveniently measurable pressures, the gravimetric, volumetric or continuous flow methods can be employed. The gravimetric and volumetric methods for determining this type of isotherm are used in the usual manner, provided that no reaction products contribute to the equilibrium pressure. The only essential difference when measuring chemical and physical adsorption using these techniques, is... 

The signals in Figure 2 are assigned to Pd (I) ions located at two different sites, A and B. Their respective g values are site A, g = 2.41, gL = 2.11 and site B, g = 2.28, g = 2.10. ESR quantitative measurements showed that about 8% of the loaded palladium was in the form of Pd(I) after hydrogen adsorption. Subsequent to the ESR experiments, the hydrogen uptake by sample (A) was measured by a volumetric method. 

Specific surface areas were determined by the volumetric method of adsorption of argon at liquid N2 temperature, using the BET equation (Parfitt and Sing, 1976). 

The volumetric method is mainly used for the purpose of determining specific surface areas of solids from gas (particularly nitrogen) adsorption measurements (see page 134). The gas is contained in a gas burette, and its pressure is measured with a manometer (see Figure 5.4). All of the volumes in the apparatus are calibrated so that when the gas is admitted to the adsorbent sample the amount adsorbed can be calculated from the equilibrium pressure reading. The adsorption isotherm is obtained from a series of measurements at different pressures. 

The application of the volumetric method for the determination of adsorption isotherms can be performed as follows. First, the adsorbate gas is introduced into the manifold volume, V and the quantity dosed is measured, usually in cubic centimeters at STP, that, is, standard temperature and pressure, specifically 273.15 K and 1.01325 x 10 Pa... 

The home-made heat-flow calorimeter used consisted of a high vacuum line for adsorption measurements applying the volumetric method. This equipment comprised of a Pyrex glass, vacuum system including a sample holder, a dead volume, a dose volume, a U-tube manometer, and a thermostat (Figure 6.3). In the sample holder, the adsorbent (thermostated with 0.1% of temperature fluctuation) is in contact with a chromel-alumel thermocouple included in an amplifier circuit (amplification factor 10), and connected with an x-y plotter [3,31,34,49], The calibration of the calorimeter, that is, the determination of the constant, k, was performed using the data reported in the literature for the adsorption of NH3 at 300 K in a Na-X zeolite [51]. 

In Figure 6.22a, the MPSD of the DAY zeolite calculated with the help of the SF method is reported. The DAY zeolite has an FAU framework, with a 12MR three-dimensional channel system with apertures of 7.4 A (see Section 2.5.1) [83], The obtained PSD for the DAY zeolite shows a clear maximum at about 7 A. In Figure 6.22b, the S-F MPSD of the SWCNT is reported [80], The calculated PSD for the SWCNT indicates that this material has a slightly heterogeneous distribution of pores with a clear, and highly populated, maximum at 13.5 A [80], The adsorption study, in both cases, was carried out using N2 adsorption at 77 K isotherms, obtained with a Quantachrome Autosorb-1 equipment for adsorption isotherm determination by the volumetric method [21],... 

The many different procedures which have been devised for the determination of the amount of gas adsorbed may be divided into two groups (a) those which depend on the measurement of the amount of gas removed from the gas phase (i.e. gas volumetric methods) and (b) those which involve the measurement of the uptake of the gas by the adsorbent (e.g. direct determination of increase in mass by gravimetric methods). Many other properties of the adsorption system may be related to the amount adsorbed, but since they require calibration they will not be discussed here. In practice, static or dynamic techniques may be used to determine the amount of gas adsorbed. 

If the solid contains no macropores, the isotherm remains nearly horizontal over a range of p/p° approaching unity and the total pore volume is well-defined. In the presence of macropores the isotherm rises rapidly near p/p° = 1 and if the macropores are very wide may exhibit an essentially vertical rise. The limiting adsorption at the top of the steep rise can be identified reliably with the total pore volume only if the temperature on the sample is very carefully controlled and there are no cold spots in the apparatus (which lead to bulk condensation of the gas and a false measure of adsorption in the volumetric method). 

The problem with sulfide catalysts (hydrotreatment) is to determine the active centres, which represent only part of their total surface area. Chemisorption of O2, CO and NO is used, and some attempts concern NIL, pyridine and thiophene. Static volumetric methods or dynamic methods (pulse or frontal mode) may be used, but the techniques do not seem yet reliable, due to the possible modification (oxidation) of the surface or subsurface regions by O2 or NO probe molecules or the kinetics of adsorption. CO might be more promising. Infrared spectroscopy, especially FTIR seems necessary to characterise co-ordinativcly unsaturated sites, which are essential for catalytic activity. CO and NO can also be used to identify the chemical nature of sites (sulfided, partially reduced or reduced sites). For such... 

Apparatus for measuring adsorption isotherms by the volumetric method. 

Experimental results by means of the volumetric method using a conventional BET system were observed to be Henry type in the temperature range 233-313 K and the pressure range P= 5-30 kPa, achieving the condition of small coverage. An example adsorption isotherm, for H-mordenite, is shown in Figure 17.3. 

The adsorption apparatus, shown in Figure 1, uses a static volumetric method to measure pure gas adsorption, and an open-flow adsorption/desorption method for mixture... 

Most adsorption data were collected by volumetric method until microbalance of high sensitivity appeared few years ago. It can hardly say which method is superior to the other, and both methods need the value of the skeleton volume of sample adsorbent. This volume has to be subtracted from the whole volume of the sample container to obtain the volume of void space, which is used for the calculation of the amount adsorbed. The skeleton volume of sample adsorbent was directly used in the calculation of buoyancy correction in gravimetric method. This volume was usually determined by helium assuming the amount of helium adsorbed was negligible. If, however, helium adsorption cannot be omitted, error would yield in the skeleton volume and, finally, in the calculated amount adsorbed. However, the effect of helium adsorption would be much less for volumetric method if the skeleton volume is considerably less than the volume of void space, but the volume of void space cannot affect buoyancy correction. In this respect, helium adsorption would result in less consequence on volumetric method especially when the skeleton volume was determined at room temperature and pressures less than IS MPa. The skeleton volume (or density) was taken for a parameter in modeling process in some gravimetric measurements. However, the true value of skeleton volume (or density) can hardly be more reliable basing on a fitted parameter than on a measured value. Therefore, one method of measurement cannot expel the other up to now, and the consequence of helium adsorption in the measured amount adsorbed should be estimated appropriately. 

Batch experiments were performed at 298 K via a standard volumetric method, using a 1.1 L glass reactor at atmospheric pressure, containing typically 0.5 g of adsorbent, polluted by liquid VOC injection, leading to an initial concentration of about 0.5 mmol.L. Equilibrium times were 1 and 2 hours respectively for Fau Y and Sil Z, after which the gas phase was sampled and analysed by chromatography (HP 5890II). Using the conventional assumption that the clay binder does not take part in the adsorption mechanism, data were reported for pure zeolite material. Reproducibility and repeatability of experimental data were checked by three different manipulators. 

Adsorption equilibrium and kinetics of Ar on the 4 adsorbents were measured by volumetric method at three temperatures. The constant volume apparatus and the calculation method have been described in detail elsewhere [3]. Small pressure steps ( 0.1 bar) were given to make sure that the measurements were in the linear range (q = Kc, where K is Henry law constant, q and c, in mmol/cc, are concentrations in adsorbed phase and gas phase). As such, constant, limiting kinetic parameters were extracted. 

The equilibrium parameters used in this study have been taken from independent single-component experiments using volumetric method at several temperatures. Adsorption isotherms on each adsorbent are shown in Figures 2 and 3. 

As with physical adsorption, there are three experimental techniques a dynamic volumetric method with chromatographic analysis, a static volumetric method and a gravimetric method. As the principles are strictly identical, they will not be discussed again here. 

As with cell shapes of a real foam, cell sizes in this material can also be characterized only by nominal (effective) values. The actual effective values depend, first, on the observation method (whether direct — macroscopic, or indirect — adsorption, volumetric, picnometric, etc.). Secondly, they depend on the particular simplified model of the structure and cell shape and thirdly on the method of processing the measured data. 

Measurement of weight changes is a very direct method of obtaining adsorption kinetics, and is also used to follow the course of other heterogeneous reactions (see Section 3). It is particularly suitable for adsorption measurements with small surface areas at relatively high pressures, where the volumetric method is insensitive. Several comprehensive reviews are available " which discuss the theory,... 

The adsorption and desorption isotherms of an inert gas (classically N2 at 77 K) on an outgassed sample are determined as a function of the relative pressure (Prei = p/Po/ the ratio between the applied pressure and the saturation pressure. The adsorption isotherm is determined by measuring the quantity of gas adsorbed for each value of p/po by a gravimetric or a volumetric method (less accurate but simpler). A surface acoustic wave device can also be used as a mass sensor or microbalance in order to determine the adsorption isotherms of small thin films samples (only 0.2 cm of sample are required in the cell) [42,43]. 

Waldman M, Vanecek M. 1982. Volumetric method for the determination of carbon disulfide in air using personal sampling and adsorption by active charcoal. Ann Occup Hyg 25 5-15. 

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