Adsorption gravimetric methods

The number of gas molecules can be measured either directly with a balance (gravimetric method) or calculated from the pressure difference of the gas in a fixed volume upon adsorption (manometric method). The most frequently apphed method to derive the monolayer capacity is a method developed by Brunauer, Emmett, and Teller (BET) [1], Starting from the Langmuir equation (monolayer adsorption) they developed a multilayer adsorption model that allows the calculation of the specific surface area of a sohd. The BET equation is typically expressed in its linear form as... 

The thermal conductivity detector used in the continuous flow method can sense signals corresponding to less than 0.001 cm of adsorption with 1 % accuracy, causing it to be considerably more sensitive to small amounts of adsorption than the volumetric or gravimetric methods. 

Gas chromatographic (GC) methods also provide possibilities to detect the irreversibly held amounts of a poison (51), although these techniques are less accurate than the gravimetric methods. The number of irreversibly adsorbed molecules can be calculated from the material balance for successively injected pulses of the poison and the eluted amounts. Alternatively, adsorption equilibrium can be attained at low temperature, the adsorbed amount being determined by frontal analysis (51). Desorption may then be carried out at the same temperature and the irreversibly held amount can be calculated either from the difference between adsorbed and desorbed amounts of a first cycle or from the difference of the adsorbed amounts of a first and a second adsorption (52). Desorption temperatures can then be raised stepwise after the first desorption and the dependence of the irreversibly adsorbed amounts on desorption temperature determined from the corresponding desorbed amounts. The accuracy of these GC measurements is limited because of the usually very pronounced tailing of the desorption trace for the systems of interest. 

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. 

Adsorption from the gas phase can be measured by either gravimetric or volumetric techniques. In the gravimetric method, the weight of adsorbed gas is measured by observing the stretching of a helical spring from which the adsorbent is himg (see Fig. 2).f Alternatively,... 

E. Loren Fuller, Jr..E. Robens, et. al. Volumetric and Gravimetric Methods for Adsorption Isotherms Evaluation, Thermochim Acta. 29 (1979) 315. 

The principles of solution thermodynamics can be applied to absolute adsorption variables without any of these complications. For absolute variables, which arise naturally in molecular simulation, the pressure is a single-valued function of n, the differential functions exhibit no singularities, and the selectivity approaches a finite value as P —> oo. Absolute adsorption may be determined experimentally by measuring excess adsorption in the usual way (volumetric or gravimetric method) at sub-atmospheric pressure where the difference between absolute and excess adsorption is negligible. 

The objective of this study is to compare the strengfti of adsorptive interaction between adsorbents and thiophene/benzene. Extremely low partial pressures at less than 10 atm would be necessary to meet this objective if isotherms were measured at ambient temperature, because the isotherms at ambient temperature are fairly flat and are difficult to compare. However, it is very difficult to obtain and control such low partial pressures experimentally. Therefore, single component isothoms for benzene and thiophene were measured at 90, 120 and 180 °C using standard gravimetric methods. A Shimadzu TGA-50 automatic recording microbalance was employed. Isosteric heats of adsorption were calculated using the Clausius-Clapeyron equation from isotherms at different temperatures. 

Volumetric versus gravimetric method of adsorption measurement... 

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. 

Gravimetric or manometric techniques have been used to establish adsorption data of gases on zeolites. Both techniques present problems, manometric equipment has an accumulation of the error and data obtained by the gravimetric method are influenced by effects associated with flow patterns, bypassing, and buoyancy. In the mixture s adsorption behaviour, isomers mixtures have the highest degree of difficulty to study. Isomers can not be differentiated in standard commercial adsorption equipment. This problem has been solved in this study by coupling a manometric apparatus with an NIR spectrometer, which allows us to measure the gas phase composition (in time, if necessaiy). In this paper we report this new approach to study the adsorption of mixtures of butane and iso-butane. 

Isotherms and dynamics of CH2CI2 adsorption for commercial resin and high silica zeolite adsorbents were measured by a flow type gravimetric method. 

Adsorption equilibrium was measured by a flow type gravimetric method. 

In this study, equilibria and isosteric heat of adsorption for the system of chlorinated hydrocarbons and Y-type zeolite were obtained with gravimetric method and chromatographic method. By comparing an experiment result with a molecular simulation result, the validity of forcefield parameters and zeohte model was exartuned... 

The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method was used to get the initial slope of the isotherms. In the simulation, GCMC method was used to calculate amounts adsorbed for various conditions. When the experiment result and simulation result of chloroform are compared, the simulation for the acid site model was most agreement with chromatographic data and baratron data. The simulation result of tetrachloroethylene with three models corresponded mostly for the non-polar molecule, and above all the acid site model was the closest to the experiment result. Therefore, to get better coincidence between experimental data and simulation, it was found to be necessary to account for aluminum rather than silanol nest. 

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. 

From a practical standpoint the measurement of highly accurate adsorptions at higher pressures poses some dificulties. In the gravimetric method only small quantities of sample can normally be used and hence the amount of external surfece actually present in the... 

The adsorption/desorption isotherms of benzene vapors were measured at 293K by gravimetric method using the McBain-Bakr balance. 

Two kinds of pitch-based ACFs (P5 and P20 Osaka Gas Co.) were used. The microporous structure was determined by high-resolution N, adsorption isotherms at 77 K using a gravimetric method. The micropore structual parameters were obtained from high-resolution a, -plot analysis with subtracting pore effect (SPE) method. The average slit pore width w was determined from the micropore volume and the surface area. The adsorption isotherms of methanol and ethanol on carbon samples were gravimetrically measured at 303 K. The sample was preevacuated at 10 mPa and 383 K for 2h. 

Determined by N2 adsorption isotherms. Determined by gravimetric method. 

The extent of adsorption of gases onto solid surfaces can be determined experimentally using a wide variety of apparatus and techniques, and the literature on this subject is extensive. In general, measurements fall into one of two categories either the volume of the gas adsorbed is determined manometrically, or gravimetric methods are used, where the mass adsorbed on the solid is determined directly. 

In the gravimetric method, the adsorbent (usually in the form of powder) is placed into a bulb, which is mounted on a sensitive balance and the bulb is then evacuated. Next, the weight increase of the adsorbent solid as a function of the absorptive gas pressure is monitored at constant temperature. More recently, the quartz crystal microbalance (QCM) technique has been applied this is very sensitive to mass increases. Quartz is a piezoelectric material and the thin crystal can be excited to oscillate in a traverse shear mode at its resonance frequency when a.c. voltage is applied across the metal (usually gold) electrodes, which are layered on two faces of the crystal. When the mass on the crystal increases upon adsorption, its resonance frequency decreases. The increase in the mass is calculated from the reduction in resonance frequency. On the other hand, adsorption on single flat surfaces can also be measured by ellipsometry, which measures the film thickness of transparent films optically using the difference between light reflection from bare and adsorbed surfaces. 

Coull, Engel, and Miller17 have developed a method in which an organic vapor-air mixture of any desired composition can be passed through the carbon bed. The exit gases are analyzed continuously by a thermal-conductivity, cell, the accuracy of which has been confirmed by gravimetric methods. The resulting data allow a ready evaluation of the performance of the carbon and yield information as to the adsorption efficiency after the break point is reached. 

Volumetric and gravimetric methods are the most exphcit and common methods used to display and quantify adsorption. 

In order to build a thermodynamic model of physical adsorption, it is important to take note of a few experimental results. The adsorption isotherms acquired through the volumetric or the gravimetric method allow us to make sure that the... 

Physical adsorption isotherms are generally obtained experimentally using a volumetric or gravimetric method. Before we tiy to obtain meaningftil results from the theoretical expressions we have arrived at using Hill s or Hill and Everett s... 

The concentration-distance curves method is based on the measurement of the distribution of the diffusant concentration as a function of time. Light interference methods, as well as radiation adsorption or simply gravimetric methods, can be used for concentration measurements. Various sample geometries can be used, for example semiinflnite solid, two joint cylinders with the same or different material, and so on. The analysis is based on the solution of Pick s equation. 

There are two main ways to measure the amount of adsorption gravimetric and volumetric. In gravimetric methods the uptake is measured as the increase in weight of a sample as the adsorbate pressure is varied. This requires great accuracy in weighing, which is usually performed using an electrobalance. In... 

---