Gas adsorption gravimetry

It is evident that two independent sets of measurements are required in the 

There are a number of potential sources of error in gravimetric measurements and particular attention is drawn to the following  

A different type of extremely sensitive gravimetric technique is based on the effe change of mass on the resonance frequency of a vibrating quartz crystal (see Fi 3.12). In this case, the adsorbent must be firmly attached to the crystal. Its area be as small as a few square centimetres and mass changes as low as 10-2 pg ca detected from the frequency shift (Krim and Watts, 1991). 

---

Figure 3.23. Gas adsorption gravimetry percentage enor in surface area as a result of neglecting totally the buoyancy effect on the adsorbent (assumptions nitrogen BET, monolayer at 100 mbar, bulk density of adsorbent from 1 to 8).

Figure 3.24. Gas adsorption gravimetry temperature difference between sample and cryostat (at 77 K) as the vacuum around the sample is progressively reduced (after Rouquerol and Davy, 1978).

Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar... 

Adsorption gravimetry lends itself to in-situ outgassing (up to 500°C with our equipment) but can only accommodate one sample at a time. A major advantage only obtained with the special assembly making use of a sinker is the permanent measurement of the density of the gas phase it allows not only to make at any time a correct buoyancy correction but it also allows to provide the users of adsorption manometry equipment with the data they need to make a safe void volume correction. 

Adsorption and diffusion of alkanes in zeolites and in well-structured porous materials like MCM-41 materials are studied widely [1,5,8,9,11], The reported difusivities however differ sometimes by orders of magnitude. These differences are sometimes attributed to the use of microscopic techniques in stead of macroscopic techniques [12]. We, however, think that a major part of the found differences must be imputed to the use of a carrier gas. Adsorption is often studied in diluted systems with methods as ZLC [3], gaschromatography [4], inverse gas chromatography [10], gravimetry [12], > ile others are not using carriers gasses at all. 

Gas adsorption equilibria can be measured by several basically different methods. In this section we are going to outline the classical ones, namely volumetry/manometry and gravimetry as well as some newer ones, oscillometry and impedance spectroscopy. Emphasis is given to the underlying physical principles. Complementary remarks deal with possibilities to measure binary coadsorption equilibria with and without gas phase analysis. Technical details of all the measurement methods are given in the subsequent chapters, Chaps. (2-6). Prior to considering the measurement methods some general remarks on experimental work with gas adsorption systems are in order. 

This chapter is organized as follows In Sect. 2 we consider pure gas adsorption measurements by both two beam and single beam balances. Section 3 is devoted to thermogravimetry. In Section 4 multicomponent gas adsorption equilibria are discussed. Finally in Sect. 5 pros and cons of gravimetry especially compared to volumetry/manometry are elucidated. A list of symbols and abbreviations used is given followed by references dted. 

Evolution of the external surface area and the two types of microporosity of atiapulgite (structural and inter-fiber) were examined as a function of a vacuum thermal treatment upt to 500°C. The methods used include controlled transformation rate thermal analysis, N2 and Ar low temperature adsorption calorimetry, water vapor adsorption gravimetry and quasi equilibrium gas adsorption procedure of N2 at 77K and CO2 at 273 and 293K. Depending on the outgassing conditions,i.e. the residual pressure, the structure folds 150 to 70 C. For lower temperature, only a part (18%) of the structural microp< osity is available to N2,13% to argon and 100% to CC>2.With water, the structure can rehydrate after the structure is folded up to an outgassing temperature of 225°C. 

We report in Figures 1 and 2, with the same presentation and scale, the experimental results obtained by adsorption manometry and gravimetry, for the three gases and the three temperatures. Incidentally, this is a good example where the formerly used representation in adsorbed volume or adsorbed mass is clearly less convenient than the more universal representation in adsorbed amount (or still more precisely in the present case, in surface excess amount ). The shapes of the adsorption isotherms, with very different curvatures from one gas to the other, are a clear indication of the increased gas/solid interaction as one passes from argon to nitrogen and then to carbon dioxide, for which the isotherms are practically of type I. 

Contrary to manometry/volumetry, very high and very low pressures of the sorptive gas do not pose a serious problem in gravimetric adsorption measurements. Ibis is a due to the fact that in gravimetry, the adsorbed mass is determined by its weight, i. e. a quantity which in principle is physically independent of the gas pressure. 

Today there are several experimental methods available to measure pure gas and gas mixture adsorption equilibria on porous rigid or swelling sorbent materials. All these methods have their specific advantages and disadvantages [1]. Choice of any of them depends mainly on the purpose of measurement and/or accuracy and reliability of data needed. For quick measurements of restricted accuracy gas expansion experiments or volumetric measurements are recommended. If high accuracy data are needed, weighing procedures, i. e. gravimetry should be used... 

---