Volumetry/manometry

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. 

As we here are mainly interested in adsorption measurement techniques for industrial purposes, i. e. at elevated pressures (and temperatures), we restrict this chapter to volumetric instruments which on principle can do this for pure sorptive gases (N = 1), Sect. 2. Thermovolumetric measurements, i. e. volumetric/manometric measurements at high temperatures (300 K - 700 K) are considered in Sect. 3. In Section 4 volumetric-chromatographic measurements for multi-component gases (N>1), are considered as mixture gas adsorption is becoming more and more important for a growing number of industrial gas separation processes. In Section 5 we discuss combined volumetric-calorimetric measurements performed in a gas sensor calorimeter (GSC). Finally pros and cons of volumetry/manometry will be discussed in Sect 6, and a hst of symbols. Sect. 7, and references will be given at the end of the chapter. 

Gas adsorption processes may last for seconds, hours or - sometimes -even days. Therefore one never can be sure whether mermodynamic equilibrium in a volumetric experiment has been reahzed. Hence the time which should elapse between opening the expansion valve and reading of instruments, especially thermometer and manometer has to be chosen according to experience or accompanying gravimetric measurements which - contrary to volumetry / manometry - also provide information on the kinetics or the sorption process, cp. Chap. 3. 

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. 

In practice, activation of a sorbent material much more easily can be accomplished in gravimetric than in volumetric sorption instruments. The main reason for this is that in gravimetry the mass of the sample can be recorded even during the activation process whereas in volumetry / manometry it normally can not. Hence in gravimetry the initial state of a sorbens is fairly well known. Also presorption in a new sorbent sample and remnant sorption after a desorption experiment easily can be checked. 

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