Page 93 - Gas Adsorption Equilibria

Chapter 2

VOLUMETRY / MANOMETRY

Abstract The physical principles and experimental techniques of pure gas- and multi-
component gas adsorption measurements by the volumetric (or manometric) method
are outlined. Examples are given. Thermovolumetric and combined volumetric–
calorimetric measurements are presented. Pros and cons of the method are discussed.
References. List of Symbols.

1. INTRODUCTION

Volumetry or, preferably “Manometry”, cp. Sect. 2.1, is the oldest method
to investigate sorption of gases in solids. Early experiments had already been
performed by C. W. Scheele (1777), Chappuis (1881), W. Ostwald (1905) and
J. Langmuir (1912), [1.1]. Some of these experiments were designed to
determine the volume of geometrically complicated objects like cauliflower or
sea-urchins. A special instrument for this purpose was designed by R. W. Pohl
about 1940 and was called a “volumimeter” [2.1]. It served as a prototype of
many of today’s volumetric instruments.

The physics of volumetric gas adsorption experiments is simple: a given
amount of sorptive gas is expanded into a vessel which includes a sorbent
sample and which initially has been evacuated. Upon expansion the sorptive
gas is partly adsorbed on the (external and internal) surface of the sorbent
material, partly remaining as gas phase around the sorbent. By a mass
balance, the amount of gas being adsorbed can be calculated if the void
volume of the sorbent, i. e. the volume which can not be penetrated by the
sorptive gas molecules is known – at least approximately.

The adsorbed mass per unit mass of sorbent is a characteristic quantity for
a porous solid. Assuming a characteristic area per single molecule adsorbed
on the surface of a solid – for at 77 K this area is for Kr at 87 K
it is [2.2, p. 173] – the surface area of the solid can be calculated
and from this the porous solid can be “characterized”. Actually, many