Page 131 - Gas Adsorption Equilibria

Chapter 3

GRAVIMETRY

Abstract The physical principles and experimental techniques of pure gas- and
multicomponent gas adsorption measurements by the gravimetric method are
outlined. Two beam microbalances and the magnetic suspension balance are
considered. Several examples are given and uncertainties of data are discussed.
Pros and cons of the gravimetric method are outlined. Lists of symbols. References.

1. INTRODUCTION

Gravimetry is a fairly new method for investigation of sorption
phenomena of gases in porous solids, although comparing masses by
weighing them in the gravity field of earth is a very old technique going back
to biblical times [3.1]. The main reason for the newness of this has been the
lack of highly sensitive balances able to measure small relative changes in the
weight of a sorbent sample of – for example – or even less. Though
at the end of the nineteenth century fairly accurate mechanical balances were
available in many European countries [3.1], it was only with the design of the
electro-magnetically compensated two beam microbalance by Th. Gast,
manufactured commercially by Sartorius, Göttingen, Germany during
1965-1995, that gravimetric gas adsorption measurements became feasible for
non-corrosive gases within a certain range of pressures (< 15 MPa) and
temperatures (< 450 K). The next milestone was the development of the single
beam magnetic suspension balance (MSB) initiated also by Th. Gast but
really accomplished by Lösch, Kleinrahm, and Wagner in the 1980s and today
commercially available from Rubotherm GmbH, Bochum, Germany [3.2-3.6].
In a MSB the gas and sorbent filled vessel is completely mechanically
decoupled from the microbalance thus allowing also measurements with
highly corrosive sorptive gases in a broad range of pressures (< 100 MPa) and
temperatures (< 1500 K). Today there are many companies around the world
offering single beam and two beam microbalances operating both under
vacuum and also high pressure conditions. A list of suppliers of vacuum