Vacuum microbalance

This method is smiple but experimentally more cumbersome than the volumetric method and involves the use of a vacuum microbalance or beam balance [22], The solid is suspended from one ann of a balance and its increase in weight when adsorption occurs is measured directly. The dead space calculation is thereby avoided entirely but a buoyancy correction is required to obtain accurate data. Nowadays this method is rarely used. 

P. A. Cutting, in Vacuum Microbalance Techniques 7,51, Plenum Press, New York (1970). 

Surface Studies with the Vacuum Microbalance Instrumentation and Low-Temperature Applications T. N. Rhodin, Jr. 

Surface Studies with the Vacuum Microbalance High-Temperature Reactions Earl A. Gulbransen The Heterogeneous Oxidation of Carbon Monoxide Morris Katz... 

High-temperature reactions with vacuum microbalance, 5 119 High-temperature reduction, 34 19 effects on titania-supported metals, 36 176-177, 180... 

A chemical investigation of the surface oxides on diamond was undertaken by Boehm et al. (35). Using a fine particle size diamond powder with a specifie surface area of 17 m /gm, the oxidation was studied by use of a vacuum microbalance. Formation of surface oxides started at a measurable rate with pure oxygen at 260°. A weight loss due to formation of carbon oxides became apparent above 360°. 

This experiment is typical of many that are conducted using classical adsorption techniques, such as the measurement of isotherms and control of evacuation temperature and pressure. There is a high probability that such a treatment will increase the number of OH groups on the surface, but there is no direct evidence of this. An improvement could be made if the sample were continuously weighed during the cycle in a vacuum microbalance (160), but even this would not be entirely free from ambiguity. 

As shown previously (Section 14.7), thermal transpiration or thermomole-cular flow arises when gas is contained in two vessels at equal pressures but different temperatures and a connection is made between the vessels. A vacuum microbalance represents just such a system. The long tube surrounding the hang-down wire separates the sample, immersed in the coolant bath from the warm upper portion of the apparatus. With vacuum microbalances the effects of thermal transpiration are further exacerbated by the temperature gradient along the hang-down wire. 

All vacuum microbalances suitable for surface studies can best be described in terms of four different classes with operating and performance characteristics unique to each class. The advent of the simple precision microbalance seems to be with Warburg and Ihmori (6) in 1886. From then on numerous designs have been described. Excellent reviews cover the steady progress in precision microgravimetry from those of Pettersson (7) (Sweden, 1915), through Emich (8) (Germany,... 

One of the classical applications of the vacuum microbalance for adsorption studies was that of Barrett, Birnie, and Cohen (32) in 1940. They measured the physical adsorption of water vapor on silica surfaces at 30°C. by direct weighing. They employed a quartz beam balance using a cemented-on tungsten wire (0.001 in. diameter) and enclosed in a high vacuum housing. The capacity of the balance was relatively high (0.5 g.)... 

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