Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLIP-GAS INTERFACE 2070.7 mm. This is greater than the normal separation of the condenser plates, 0.1to 0.3 mm. The vibrating electrode is attached to the glass. The fixed electrode isflexibly supported so that it can be moved to one side to allow evaporation of ametal Mm onto the vibrating electrode. Adequate screening against stray electromagneticor electrostatic currents is essential, as for the steady condenser method.A major difficulty with contact potential work is the provision of a completelyinert reference electrode. Poisoned nickel has been used131, but aged gold orplatinum is more widely applicable. Bewig and Zisman' have described referenceelectrodes of gold and platinum coated with Teflon resin. These electrodes weremore stable than the bare metals in wet and dry oxygen, nitrogen and in carbondioxide, hydrogen and helium.2.1.6 Measurement of magnetic changesChemisorption often involves the unpaired electrons in solids, causing a changein the magnetic properties due to a change in the number of unpaired electronsper adsorbed atom or molecule. Because the magnetic properties are bulk properties,the surface-to-volume ratio must be high in order to make the changes detectable.Powder samples are often the most suitable for this reason. The classicalmethod of following magnetic changes is by measurement of magnetic susceptibility,but more recently the techniques of nuclear magnetic resonance (NMR) and electronspin resonance (ESR) have been employed. At the present time, magnetic methodshave been more widely used to study finite changes in the magnetic propertiesrather than the kinetics of the adsorption process.(a) Measurement of magnetic susceptibilityIf a substance is placed in a magnetic field, the intensity of magnetization maybe slightly smaller (diamagnetic), or larger (paramagnetic) than that produced ina vacuum by the same field. A few materials are ferromagnetic, i.e. the intensityof magnetisation is very much larger than for diamagnetic and paramagneticsubstances, and depends upon field strength in a complicated way. There are anumber of methods for measuring magnetic susceptibility' 34. One which has beenmodified for making chemisorption measurements is the Faraday Method' "* ' 36.If the sample is placed in an inhomogeneous magnetic field with an axis ofsymmetry, such as that formed by two appropriately shaped pole pieces, a forcef will be exerted on the sample whereHere, m is the mass of the sample, x is the mass susceptibility, H the field strengthand aH/aS the field gradient. Selwood et a1.135s136 measured the force by theReferences pp. 270-278