Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

198 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSconstruction and applications of vacuum microbalances suitable for adsorptionmeasurements.The term vacuum microbalance is applied to instruments which can work undervacuum conditions of at least low6 torr, and in some cases down to torr, andfollow weight changes of the order of a few micrograms (pg = g). Foradsorption measurements with small surface areas, balances capable of detectingchanges of lo-' pg are desirable. Several types of balance have been used foradsorption studies. The simplest is the helical spring balance employing a springof silicag9, tungsten' O0 orcopper-beryllium all~y'~', as used by McBainet~Z.~~~'~'.Extension of the spring is proportional to the total load, so that increased sensitivityis only available at the expense of reduced capacity. In general, spring balancescannot be made as sensitive as other types and the buoyancy correction cannot beeliminated. Another troublesome feature is their sensitivity to vibration, especiallyin high vacuum where only internal friction effects damp the oscillations. Magneticdamping can be employed, but this reduces the useful load. Quartz is very fragileand must be carefully thermostatted if errors due to temperature changes are to beeliminated.Beam microbalances are of two types, pivotal balances and torsion microbalances.Pivotal microbalances have been used by Gregg'O' and by Czandernaand Honig' O4-Io6. The balance described by Czanderna106 will operate underultra high vacuum conditions with a load of up to 20 g, and can measure weightchanges of 2 x g. Torsion microbalances are suspended by a fine wire, usuallytungsten, which acts as the primary fulcrum. This type of microbalance has beenmost widely used and is described by Gulbransen9' and Rhoding6. By using specialcare in ensuring uniformity of cross section of the beam and equality of arm length,balances which can detect weight changes of to g can be constructed.The sensitivity can be made dependent on, or independent of load by suitabledesign. Wolsky and Zdanucklo7 used a thin silica beam which sagged whenloaded, thus enabling the sensitivity to increase with increasing load. One disadvantageof torsion microbalances is their tendency to undergo sideways yawing.This is often caused by shock or vibration which must therefore be avoided.Buoyancy corrections can be minimised with beam microbalances by trying tomake both limbs of the balance identical and using a counterweight with the samedensity as the sample. A more serious problem is the spurious mass changesproduced by thermomolecular flow. This effect has been treated theoretically byThomas and Poulis'os, and occurs at pressures between and 1 torr, with amaximum effect between lo-' and lo-' torr. This is the Knudsen pressure range,where the mean free path of the gas molecules is comparable with the dimensionsof the sample and the annular space between sample and balance tube. In order toeliminate these spurious mass changes, very elaborate thermostatting is necessary.One way to avoid this problem is perform the experiments in the presence of aninert gas in order to increase the total pressure above the Knudsen range.