Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

206 EXPERXMENTAL METHODS FOR HETEROGENEOUS REACTIONSI DewarsealVibratlnelectroc%--Fig. 15. Cell for themeasurement of contact potential difference by the vibrating condensermethod. From ref. 131.The inner reference cylinder can be moved vertically and maintained concentricwith the outer section. With the inner cylinder raised, a metal film can be evaporatedto form the outer cylinder of the capacitor. The reference surface is prepared bycoating the outside of the glass cylillder with a mixture of stannous oxide+ 1antimany pentoxide. For most evaporated films, the condenser has a capacityof 30 pf. It is essential to screen effectively from electromagnetic and electrostaticinterference. This is done by earthing the fllament, an inner stannous oxide coatingand an outer coating of liquid platinum.The contact potential is found by applying an opposing potential until the nullpoint is reached. It is essential that this potential is applied rapidly, in a timemuch smaller than the time constant of the circuit comprising the condenser andthe external resistance. This is achieved by an integrating dc amplifier coupled to aservo motor which actuates a helipot’30. A reproducibility of k0.03 volts isclaimed, with results in good agreement with other literature values.A cell suitable for measurements by the vibrating condenser method was devisedby MignoletI3l and is shown in Fig. 15. The principle is that of an electricallydriven hollow tuning fork with the tail rigidly fixed in an iron block. The fundamentalvibration frequency is about 450 c. sec” and it will stand an amplitude of