Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

268 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSmillidebye units. The determination of surface potentials is often carried outsimultaneously with measurements of area as the film is compressed. Film balancesare often designed with provision for carrying out these measurements. Inhomogeneityof the film, or surface contamination, will cause a variation in AY over thefilm and surface potential measurements can be used to scan the film surface.Measurement of surface potentials can be accomplished by the ionising electrodeor the vibrating condenser method. The former is simpler experimentally, but thelatter can also be used for liquid-liquid interfaces.(a) The ionising electrode methodThis method has been extensively used by Schulman and Ridea1377 and isindicated diagrammatically in Fig. 47. A small amount of radioactive material,commonly polonium, is mounted on or near the air electrode. The a-particleradiation ionises the air between the electrode and the liquid, making it conducting.An electrometer can then be used to measure the potential difference across thegap. Normally, the potential difference is balanced against a standard cell witha potentiometer to obtain a null reading. The electrode E is usually silver-silverchloride, although others have been tried. The main requirement is freedom fromappreciable drift during the duration of the experiment. There are references inthe literature to the preparation of radioactive sources, but they are now readilyavailable commercially.-Fig. 47. Ionising electrode method for the determination of surface potentials. Po, poloniumelectrode; E, reference electrode; G, galvanometer. From ref. 1.The range of polonium a-particles in air is 3-8 cm, so that effective ionisationcan be achieved with the electrode up to 4 cm from the surface. If the radioactivematerial is covered with a protective coating, the effective range will be reduced.Polonium (210Po) is convenient because it emits negligible p- and y-radiation andspecial shielding is not required. Its disadvantage is its short half-life (140 days).Radium (226Ra) has a half-life of 1590 years, but requires greater handling precautionsbecause of its p- and y-emission. Recently, plutonium (' 34Pu, half-life24000 years) and americium (241Am, half-life 458 years) have been used becauseshielding problems are minimised. However, in handling any radioactive materialthe necessary precautions must be observed246* 249.