Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

62 EXPERIMENTAL METHODS FOR SLOW REACTIONS(b) Chemical actinometersChemical actinometers are simpler to use than thermopiles and are often morereliable because of the extreme care needed to calibrate and set up the latter.However, they first have to be calibrated for their useful wavelength range andthe corresponding quantum yield Cp. Several gaseous actinometers are given inTable 9 together with the relevant details of Cp and wavelength. Any chemicalactinometer is most conveniently used when it and the system being studied canbe considered as completely absorbing. Apart from one exception, the actinometerscited are considered to be the most reliable over the vacuum-near uv range. Thereis a distinct lack of actinometers for the visible region. Only one reaction has beencited as such. As a liquid phase actinometer, the long-standing uranyl oxalateactinometer is now being replaced by the 1000 times more sensitive ferrioxalateactinometer”’, although Porter and Volman16’ claim that with GSC (FID) estimationof CO production, the uranyl oxalate system is the most sensitive available.(c) PhotocellsLike chemical actinometers, photocells have to be calibrated against a thermopile-galvanometersystem; this has to be done frequently as there tends to be somevariation with time. Under these conditions, they can be used to measure the absoluteintensity of monochromatic light. The cell best suited for photochemicalstudies is the photoemissive type, which operates via the photoelectric emissionof electrons from an irradiated-TMsurface. The metallic cathode is mounted either ina vacuum or in a small pressure of one of the inert gases. The cell may involve asingle phototube or a multielement photomultiplier. An amplification of about lo6is achieved with the latter.;.BGlass bulbs are suitable only for the visible part of the spectrum but can be extendedto theuv by fitting quartz windows. However, the sensitivity drops with decreasingwavelength. A much better technique is to coat the face of the phototube witha layer of fluorescent material which extends the useful range to 850A161. Sodium0- ----____ - --- GalvanometerFig. 49. Basic photocell circuit. From ref. 22c.-