Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

44 EXPERIMENTAL METHODS FOR SLOW REACTIONSdeterioration in the lamp and to follow the intensity, respectively. Melville" foundthat, with different pressures of Inert gas in the low-pressurelamp, maximum intensityof output was obtained at 180mA (Fig. 34). Heidt and Boyles report that to main-tain an intensity to & 1 %, the current through the lamp must be controlled to- $0.6 7; at 120-130 mA and k0.8 at MmA, and the temperature controlled tof0.1" at 130mA and +0.05" at 15mA' 17. However, the lamp may be heated to600" C without serious change in light intensity" '. Gomer and Kistiakow~ky'~~discuss the control of intensity for medium-pressure lamps. All lamps are subjectto some "ageing" as far as intensity is concerned, and the latter may be seriouslyaffected if the quartz or other material is not kept clean by wiping with diluteacetic acid or absolute alcohol.The lamps may be operated on AC or DC, but for polynitxisation studies andradical lifetime measurements DC must be used. For a low-pressure lamp theintensity falls to nearly zero 120 times per second for a 60 cycles AC source'24;the high-pressure lamp also tends to be unstable with AC125. Circuits (Fig. 35)are shown for medium-pressure'26 and high-pressure'' lamps; operating proceduresare given quite clearly in these last two references. The low-pressure lampreaches stable operating conditions quite quickly, but the other two types requiresom- time to do so (10-15 min). The high-pressure lamp also has a very shortlifetime.Fig. 3 I . Typical medium-pressure lamp. Hanovia Chemical and Manufacturing CO., 500 w. Seeref. 9, p. 698.