Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ANALYSIS 89netic resonance has least direct application to reactions in the gas phase; althoughits potential has been discussedz8’, no in situ measurements have been made. Insolution kinetics, where it has been used extensively, the situation is very different.However, it should be helpful for product identification, particularly from thepoint of view of site attack. It appears to have some potential use in combustionstudies286. The other regions of the spectrum are discussed in three sections below.4.4.2 uv, visible and IRMcDowell and Thomasz8’ have used the following set-up (Fig. 60) for the spectrophotometricanalysis for nitrogen dioxide. Light from a sodium lamp is focussedby lenses onto EEL selenium photocells either through the RV onto PI orthrough an iris diaphragm onto P,. R1 and R, are variable resistances and G agalvanometer. When the external resistance is 1000 R, the response curves of thephotocells become logarithmic. Since R, is proportional to the optical density(= Ecd, where E is the decadic molar extinction coefficient, c is the concentrationand d is the path length), R, is also proportional to the pressure of NO,. Goldenet ~ 1 . ’ have ~ ~ modified a Cary Model 15 spectrophotometer for the direct measurementof methyl iodide, hydrogen iodide and iodine in the equilibrium systemI,+CH,MeI+HIMeasurements were made at 500,350,257,245 and 235 mp. 350 mp is an “opticalwindow” for this system and therefore a reference point. Calibrations were madewith the substances concerned. Beer’s law was obeyed up to 30 torr. I, was directlydetermined from the measurement at 500mp. The absorbance for I, was subtractedat the other wavelengths, leaving values from which the pressures of Me1and HI could be determined. In this way, lo-’ pmole of each could be detected.Other spectrophotometric estimations are cited in ref. 22c. Morse and Kaufman289have measured the concentrations of oxygen, nitrogen and hydrogenReferences pp. 104-111Fig. 60. Spectrophotometric determination of NOI. From ref. 287.