Conventional and slow reactions

The most widely used routine spectroscopic methods are those based on the absorp-tion/emission of UV-vis light. The UV-vis method is generally preferred over IR and NMR spectroscopy because (a) it is more sensitive, so more dilute solutions may be used, and (b) water and other common solvents present fewer problems with UV-vis than with IR and NMR methods. It is rare, but not impossible, that the light source used initiates some photochemical process, and this should be checked. 

A reaction which is complete within, say, a day may be monitored for most of its course whereas a slower reaction would usually be studied using the initial rates method (see Section 3.3.1) using data collected during the initial stages - perhaps just the first 1% of the reaction. 

Laboratory spectrophotometers and fluorimeters which allow monitoring of reactions that take place within a few seconds are fairly routine now, and time resolutions of tenths of a second are available detailed descriptions are available in texts on spectroscopy [26]. Solution cells may be glass or plastic for light in the visible range, but quartz cells are needed for UV work. In an absorption spectrophotometer, the light source and photodetector are in line. In a fluorimeter, the detector is at 90° to the incident light, so cells must have four optical faces. 

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