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2.1 Colorimetry

In this technique, normally only used with pure solutions, i.e those having no suspended matter present, some of the incident light is absorbed by the species in solution. So the intensity of the emerging light, which passes straight through, to Detector I. is reduced, or "attenuated". The extent of this attenuation is, in most cases, directly proportional to the concentration of species present, a fact which is expressed as the Lambert-Beer Law. If a solution is coloured, even if not strongly, this technique can almost certainly be used. Even, in some cases, an apparently colourless solution can be analysed in the same way, using ultra-violet light (which is invisible to the naked eye) but which is absorbed by many organic compounds. Even when a solution is not coloured, this technique can be used, by adding certain dyes or other complexing agents, which form very brightly coloured solutions. Use of these can make the method extremely sensitive, often allowing analysis of solutions well below the 1 ppm concentration.

In its very simplest form, this technique uses daylight or artificial light as a source, employs a low-cost gelatin filter to produce (approximately) monochromatic light, and measures absorption by visual comparison, either with a known standard concentration solution or even with a coloured shade card. However most colorimeters have their own light source, use glass filters and measure light intensity with a photo-detector.

In even more sophisticated (though still reasonably priced) instrument, is the spectrophotometer, which uses an optical prism or grating to generate monochromatic light of any desired wavelength in the visible, or in better instruments, u.v- visible range of the spectrum. Such an instrument can do everything a colorimeter can do, and beyond that, can record a complete spectrum. Though colorimetry is mainly used for determination of metal ions, it can also be used for analysis of some organic Metal Finishing additives such as saccharin or coumarin.

Further details of the principles of colorimetry, equipment available and some 140 specific analytical methods, are included in the book by Knowles. Click here for a full listing of contents. Individual methods or sections of this book are available by post or email. Price - US$2.00 per page, plus shipping at cost (email free of charge).