You can't measure something unless you can see it. Scientists have loads of instruments to detect things by all kinds of methods, but the most popular and simplest has to be UV-vis spectroscopy. Shine some light over your stuff, see how much gets through, you know something about what's there. UV light is particularly popular - anyone who's ever done DNA work has used 260nm light for this purpose.
One problem, though, is that just about everything absorbs some UV light - 260nm is pretty well-behaved, but not without its difficulties. Colored things, however, are a bit rarer. If you can selectively make your stuff into colored stuff, you're on your way to being able to detect it selectively.
2,4-dinitrophenylhydrazine will form a complex with just about any aldehyde or ketone. It's usually a solid, too, which means you have a melting point to work with. Used to be popular, now it's pretty much something you do sophomore year of college in organic lab.
2,4-DNPH, however, hasn't gone away - it still finds use in industry and government labs today.
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Fluorescence is overall better than absorbance for detection because the background can be subtracted before the measurement is taken. Net single photons can be counted. Good qualitatively, mediocre quantitatively.
Hello.
Are you sure of your Lewis form?
I would have put the hydrazine group in meta.
(sorry for my not pretty good english...)
See you.
Parapluie
some of these classical reagents are tried and tested, and all the problems have been sorted. -plus they are cheap.
2,4DNP is still used after all it's stable, easy to store, works very well, has some useful spectra, and inexpensive.