It can be prepared by the reaction of hydrazine sulfate with 2,4-dinitrochlorobenzene:[1] DNPH is a reagent in instructional laboratories on qualitative organic analysis.

A positive test is signalled by the formation of a yellow, orange or red precipitate of the dinitrophenylhydrazone.

Mechanistically, it is an example of addition-elimination reaction: nucleophilic addition of the -NH2 group to the C=O carbonyl group, followed by the elimination of a H2O molecule:[3] DNP-derived hydrazones have characteristic melting points, facilitating identification of the carbonyl.

DNPH does not react with other carbonyl-containing functional groups such as carboxylic acids, amides, and esters, for which there is resonance-associated stability as a lone-pair of electrons interacts with the p orbital of the carbonyl carbon resulting in increased delocalization in the molecule.

For this reason, it’s supplied damp or ‘wetted’ when a school purchases it from a chemical supplier.