Amidol

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Amidol is a colourless crystalline compound, C₆H₃(NH₂)2OH·2HCl, used as a photographic developer. It was introduced as a developing agent for photographic papers in 1892. It is unusual amongst developers as it works most effectively in slightly acid conditions rather than the strongly alkaline conditions required for most other developers. As Amidol ages it changes colour to a dark red-brown. Developing dishes and equipment used to prepare Amidol solutions are also frequently stained brown, a stain that is very persistent.

Prints developed in Amidol are typically a very warm brown-black colour but overdevelopment can quickly lead to chemical fogging

Chemically, it is 2,4-diaminophenol dihydrochloride. Its CAS number is [137-09-7] and its SMILES structure is NC1=CC(N)=C(O)C=C1.