Oxidation of Phenols to Benzoquinones
Oxidation of Phenols to Benzoquinones: Applications
The quinone-hydroquinone redox system serves as the basis for the so-called quinhydrone electrode, a workhorse for pH measurements.
Hydroquinone itself is an important photographic developer. Photographic developers are used to enhance the sensitivity of films that contain silver bromide as light-sensitive compound. Exposure to light splits silver bromide into silver atoms and bromine radicals.
Finely dispersed silver in its amorphous state is black. Bromine radicals react with the gelatine in which AgBr crystalls are embedded. Developers increase the blackening of the film and thus light-sensitivity. Crystalls of silver bromide that contain even just one silver atom as seed are completely reduced to silver.
In the reduction process, silver seeds act as autocatalysts. Pure AgBr is either not reduced or only after a longer induction period. In the subsequent fixing process, unreacted AgBr is transformed with sodium thiosulfate into water-soluble complexes which are removed by rinsing.