![]() What seems clear, though, is that MQ and PQ developers are synergistic because the metol/phenidone electron donors at the surface of the crystals are replenished by hydroquinone.ĭevelopers require a soup of other chemicals to ensure that they act efficiently. A third theory has it that the crystal is an electrical circuit which is put into play by the developer, depositing silver through electrolysis. Another theory has it that the negative charge on the surface of the halide cystal repels the electrons provided by the developing agent except where silver has already been deposited on the surface (the latent image). One theory has it that the developing agent injects electrons into the halide crystals thereby effecting the deposition of more metallic silver in an autocatalytic process. metallic silver sensitve points) catalyses image formation is much in dispute. And Hydroquinone-only developers are very specialist, often only used in current darkrooms for lith development in developers like Kodak D-85, Ansco 70 and NewLith. The benzene rings gives great solubility and stability of the developing agent, which allows for regeneration.īut this diagram, often reproduced in textbooks, is a very great simplification. The diagram below gives the reaction between Silver bromide and Hydroquinone: There are usually one or two electron-rich groups of atoms, attached to the ring, which provide the necessary electron to initiate development. Both are indispensible to the darkroom practitioner.Ĭommon to these developers are benzene rings. One might think that without Hydroquinone, D-23/D-25 would be low contrast developers but this view is unjustified.įor further information on the ‘pros and cons’ of each developer I recommend ‘The Film Developing Cookbook’ by Anchell and Troop also ‘The Darkroom Cookbook’ by Anchell. ![]() Metol-only developers are also used, such as D-23. Examples include HC-110, Acutol and Microphen. PQ developers tend to give an increase in speed. A well-known MQ developer is Kodak’s D-76 (or Ilford’s ID11). Metol and phenidone are super-additive with Hydroquinone, meaning that they act synergistically together. It is often paired with metol, (MQ developers) or phenidone (PQ developers) where it regenerates metol and phenidone respectively. It is relatively unstable, being easily oxidised in air. Hydroquinone adds density and contrast when paired with with other agents. Less comonly used, although important for certain applications are Catechol, Pyrogallic acid, Para-Aminophenol and Amidol. The most commonly used developing agents are Hydroquinone, Metol, Ascorbic acid and Phenidone. Phenidone can substitute for Metol, but resulting in different outcomes. Ascorbic acid is the developing agent in Kodak’s XTOL and can substitute for Hydroquinone. p-Phenylenediamine forms the basis of Diamine and is used in colour development. The structure of Metol is based on a aminophenol. Hydroquinone is found in many developers combined with other developers. In general, black and white developing agents have a di- or poly- hydroxybenzene structure, that is, a benzene carbon ring with two or more OH groups.Ĭommonly used developers. The evolution of developers has been a see-saw of balancing between many objectives. The chemical has to tolerate a certain pH range, be soluble, operate within a certain temperature range, be stable and so on. However, not any chemical that supplies electrons will do. When a grain with a latent image center is immersed in developer solution after exposure, these few silver atoms catalyze the reduction of the entire silver halide grain to approximately 1 billion atoms of metallic silver. The main function of the developing agent is to supply electrons which are required for the reduction of silver ions into metallic silver. A wide range of chemicals can be used to develop images, both organic and inorganic. Let’s start with looking at developing agents. ![]() In this post I want to move onto the next stage: developing an image. It is thought that a mimimum structure of 4 silver atoms is required in order that the next step ensues. The process repeats until a structure of 4 silver atoms form a latent sensitive point. This attracts interstitial silver ions and metaiilic silver is formed, which in turn can act as a hole. The photo-electron gets trapped in holes caused by imperfections inn the lattice, making it negatively charged. The resulting photo-electron and bromine can move through the halide lattice crystal. A photon of light excites an electron of the bromine atom in the silver halide.
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