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In film developing, photographic developer (or just developer) is a chemical that makes the latent image on the film or print visible. It does this by reducing the silver halides that have been exposed to light to metals of elemental silver in the gelatine matrix. As a generalisation, the longer a developer is allowed to work, the greater the degree of reduction of the silver halide crystals to silver and therefore the darker the image.
Formulae
For black and white photography, the developer is often a mixture of metol and hydroquinone. These are made up in aqueous solution with sodium carbonate to create the appropriately high pH and with sodium sulfite to delay oxidation of the developing agents by atmospheric oxygen. Hydroquinone is superadditive with metol, meaning that it acts to "recharge" the metol after it has been oxidized in the process of reducing silver in the emulsion. Sulfite in a developer not only acts to prevent aerial oxidation of the developing agents in solution, it also facilitates the replenishment of metol by hydroquinone (reducing compensation and adjacency effects) and in high enough concentrations acts as a silver solvent, meaning that it blurs the appearance of grain and fine detail in the negative.
Most developers also contain small amounts of potassium bromide to modify and restrain the action of the developer to enable penetration of the developer throughout the emulsion before over-development occurs on the surface - chemical fogging. This also changes the colour of the silver image from sepia (low bromide) through black to blue (high bromide). Developers for high contrast work have higher concentrations of hydroquinone and lower concentrations of metol and tend to use strong alkalis such as sodium hydroxide to push the pH up to around pH14.
Because Metol is difficult to dissolve in alkaline solutions, instructions for mixing developer formulae almost always list metol first. It is important to dissolve chemicals in the order in which they are listed. Some photographers add a pinch of sodium sulfite before dissolving the metol to prevent oxidation, but large amounts of sulfite in solution will prevent metol from dissolving.
Because metol is relatively toxic and can cause skin sensitization, modern commercial developers often use phenidone instead. Hydroquinone, while only moderately toxic, can be an environmental hazard, so some modern developers replace it with ascorbic acid, or Vitamin C. The latter, however, suffers from poor stability, particularly when dissolved in water containing minerals. Distilled water is recommended for mixing any developer stock solution, but this is particularly important for those containing ascorbate. Ascorbate developers may have the advantage of being compensating and sharpness-enhancing, as oxidation by-products formed during development are acidic, meaning they retard development in and adjacent to areas of high activity. This also explains why ascorbate developers have poor keeping properties, as oxidized ascorbate is both ineffective as a developing agent and lowers the pH of the solution, making the remaining developing agents less active.
Other developing agents in use are paraminophenol, glycin, pyrogallol and catechol. In the proper formula, the latter two substances not only reduce silver, they stain the gelatin of the film in proportion to the image density, a property that is highly sought after by some photographers because it increases negative contrast in relation to density, meaning that highlight detail can be captured without "blocking" (reaching high enough density that detail and tonality are severely compromised). Hydroquinone shares this property. However, the staining effect only appears in solutions with very little sulfite, and most hydroquinone developers contain substantial quantities of sulfite.
Other modern constituents include small amounts of wetting agents and small amounts of alcohol, often propanol.
In the early days of photography, a wide range of developing agents were used, including Chlorohydroquinone, ferrous oxalate, hydroxylamine, lactate of iron, ferrous citrate, Eikonogen, atchecin, resorcinol, antipyrin , acetanilid and Amidol (which unusually required midly acidic conditions).
Development
The developer reduces the silver halides in the latent image in the exposed photograph into reduced, opaque, black silver metal. The image is then fixed using photographic fixer.
The mechanism by which this reduction occurs preferentially on those halide grains containing the silver atoms of the latent image is complex. The developer molecule (typical a relatively simple benzene-ring molecule) may act as an 'electron bridge'. (See www.kodak.com for more detailed discussion).
The time over which development takes place, and the type of developer, affect the relationship between the density of silver in the developed image and the quantity of light. This study is called sensitometry and was pioneered by F Hurter & V C Driffield in the late 1800s.
Color development
In color and chromogenic black and white photography, a similar development process is used except that the reducing silver catalyses the production of dye-stuffs in the emulsion. There are three distinct processes used here. The C41 process is used for almost all colour negative films and in this process dye couplers in the emulsion react with dye formers in the developer to generate the visible dyes. An almost identical process is then used to produce colour prints from films. The developing agents used are derivates of paraphenylene diamine.
Reversal film development
In many color transparencies, the film is first processed in an MQ (Metol-Hydroquinone) black and white developer for 6:00 at 100.4F (38C); with more time yielding "push" processing to increase the apparent film speed. The first developer is the most critical step in Process E-6. The solution is essentially a black-and-white film developer, because it forms only a negative silver image in each layer of the film; no dye images are formed. Then, the film goes directly into the first wash for 2:00 at 100F, which acts as a controlled stop bath. Next, the film goes into the reversal bath. This step prepares the film for the color-developer step. In the reversal bath, a chemical reversal agent is absorbed into the emulsion. No chemical reaction takes place until the film enters the color developer. The reversal process can also be carried out using 800 footcandle-seconds of light, which is used by engineers to troubleshoot reversal bath problems. Next, the film is developed to completion in a color developer bath. When film enters the color developer, the reversal agent absorbed by the emulsion in the reversal bath chemically exposes the unexposed silver halide. The color developing agent then reacts with the silver halide to form metallic silver. (The metallic silver image formed in the first developer is not a part of the reaction that takes place in this step.) The color developer acts on the chemically exposed silver halide to form a positive silver image. As the metallic silver image is formed, the oxidized color developing agent reacts with the color couplers in each of the three layers of the film to form colored dyes. The dyes form only at the sites where the image is being converted to metallic silver. Next, the film goes into the pre-bleach (formerly conditioner) bath, which has formaldehyde and EDTA to "kick off" the bleach. Next, the film goes into a bleach solution. The bleach converts metallic silver into silver bromide, which is converted to soluble silver compounds in the fixer. During bleaching, iron (III) EDTA is changed to iron (II) EDTA. Fe+++ EDTA + Ag° + Br−→ Fe++ EDTA + AgBr before fixing, and final wash. The most common processing chemistry for such films is E6, derived from a long line of developers produced for the Ektachrome range of films.
The steps in the E-6 process per Kodak Process Control Manual Z-6 are as follows:
gamma.
Proprietary methods
The Kodachrome process is a proprietary process in which all the dyes are added to the emulsion during development.
In colour print development, the Cibachrome process also uses a print material with the dye-stuffs present and which are bleached out in an appropriate places during developing. The chemistry involved here is wholly different from C41 chemistry; (it uses azo-dyes which are much more resistant to fading in sunlight).
"Photographic developer"