In the processing of photographic films, plates and papers, a developer is a chemical solution that converts the latent image to a visible image. This may be a negative image, with the emulsion darkened by light, requiring a further imaging process to produce a print where highlights and shadows are not reversed, or a positive image which can be viewed or projected. In the gelatin silver process, the developer achieves this conversion by reducing the silver halides, which are pale-colored, into silver metal, which is black when in the form of fine particles. Although the developer will react with all grains of silver halide, it acts much more quickly on those that have been exposed to light, which is how it produces an image: when left in developer, all the silver halides will eventually be reduced and turn black. The longer a developer is allowed to work, the darker the image. To produce a usable image, development must be stopped at the appropriate time. The following sections first describe monochrome, black-and-white, development, then colour processes which add coloured dyes to the silver blackening process.
Chemical composition of developers
Main components The developer typically consists of a mixture of chemical compounds prepared as an aqueous solution. For black-and-white photography, three main components of this mixture are::115
Developing agents such as metol (monomethyl-p-aminophenol hemisulfate), phenidone (1-phenyl-3-pyrazolidinone), dimezone (4,4-dimethyl-1-phenylpyrazolidin-3-one) and hydroquinone (benzene-1,4-diol). Dimezone is thought to resist oxidation in solution better than Phenidone A, but it is not as available as Phenidone A. Dimezone is also known as Phenidone B. Alkaline agents such as sodium carbonate, borax, or sodium hydroxide to raise the pH. Sodium sulfite to delay oxidation of the developing agents by atmospheric oxygen. Notable standard formulas include Eastman Kodak D-76 film developer, D-72 print developer, and D-96 motion picture negative developer. Hydroquinone is superadditive with metol, meaning that it acts to "recharge" the metol after it has been oxidised 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 regeneration of metol by hydroquinone (reducing compensation and adjacency effects) and in high enough concentrations acts as a silver halide solvent. As well as the developing agent, the activator and the preservative, most developers contain small amounts of potassium bromide to modify and restrain the action of the developer to suppress chemical fogging. 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 increase the pH up to around 11 to 12. Developers also contain a water softening agent to prevent calcium scum formation (e.g., EDTA salts, sodium tripolyphosphate, NTA salts, etc.). Metol is difficult to dissolve in solutions of high salt content and instructions for mixing developer formulae therefore 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 make the metol very slow to dissolve. Because metol is relatively toxic and can cause skin sensitisation, modern commercial developers often use phenidone or dimezone S (4-hydroxymethyl-4-methyl-1-phenyl-3-pyrazolidone) instead. Dimezone, Dimezone S, is a white crystalline powder that is soluble in water and polar solvents. DD-X, HC-110, TMax developer, and PQ Universal developer are a few common film developers that use Dimezone as a developing agent. Dimezone is acutely toxic and an irritant. Hydroquinone can also be toxic to the human operator as well as environment; some modern developers replace it with ascorbic acid, or vitamin C. This, however, suffers from poor stability. 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 oxidised ascorbate is both ineffective as a developing agent and lowers the pH of the solution, making the remaining developing agents less active. Recently, claims for practical methods to improve the stability of ascorbate developers have been made by several experimenters.
Other developing agents Other developing agents in use are p-aminophenol, glycin (N-(4-hydroxyphenyl)glycine), pyrogallol, and catechol. When used in low sulfite developer composition, the latter two compounds cause gelatin to harden and stain in the vicinity of developing grains. Generally, the optical density of the stain increases in the heavily exposed (and heavily developed) area. This is 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. In the early days of photography, a wide range of developing agents were used, including chlorohydroquinone, ferrous oxalate,:131 hydroxylamine, ferrous lactate, ferrous citrate, Eikonogen, atchecin, antipyrin, acetanilid and Amidol (which unusually required mildly acidic conditions).
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