Editing Photographic developer (section)

==Chemical composition of developers==
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:<ref name="Wall">Wall, E. J. (1890). ''Dictionary of Photography''. London, England: Hassel, Watson and Viney.</ref><sup>:115</sup>

*Developing agents. Popular developing agents are [[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 agent such as [[sodium carbonate]], [[borax]], or [[sodium hydroxide]] to create the appropriately high [[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.<ref>"[https://archive.org/stream/KodakDataBookVolume12/KodakDataBookVolume12_djvu.txt Full text of "Kodak Data Book Volume 1 & 2"]". Accessed 30 September 2017</ref>

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. The original lithographic developer contained formaldehyde (often added as paraformaldehyde powder) in a low sulfite/bisulfite solution.

Most developers also contain small amounts of [[potassium bromide]] to modify and restrain the action of the developer<ref name=Wall/><sup>:218-219</sup> to suppress ''chemical fogging''. Developers for high contrast work have higher concentrations of hydroquinone and lower concentrations of metol and tend to use strong [[alkali]]s such as [[sodium hydroxide]] to push the pH up to around pH 11 to 12.

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.{{Citation needed|date=November 2017}}

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]],<ref name=Wall/><sup>:131</sup> [[hydroxylamine]], [[ferrous lactate]], [[ferrous citrate]], [[Eikonogen]], [[atchecin]], [[antipyrin]], [[Acetanilide|acetanilid]] and [[Amidol]] (which unusually required mildly acidic conditions).

Developers also contain a water softening agent to prevent calcium scum formation (e.g., EDTA salts, sodium tripolyphosphate, NTA salts, etc.).

The original lithographic developer was based upon a low sulfite/bisulfite developer with formaldehyde (added as the powder paraformaldehyde). The very low sulfite, high hydroquinone and high alkalinity encouraged "infectious development" (exposed developing silver halide crystals collided with unexposed silver halide crystals, causing them to also reduce) which enhanced the edge effect in line images. These high energy developers had a short tray life, but when used within their tray life provided consistent usable results.

Modern lithographic developers contain hydrazine compounds, tetrazolium compounds and other amine contrast boosters to increase contrast without relying on the classic hydroquinone-only lithographic developer formulation. The modern formulae are very similar to rapid access developers (except for those additives) and therefore they enjoy long tray life. However, classic lithographic developers using hydroquinone alone suffers very poor tray life and inconsistent results.