Most developers also 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 push the pH up to around pH 11 to 12.
Because Metol is difficult to dissolve in solutions of high salt content, 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 make it very slow for Metol to dissolve.
Because Metol is relatively toxic and can cause skin sensitisation, modern commercial developers often use Phenidone or Dimezone S instead. 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 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, hydroxylamine, ferrous lactate, ferrous citrate, Eikonogen, atchecin, antipyrin, acetanilid and Amidol (which unusually required mildly acidic conditions).
Developers also contain water softening agent to prevent calcium scum formation (e.g., EDTA salts, sodium tripolyphosphate, NTA salts, etc.).
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.
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.
In colour negative films, there are 3 types of dye couplers. There are the normal cyan, magenta and yellow dye forming couplers, but also there is a magenta coloured cyan masking coupler and a yellow coloured magenta masking coupler. These form respectively normal cyan dye, and magenta dye, but form an orange positive mask to correct colour. In addition, there is a third type of coupler called a DIR (Developer Inhibitor Release) coupler. This coupler releases a powerful inhibitor during dye formation, which affects edge effects and causes effects between layers to enhance overall image quality.
Next, the film is developed to completion in the colour developer bath, which contains CD-3 as the colour developing agent. When film enters the colour developer, the reversal agent absorbed by the emulsion in the reversal bath chemically fogs (or "exposes") the unexposed silver halide (if it has not already been fogged by light in the previous step). The colour developer acts on the chemically exposed silver halide to form a positive silver image. (The metallic silver image formed in the first developer, which is a negative image, is not a part of the reaction that takes place in this step. What is being reacted in this stage is the "leftover" of the negative image, that is, a positive image.) As the colour development progresses, metallic silver image is formed, but more importantly, the colour developing agent is oxidised. Oxidised colour developer molecules react with the couplers to form colour dyes in situ. That is, colour dye is formed at the site of development in each of the three layers of the film. Each layer of the film contains different couplers, which react with the same oxidised developer molecules but form different colour dyes. Next, the film goes into the pre-bleach (formerly conditioner) bath, which has a precursor of formaldehyde (as a dye preservative) 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.
Standard black and white stock can also be reversal processed to give black and white slides. After 'first development,' the initial silver image is then removed (e.g. using a potassium bichromate/sulfuric acid bleach, which requires a subsequent "clearing bath" to remove the chromate stain from the film). The unfixed film is then fogged (physically or chemically) and 'second-developed'. (See say M J Langford Advanced Photography Focal Press (1980) p345 for materials details; kits containing the necessary fogging and bleach agents are also available commercially). However the process works best with slow films such as Ilford Pan-F processed to give a high gamma. Kodak's chemistry kit for reversing Panatomic-X ("Direct Positive Film Developing Outfit") used sodium bisulfate in place of sulfuric acid in the bleach, and used a fogging developer that was inherently unstable, and had to be mixed and used within a two hour period. (If two rolls, the maximum capacity of a single pint of redeveloper, were to be processed in succession, the redeveloper had to be mixed while the first roll was in the first developer.)
In colour print development, the Cibachrome process also uses a print material with the dye-stuffs present and which are bleached out in 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).
The chemical process can be described as ferro sulfate splitting into iron and sulfurous acid leaving a radical oxygen molecule that combines with the silver forming silver oxide.