Photographic developer
The special feature of photography is that the developer acts more quickly on those particles of silver halide that have been exposed to light.
The developer typically consists of a mixture of chemical compounds prepared as an aqueous solution.
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.
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.
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.
Recently, claims for practical methods to improve the stability of ascorbate developers have been made by several experimenters.
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).
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.
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.
[4] The light sensitive layer or emulsion consists of silver halide crystals in a gelatin base.
The areas with the most light exposure use up the tiny amount of developer in the gelatin and stop making silver crystal before the film at that point is totally opaque.
There is less contrast, but time is not critical and films from several customers and different exposures will develop satisfactorily.
[5] In colour and chromogenic black-and-white photography, a similar development process is used except that the reduction of silver simultaneously oxidizes the paraphenylene colour developing agent which then takes part in the production of dye-stuffs in the emulsion by reacting with the appropriate couplers.
This black and white developer is used for 6:00 at 100.4°F (38°C), with more time yielding "push" processing to increase the apparent film speed by reducing the Dmax, or maximum density.
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.
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.
