Photographic developer solutions may contain more than one developing agents, such as

Metol Metol (or Elon) is a trade name for the organic compound with the formula OC6H4NH2(CH3)SO4. It is the sulfate salt of ''N''-methylaminophenol. This colourless salt is a popular photographic developer used in black & white photography.Gerd Löb ...

and

hydroquinone Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''para'' ...

, or

Phenidone Phenidone (1-phenyl-3-pyrazolidinone) is an organic compound that is primarily used as a photographic developer. It has five to ten times the developing power as Metol. It also has low toxicity and unlike some other developers, does not cause derm ...

and

. This is because they work together to a synergistic effect, called superadditive development.

Definition

Superadditive development commonly means that the mathematical

superadditive In mathematics, a function f is superadditive if f(x+y) \geq f(x) + f(y) for all x and y in the domain of f. Similarly, a sequence \left\, n \geq 1, is called superadditive if it satisfies the inequality a_ \geq a_n + a_m for all m and n. The ter ...

inequality holds for the rate of development. That is, the rate of development of the two (or more) agents together is greater than the sum of the rate of each agent used alone. There are cases that ''density'' superadditivity holds without ''rate'' superadditivity. Note that the term superadditive development is usually ''not'' used to mean density superadditivity: the image density obtained from the combined agents is greater than the sum of the density from each agent used alone.

Mechanism

There were several historical theories about the mechanism of superadditive development, until G. I. P. Levenson elucidated the regeneration mechanism. It is useful to review modern treatment of the development mechanism described as an electrode process. Of the two developing agents that are superadditive, the following is usually the case: # Agent 1 has lower reduction potential but much higher adsorption to the silver halide crystals. # Agent 2 has higher reduction potential but much poorer adsorption to the silver halide crystals. Agent 1 is the developer that reduces the silver halide crystals. Oxidation product of Agent 1 is reduced back to the original form by Agent 2. In other words, the ultimate source of the electrons used to develop image comes from Agent 2. For this reason, in modern convention, Agent 1 is usually called ''electron transferring agent'' or ETA. Agent 2 is usually called the main developing agent. As obvious from the above mechanism, developing agent with very stable semiquinone radical form (the first oxidized form of the ETA) tends to make more superadditive combination. Also obvious from the above mechanism is that Agent 1 has to have an intermediate reduction potential ''in the developer solution'' between the Fermi energy level of silver (the developing image silver) and the reduction potential of Agent 2 in the developer solution. Examples of Agent 1 include:

, Dimezone S,

p-aminophenol 4-Aminophenol (or ''para''-aminophenol or ''p''-aminophenol) is an organic compound with the formula H2NC6H4OH. Typically available as a white powder, it is commonly used as a developer for black-and-white film, marketed under the name Rodinal. Re ...

glycin Glycin, or N-(4-hydroxyphenyl)glycine, is N-substituted p-aminophenol. It is a photographic developing agent used in classic black-and-white developer solutions. It is unrelated to the amino acid glycine. It is typically characterized as thin pl ...

, Eikonogen. These agents have nitrogen atoms that give strong affinity to silver halide (argentophilic) as well as hydrophobic end, effectively acting as a ''surfactant'' between the developer solution and silver halide crystals. Examples of Agent 2 include:

and

ascorbic acid Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) an ...

. These agents have stronger reduction potentials than those of Agent 1 group, but they have very poor adsorption on silver halide crystals, due to lack of argentophilic end. However, there are many developer combinations that exhibit superadditive development.

Hydroquinone developers

References

Science of photography Photographic processes {{photography-stub