Superadditive developer
Encyclopedia
Photographic developer solutions may contain more than one developing agents, such as Metol
and hydroquinone
, or Phenidone
and hydroquinone
. This is because they work together to a synergistic effect, called superadditive development.
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.
Of the two developing agents that are superadditive, the following is usually the case:
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: Phenidone
, Dimezone S, Metol
, p-aminophenol, glycin
, 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: hydroquinone
and ascorbic acid
. 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.
Metol
Metol is the chemical compound with the name monomethyl-p-aminophenol hemisulfate. It is a developing agent used in black & white photographic developers...
and hydroquinone
Hydroquinone
Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H42. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid...
, or Phenidone
Phenidone
Phenidone is an organic chemical compound whose primary use is 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 dermatitis upon skin contact.Phenidone is Ilford's trademark for this...
and hydroquinone
Hydroquinone
Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H42. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid...
. This is because they work together to a synergistic effect, called superadditive development.
Definition
Superadditive development commonly means that the mathematical superadditive 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 regenereation 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: Phenidone
Phenidone
Phenidone is an organic chemical compound whose primary use is 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 dermatitis upon skin contact.Phenidone is Ilford's trademark for this...
, Dimezone S, Metol
Metol
Metol is the chemical compound with the name monomethyl-p-aminophenol hemisulfate. It is a developing agent used in black & white photographic developers...
, p-aminophenol, glycin
Glycin
Glycin, or N-glycine, is N-substituted p-aminophenol. It is a photographic developing agent used in classic B&W developer solutions. It is a derivative of the amino acid glycine. When fresh, it is typically characterized as thin plates of white or silvery powder, turning brown with age...
, 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: hydroquinone
Hydroquinone
Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H42. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid...
and ascorbic acid
Ascorbic acid
Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...
. 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.