Antifoggant
From Silvergrain Labs
Contents |
Mechanism
The exact mechanisms of organic antifoggants are not completely understood, but there are two proposed mechanisms:
Solubility depressor
Most antifoggants make very sparingly soluble compound with silver ion. If the solubility constant is smaller than that of the silver halide used in the emulsion, then the agent acts as an antifoggant by lowering the cathode potential of the developing silver electrode.
Adsorption competitor
Most antifoggants strongly adsorb on the surface of silver halide crystals. If this adsorption is stronger than adsorption of the developing agent, the agent acts as an antifoggant by drsorbing some of the developer molecules.
Useful compounds
All of the compounds listed below are heterocyclic nitrogen compounds. It is probably safe to say that ALL of indispensable organic antifoggants are heterocycles containing one or more nitrogen atom. Heterocyclic compounds are still important research area in chemistry, especially in relation to biochemistry and pharmacology.
E. J. Birr, who discovered 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (TAI) as am emulsion stabiliser, left a famous quote "you must master the heterocyclic chemistry if you want some success in synthesis and in photographic application" (c. 1930).
1-phenyl-5-mercaptotetrazole
(1-Phenyl-1H-tetrazole-5-thiol, abbr. PMT, CAS 86-93-1, MW 178.21)
This is a very potent antifoggant and also a very powerful emulsion stabilizer.
As an emulsion stabilizer, it is used most commonly for chloride emulsions (with or without bromide and iodide). When used in small quantity with cubic (100) crystal emulsions, it may enhance speed. However, when it is used with octahedral (111) crystal emulsions, it can only decrease speed. It may be used effectively with (111) or tabular grain bromide emulsions, in very small quantity, and therefore usually used in combination with other agents.
1H-benzotriazole
(1,2,3-benzotriazole, abbr. BTA, CAS 95-14-7, MW 119.12)
One of the least expensive antifoggants and extensively used in black-and-white developers and emulsions. It is not very effective with many color developers; color materials (and developers) generally utilize more strongly adsorbing antifoggants, such as 5-methylbenzotriazole.
Although not very common, benzotriazoles with halogenated benzene ring are very potent and effective. These variants are also known to increase the speed of AgCl (100) crystals, but not (111) crystals.
5-methylbenzotriazole
(abbr. MBTA)
This methylated benzotriazole has more strong adsorption to AgX crystals and therefore more effective with aromatic diamine developers used for color processing. (Phenylenediamines adsorb very strongly on AgX crystal surface, and effective antifoggants must adsorb more strongly than the developer.)
5-nitrobenzimidazole
(aka. 6-nitrobenzimidazole, abbr. NBI, usually available in nitrate salt)
Common developer antifoggants and emulsion stabilizer for chlorobromide and bromide emulsions.
2-mercaptothiazoline
2-mercaptobenzimidazole
(abbr. MBI)
It was a common antifoggant and emulsion stabilizer from 1920s to 1930s, until better compounds were discovered. It sufferes emulsion speed in many cases.
This compound is only very sparingly soluble in water, and it is not very easy to use in processing solutions. However, 5-sulfonic acid of this compound is soluble, and is commonly used as antistaining agent in replenished developers. Also, similarly solubilized derivatives of 2-mercaptoimidazole (e.g., 6-amino-4-hydroxy-2-mercaptoimidazole) are commonly used as antistaining agents in replenished developers.
4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene
(modern name 7-Hydroxy-5-methyl[1,2,4]triazolo[1,5-a]pyrimidine, old name 5-methyl-7-hydroxytriazaindolizine, abbr. TAI, CAS 2503-56-2, MW 150.14)
4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene
This compound is only slightly adsorbed on AgBr surface. It is adsorbed on AgCl surface. The adsorption is rather selective of a crystallographic surface.
This agent is mostly used as an emulsion stabilizer in bromide emulsions (with or without iodide component). It was discovered by E. J. Birr and was used exclusively by German manufacturers until 1945, when this knowledge became public. (Gold sensitization of Koslowski was another example of pre-WWII German technology that became publica in 1945.) This emulsion stabilizer is very effective and does not decrease emulsion speed, so it is still extensively used in its original form, or as slightly modified derivatives, in most bromide and iodobromide emulsions.
