Glycin
From Silvergrain Research
Photographic glycin is N-(4-hydroxyphenyl)glycine (CAS 122-87-2), and it is different from glycine in other worlds
N-(4-hydroxyphenyl)glycine (photographic glycin)
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Glycine and "glycin"
- glycine = aminoacetic acid
- glycin = N-(4-hydroxyphenyl)glycine
In chemistry and biochemistry, glycine is aminoacetic acid. Aminoacetic acid forms complex ions with silver and may be used as a silver halide complexing (solvent) agent in developers.
However, in photographic chemistry, especially among darkroom workers, glycin without the ending "e" commonly referrs to N-(4-hydroxyphenyl)glycine (sometimes called p-hydroxyphenylglycine). The N-(4-hydroxyphenyl)glycine has enough potential to reduce silver halides to metallic silver, and used to be popular as a developing agent. This may be confusing for those who are outside photographic practice, or trying to buy chemicals from non-photographic chemical suppliers. In fact, I don't know of non-photographic supplier of N-(4-hydroxyphenyl)glycine.
Synthesis and current supplier
The only current supplier of glycin is Photographer's Formulary. Considering possible ways to synthesize this compound, it is most likely that the Formulary is reacting p-aminophenol and chloracetic acid to carboxylate the amine in the p-aminophenol to produce glycin. Synthesis of glycin by this method requires no special equipment besides safety devices. However, chloracetic acid is very nasty to handle in all but well equipped laboratory or plant.
"Weakened Metol"
Glycin once was a popular developing agent, and for this reason, it's described with very favorable words in old literature. They are also blindly cut and pasted in some darkroom cookbook literatures. However, the reality is that glycin disappeared from mainstream because Metol was superior developing agent. Glycin is "chemically weakened Metol." At least in early age of Metol production, Metol was manufactured from glycin by decarboxylation (removal of -COOH at the right end of the picture above). That is, they used to make Metol out of glycin because of Metol's superiority. (In that sense, it is probably better to call Metol a "chemically disinhibited glycin.") That is, the photographic characteristics of glycin and Metol can be matched rather easily by adjusting the quantity, pH and other usual variables of developer formulation. MQ developers with different M-to-Q ratios and different pH could cover a very wide range of developer needs.
Therefore, using glycin is literally reinventing inferior wheels.
Popular "classic" uses of glycin
There are two major places where glycin is highly praised. One is very long shelf life as well as tray life of the print developer solution. This author found that the same could be achieved with almost any developing agent, given suitable stabilizing agent is used in the developer. It is particularly proven true with ascorbate developer DS-14, which can be left in an open tray overnight with no ill effect.
Another praise given for glycin is warm tone print developer. However, hydroquinone and its derivatives are superior warm tone developer, although they require longer development time, higher pH or both. Again, an MQ formulation with suitable MQ ratio would solve this problem.
Glycin was once used as a surface developer by T.H.James and other researchers of emulsion sensitivity and development mechanism. This was because it was possible to use this agent with little or no sulfite preservative, which is a mild silver halide solvent. However, glycin itself has some solvent effect, and therefore James and others moved on to Metol-Ascorbate developer MAA-1 for the research purpose. These may be a reason why glycin was reputed for accutance developing agent, but in reality, it is easy to formulate Metol developer of comparable accutance and overall better image quality.
Outside of photography, N-(4-hydroxyphenyl)glycine is used in analytical chemistry, in tests for iron, phosphorus, etc. In terms of toxicology, kidney damage is reported after a prolonged exposure to this agent.
