Hubbry Logo
MetolMetolMain
Open search
Metol
Community hub
Metol
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Metol
Metol
Metol
View on Wikipedia
from Wikipedia

Metol
Names
IUPAC name
4-(methylamino)phenol sulfate
Other names
N-methyl-p-aminophenol sulfate, Pictol, p-(methylamino)phenol sulfate, monomethyl-p-aminophenol hemisulfate, Metol, Elon, Rhodol, Enel, Viterol, Scalol, Genol, Satrapol, Photol.
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.000.216 Edit this at Wikidata
KEGG
UNII
  • InChI=1S/2C7H9NO.H2O4S/c2*1-8-6-2-4-7(9)5-3-6;1-5(2,3)4/h2*2-5,8-9H,1H3;(H2,1,2,3,4) checkY
    Key: ZVNPWFOVUDMGRP-UHFFFAOYSA-N checkY
  • InChI=1/2C7H9NO.H2O4S/c2*1-8-6-2-4-7(9)5-3-6;1-5(2,3)4/h2*2-5,8-9H,1H3;(H2,1,2,3,4)
    Key: ZVNPWFOVUDMGRP-UHFFFAOYAW
  • O=S(=O)(O)O.Oc1ccc(NC)cc1.Oc1ccc(NC)cc1
Properties
(C7H10NO)2SO4
Molar mass 344.38 g/mol
Melting point 260 °C (500 °F; 533 K)
Hazards
GHS labelling:[1]
GHS08: Health hazardGHS07: Exclamation markGHS09: Environmental hazard
Warning
H302, H317, H373, H410
P260, P280, P301+P312+P330
Safety data sheet (SDS) Oxford MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Metol is a trade name for the organic compound with the formula [HOC6H4NH2(CH3)]2HSO4. It is the sulfate salt of N-methylaminophenol. This colourless salt is a popular photographic developer used in monochrome photography.[2]

Synthesis and degradation

[edit]

Several methods exist for the preparation of N-methylaminophenol. It arises by decarboxylation of N-4-hydroxyphenylglycine (Glycin). It can be obtained by reaction of hydroquinone with methylamine.[3]

Being an electron-rich arene, metol is readily degraded by hydrogen peroxide.[4]

Application

[edit]

Metol is an excellent developing agent for most continuous tone developer applications, and it has been widely used in published developer formulas as well as commercial products. However, it is difficult to produce highly concentrated developer solutions using Metol, and therefore, most Metol developers are supplied in dry chemical mix. A developer containing both Metol and hydroquinone is called an MQ developer. This combination of agents provides greater developer activity since the rate of development by both agents together is greater than the sum of rates of development by each agent used alone (superadditivity). This combination is very versatile; by varying the quantities of Metol, hydroquinone, and restrainer, and adjusting the pH, the entire range of continuous tone developers can be made. A popular variation of this is known as 'Dr. Beers.' Two separate solutions are made up: one with Metol; one with hydroquinone. By mixing the two in varying proportions, various levels of contrast can be obtained. Therefore, this form of Metol replaced most other developing agents except for hydroquinone, Phenidone (which is more recent than Metol), and derivatives of Phenidone. Notable formulas include Eastman Kodak D-76 film developer, D-72 print developer, and D-96 motion picture negative developer.

History

[edit]

Alfred Bogisch, working for a chemical company owned by Julius Hauff, discovered in 1891 that methylated p-aminophenol has more vigorous developing action than p-aminophenol. Hauff introduced this compound as a developing agent. The exact composition of Bogisch and Hauff's early Metol is unknown, but it was most likely methylated at the ortho position of the benzene ring (p-amino-o-methylphenol), rather than at the amino group. Some time later, Metol came to mean the N-methylated variety, and the o-methylated variety fell out of use. Aktien-Gesellschaft für Anilinfabrikation (AGFA) sold this compound under the trade name Metol, which became by far the most common name.

Because it has been in use for this purpose for over 100 years, and often by amateur photographers, there is a substantial body of evidence regarding the health problems that contact with Metol can cause. These are principally local dermatitis of the hands and forearms. There is also some evidence of sensitization dermatitis.[5]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Metol

View on Grokipedia
from Grokipedia
Metol is a for the 4-(methylamino)phenol hemisulfate, a white to faintly beige crystalline salt with the (C₇H₉NO)₂·H₂SO₄ and a molecular weight of 344.39 g/mol. Primarily utilized as a developing agent in black-and-white photographic processes, it reduces exposed grains in film and paper emulsions to metallic silver, enabling the formation of visible images. Introduced in 1891 by chemist Alfred Bogisch and marketed by Hauff, Metol represented an advancement in alkaline developers for gelatine dry plates, offering finer grain and more vigorous action compared to earlier agents like . By , it gained prominence in combination with as Metol-Hydroquinone (M-Q) developers, a formulation developed by the brothers and Seyewetz, which provided enhanced contrast and superadditive effects for both film and print development. This pairing became a standard in throughout the , though Metol alone produces softer, neutral-toned results suitable for work and fine-detail rendering. Chemically, Metol is highly soluble in water (up to 50 mg/mL) and ethanol but insoluble in ether, with a melting point of approximately 260 °C (decomposition). It is light-sensitive in pure form and typically stored as a stable hemisulfate salt to prevent oxidation. While its primary application remains in traditional photographic chemistry, Metol has also appeared in some medical and analytical contexts, though these are secondary to its role in imaging. Modern digital alternatives have reduced its prevalence, but it endures among analog photographers for its reliable performance in custom developer formulas.

Chemical properties

Structure and nomenclature

Metol is the hemisulfate salt of 4-(methylamino)phenol, consisting of two molecules of the base associated with one molecule of . Its molecular formula is \ce(C7H9NO)2⋅H2SO4\ce{(C7H9NO)2 \cdot H2SO4}, equivalent to \ceC14H20N2O6S\ce{C14H20N2O6S}. In this structure, the anion \ceSO42−\ce{SO4^2-} pairs with two protonated 4-(methylamino)phenol cations, where typically occurs on the amino group. The core structure features a ring with a hydroxyl group (-OH) attached at the 1-position and a methylamino group (-NHCH₃) at the para 4-position relative to the hydroxyl. This arrangement results in a phenolic amine derivative, with the two identical units linked ionically through the . The IUPAC name for Metol is 4-(methylamino)phenol (2:1), also referred to as 4-(methylamino)phenol hemisulfate. Common synonyms include N-methyl-p-aminophenol and p-methylaminophenol . Metol is derived from the parent compound p-aminophenol through N-methylation of the amino group, modifying its reactivity while retaining the para-substituted phenolic core.

Physical and chemical properties

Metol appears as a to faintly crystalline powder. Its is 344.38 g/mol. The compound has a of 260 °C, at which point it decomposes. Metol exhibits high solubility in water, approximately 50 g/L at 20 °C, and is also soluble in ethanol. It is insoluble in non-polar solvents such as ether and chloroform. Aqueous solutions of Metol are acidic, with a pH of 3.5–4.5 for a 50 g/L concentration at room temperature. Chemically, Metol serves as a , primarily due to its phenolic hydroxyl and amino groups, which facilitate donation in oxidation-reduction reactions. These functional groups render the molecule an electron-rich arene, making it susceptible to oxidation. Under normal storage conditions, Metol remains stable, though it is light-sensitive and can decompose in the presence of strong acids or bases.

Synthesis and degradation

Synthesis methods

Metol, the sulfate salt of N-methyl-4-aminophenol, is primarily synthesized industrially through the reaction of with , followed by treatment with to form the desired salt. This method leverages the reactivity of the phenolic hydroxyl group in toward under high-temperature conditions. The process begins by dissolving hydroquinone in water, optionally with a small amount of sodium sulfite as an antioxidant, in an autoclave. Aqueous methylamine (typically 30% solution) is then added gradually over several hours while heating to 150–225 °C under pressure (around 120 psi at lower temperatures), with the reaction continuing for an additional 8–10 hours after addition. The high temperature facilitates the displacement, yielding N-methyl-4-aminophenol. The reaction can be represented as: \ceC6H4(OH)2+CH3NH2−>HO−C6H4−NHCH3+H2O\ce{C6H4(OH)2 + CH3NH2 -> HO-C6H4-NHCH3 + H2O} Subsequent acidification with sulfuric acid converts the base to the stable bisulfate salt: \ce2HO−C6H4−NHCH3+H2SO4−>(HO−C6H4−NHCH3)2⋅H2SO4\ce{2 HO-C6H4-NHCH3 + H2SO4 -> (HO-C6H4-NHCH3)2 \cdot H2SO4} Reactions are typically conducted at these elevated temperatures to ensure complete conversion while minimizing tar formation; lower temperatures extend reaction time but reduce byproducts. To mitigate side reactions such as over-methylation or oxidation, excess (about 2 equivalents) is employed, and the addition is controlled to avoid local excesses. After cooling, the crude salt crystallizes from the solution. Yields typically range from 70% to 85%, with optimized conditions achieving up to 75% based on . Purification involves filtration of the crude crystals, washing with to remove impurities, and recrystallization from hot water or , resulting in colorless crystals with purity exceeding 98%, essential for its use in photographic developers. The of Metol in hot water facilitates this step without requiring additional solvents. An alternative laboratory or supplementary route involves the thermal decarboxylation of N-(4-hydroxyphenyl)glycine (also known as glycin) at 200–250 °C in the presence of an acid catalyst, directly yielding N-methyl-4-aminophenol, which is then sulfated as above. This method exploits the instability of the group under heating, providing a route from derivatives, though it is less prevalent in large-scale production due to the availability of .

Degradation pathways

Metol, the sulfate salt of N-methyl-p-aminophenol, undergoes oxidative degradation primarily through reactions involving hydroxyl radicals generated by oxidants such as (H₂O₂). This process is facilitated by the electron-rich phenolic ring, which enables rapid , leading to the formation of quinone imines as key intermediates and the release of ions. In like UV/H₂O₂ photolysis, the degradation follows pseudo-first-order kinetics, with hydroxyl radicals attacking the aromatic structure to produce hydroxylated byproducts and ultimately ring cleavage products. A representative reaction for oxidative breakdown by H₂O₂ can be summarized as: (\ceHO−C6H4−NHCH3)2⋅\ceH2SO4+\ceH2O2→oxidized dimers+\ceNH(CH3)2+\ceSO42−+\ceH2O(\ce{HO-C6H4-NHCH3})_2 \cdot \ce{H2SO4} + \ce{H2O2} \rightarrow \text{oxidized dimers} + \ce{NH(CH3)2} + \ce{SO4^2-} + \ce{H2O} This pathway is accelerated under acidic to neutral conditions (3–7), achieving significant mineralization, with no detectable aromatic compounds remaining after extended reaction times in systems (Fe²⁺/H₂O₂). Optimal conditions for such degradation include H₂O₂ concentrations around 0.2 M and Fe²⁺ at 9×10⁻⁴ M, resulting in up to 50% reduction within 2 hours. Photodegradation of Metol occurs slowly upon exposure to light, particularly in aqueous solutions, where it promotes oxidation and generates colored byproducts from partial breakdown of the phenolic moiety. This natural photo-oxidation is enhanced in the presence of oxygen but remains limited without additional oxidants, contrasting with faster catalyzed processes. Half-lives under oxidative conditions with H₂O₂ or UV assistance range from approximately 35 minutes at 9 to 180 minutes at 7, highlighting pH-dependent reactivity. Biological degradation of Metol in is limited due to its non-biodegradable nature as a nitrogen-containing organic , resulting in within aquatic environments. Microbial breakdown is minimal, attributed to the compound's low volatility and stable aromatic structure, which resists enzymatic attack in typical systems. In neutral water without oxidants, Metol exhibits longer , with estimated half-lives on the order of weeks.

Applications

Role in photographic development

Metol serves as a key in the development of black-and-white photographic materials, where it facilitates the conversion of exposed crystals into metallic silver grains that form the visible image. In alkaline solutions, Metol ionizes, allowing its amino-phenol functional groups to donate electrons to silver ions at the sensitivity specks of exposed halides, thereby reducing them to neutral silver atoms while the developer itself oxidizes. This selective reduction targets only the sites created by light exposure, preserving unexposed areas for later removal during fixing. One of Metol's primary advantages lies in its ability to produce fine-grain images with minimal chemical fog, making it ideal for achieving high detail and sharpness in negatives and prints. It also enables effective control of image contrast, developing to relatively low gamma values that emphasize shadow detail without excessive highlight density. When combined with in MQ formulations, Metol exhibits superadditivity, where hydroquinone regenerates oxidized Metol, resulting in a synergistic increase in development rate and overall activity—often accelerating the process significantly compared to either agent alone—while enhancing contrast. This pairing leverages Metol's rapid initial action with hydroquinone's sustained energy, yielding balanced . Development with Metol proceeds at a moderate pace, typically requiring 5 to 15 minutes at 20 °C for most fine-grain films, depending on the specific and type. Its application is confined to processes, as the colored oxidation products of Metol interfere with the dye-forming reactions essential for . Historically, Metol gained preference over earlier agents like for its non-staining properties and consistent sharpness, avoiding the tanning effects and variability associated with pyrogallol-based developers.

Developer formulations and combinations

Metol is commonly employed in metol-hydroquinone (MQ) developers, which typically incorporate Metol at concentrations of 1-3 g/L alongside at 5-12 g/L, dissolved in an alkaline medium buffered by (as a and solvent) and either (for higher activity) or (for milder action). These formulations leverage the superadditive between Metol and hydroquinone to achieve balanced development with fine grain and moderate contrast. A seminal example is D-76, a fine-grain developer featuring at 2 g/L and at 5 g/L in a borax-buffered solution, yielding negatives with a gamma of 0.6-0.8 suitable for films like Tri-X. ID-11 mirrors this composition closely, using the same concentrations of (2 g/L), (5 g/L), (100 g/L), and (2 g/L), and is recommended for similar general-purpose development. For higher contrast applications, such as paper development on Multigrade, D-72 employs elevated levels of (3 g/L) and (12 g/L) with (80 g/L) to produce sharper, more vigorous results.
DeveloperKey Components (per liter)Typical Use
D-76 / ID-11 2 g, 5 g, () 100 g, 2 gFine-grain development, gamma 0.6-0.8
D-72 3 g, 12 g, () 45 g, () 80 gHigh-contrast paper or , increased vigor
-only formulations, such as D-23 ( 7.5 g/L, 100 g/L), omit to deliver softer gradients and minimized grain, ideal for low-contrast scenes or slow emulsions. In modern variants, phenidone substitutes for at lower concentrations (e.g., 0.1-0.5 g/L) while retaining , as seen in developers like ID-68, to reduce sensitization risks without sacrificing activity. These developers are prepared as stock solutions by dissolving components sequentially in warm water (around 52°C) starting with , then Metol, , and , before topping to volume with cooler water; stocks are typically diluted 1:1 for standard use or 1:3 for enhanced sharpness and economy. Refrigerated in full, airtight bottles, they maintain efficacy for 6-12 months, though partial filling accelerates oxidation and shortens usability to about 2 months.

History

Discovery

Metol was discovered in 1891 by German chemist Alfred Bogisch while working for the chemical firm Julius Hauff und Co. in . This invention occurred during a period of intense research into new photographic developers, driven by the growing popularity of and the need for agents that enabled faster processing of plates and films without sacrificing image quality. Bogisch synthesized the compound through of p-aminophenol, producing N-methyl-p-aminophenol , which exhibited superior developing properties compared to existing agents. He tested it on emulsions, observing that it produced images with finer grain and greater vigor than , a commonly used developer at the time. These initial experiments highlighted Metol's potential for producing sharp, fine-grained negatives, marking a significant advancement in photographic chemistry. Early findings were reported in German photographic journals around 1892. In the same year, Julius Hauff filed patents for its use in photographic development, securing for the innovation and paving the way for its broader adoption.

Commercialization and trade names

Metol was first commercialized in 1891 by the German chemical company owned by Julius Hauff, where chemist Alfred Bogisch discovered its enhanced developing properties and patented it under the trade name . Shortly thereafter, the Aktien-Gesellschaft für Anilinfabrikation (AGFA) introduced its own version under the trade name , which quickly gained prominence and became the dominant brand in the photographic industry by the early 1900s, with exports reaching global markets including , the , and beyond. In the United States, adopted Metol into its developer formulations following , marketing a purified variant under the Elon starting in the ; this integration helped standardize Metol-based developers like Kodak D-76. Other manufacturers used additional s, such as Genol by and various regional brands like Armol and Planetol, reflecting its widespread industrial adoption during the peak of black-and-white film photography in the mid-20th century. Production of Metol expanded significantly in during the early , supporting the growing demand for photographic chemicals, though exact output figures from that era are scarce. By the , Metol reached its height of use in combination developers, but its popularity waned with the shift to and in the late . The recent resurgence of since the 2010s has revived demand, ensuring continued availability from specialized suppliers such as Photographers' Formulary, which offers Metol in bulk for contemporary film developers.

Safety and environmental considerations

Health hazards

Metol poses several health risks to humans, primarily through , contact, and , with effects ranging from acute to chronic and potential organ damage. from is moderate, classified as under GHS criteria (H302), with an oral LD50 of 565 mg/kg (mice). Symptoms may include and upon exposure. contact presents a significant risk due to its classification as a strong sensitizer (GHS H317), where prolonged or repeated exposure can lead to , manifesting as an eczema-like rash with pruritus and localized redness. This condition has been documented in case reports among photographers handling Metol-based developers. Inhalation of Metol dust may the , though it is not classified as highly hazardous by this route; exposure should be minimized, particularly in powder form. Chronic exposure raises concerns for specific target organ toxicity (GHS H373), with potential damage to organs through prolonged or repeated contact, including effects on blood cells. Occupational cases of were commonly reported among darkroom workers prior to the , when Metol was widely used in . To mitigate risks, protective measures include wearing impermeable gloves (e.g., or ), ensuring adequate ventilation, and avoiding direct contact with skin or eyes. For individuals with known sensitivity, alternatives such as phenidone can be substituted in developer formulations to reduce the risk of allergic reactions.

Environmental impact

Metol, primarily used as a photographic developing agent, enters the environment mainly through effluents from darkrooms, film processing laboratories, and industrial photographic facilities. These discharges contain residual Metol from developer solutions, with concentrations varying based on usage and dilution. In aquatic environments, Metol exhibits moderate persistence, influenced by factors such as and microbial activity. Its low potential, indicated by a log Kow value of approximately 0.79, limits uptake in , though it remains a concern for chronic exposure in receiving waters. Metol is highly toxic to aquatic life, with LC50 values around 0.25 mg/L for fish such as fathead minnows (Pimephales promelas) over 96 hours and 0.24 mg/L for , alongside LOEC values of 0.3 mg/L for like Isochrysis galbana. This toxicity stems from its interference with transport processes in , leading to in exposed . The compound is classified under GHS as H410, denoting very toxic to aquatic life with long-lasting effects. Recent studies (as of ) have developed ultrasensitive sensors for detecting metol in water, underscoring its recognition as an environmental pollutant. In the , Metol falls under broader wastewater regulations for hazardous substances, with restrictions on discharges to surface waters via standards like the Urban Wastewater Treatment Directive, which mandates treatment to mitigate persistent pollutants. is enhanced in systems, where it is inherently biodegradable under aerobic conditions with adapted microbial consortia, achieving significant removal rates. Mitigation strategies include pre-disposal neutralization using oxidants like (H₂O₂/Fe²⁺) to degrade Metol effectively, alongside programs for spent developer solutions to reduce volumes.

References

  1. https://www.sigmaaldrich.com/US/en/product/sial/69750
  2. http://www.earlyphotography.co.uk/site/gloss9.html
  3. https://unblinkingeye.com/Articles/Developers/developers.html
  4. http://www.michaelandpaula.com/mp/developerintro.html
  5. https://cameo.mfa.org/wiki/Metol
  6. https://www.chemeurope.com/en/encyclopedia/Metol.html
  7. https://www.chemicalbook.com/ChemicalProductProperty_EN_CB7737545.htm
  8. https://timlaytonfineart.com/black-and-white-developer-formulas/
  9. https://pubchem.ncbi.nlm.nih.gov/compound/5930
  10. https://www.chemspider.com/Chemical-Structure.5717.html
  11. https://www.sigmaaldrich.com/US/en/product/aldrich/320013
  12. https://www.chemicalbook.com/ChemicalProductProperty_US_CB7737545.aspx
  13. https://pubchem.ncbi.nlm.nih.gov/compound/Metol
  14. https://www.carlroth.com/downloads/sdb/en/4/SDB_4315_IE_EN.pdf
  15. https://www.freestylephoto.com/static/pdf/msds/formulary/Formulary_-_Metol_-_MSDS.pdf
  16. https://chemiis.com/product/metol/
  17. https://www.webqc.org/compound.php?compound=Metol
  18. https://patents.google.com/patent/US1297685A/en
  19. https://patents.google.com/patent/US2090651A/en
  20. https://chemcess.com/4-aminophenol-derivatives/
  21. https://doi.org/10.1016/S0043-1354(99)00183-9
  22. https://doi.org/10.1016/S0043-1354(99)00339-5
  23. https://doi.org/10.1016/S0043-1354(00)00297-5
  24. https://pubs.acs.org/doi/10.1021/cr60095a001
  25. https://www.digitaltruth.com/devchart.php
  26. https://afterness.com/kod_d76.html
  27. https://www.digitaltruth.com/data/formula.php?FormulaID=164
  28. https://skrasnov.com/wp-content/uploads/2016/07/kodak-chemicals-and-formulae.pdf
  29. https://www.digitaltruth.com/data/formula.php?FormulaID=118
  30. https://www.ilfordphoto.com/amfile/file/download/file/1855/product/579/
  31. https://business.kodakmoments.com/sites/default/files/wysiwyg/pro/chemistry/j78.pdf
  32. https://www.belindajiao.com/blog/kodak-d76-developer
  33. https://www.feuerbach.de/historie/begehbares-feuerbacher-gedaechtnis/hauff
  34. https://www.benchchem.com/product/B085996
  35. https://www.freestylephoto.com/1007702-Formulary-Metol-100-grams
  36. https://petapixel.com/2024/12/18/analog-photography-in-2024-films-best-year-in-decades/
  37. https://www.merckmillipore.com/INTL/en/product/4-Methylaminophenol-sulfate%2CMDA_CHEM-107299
  38. https://www.carlroth.com/medias/SDB-4315-IE-EN.pdf
  39. https://www.gfschemicals.com/Portals/0/msds/2403msds.pdf
  40. https://pubmed.ncbi.nlm.nih.gov/25775668/
  41. https://www.echemi.com/sds/4-methylaminophenolsulfate-pid_Seven40877.html
  42. https://www.researchgate.net/publication/273637469_Photo_Developer_Allergic_Contact_Dermatitis_in_a_Photographer_Following_Paraphenylenediamine_Sensitization_from_a_Temporary_Henna_Tattoo
  43. https://www.bhphotovideo.com/c/product/123915-REG/Photographers_Formulary_10_0870_100G_Phenidone_100_Grams.html
  44. https://www.sciencedirect.com/science/article/abs/pii/S0043135499003395
  45. https://www.epa.gov/sites/default/files/2015-11/documents/photographic-eg_dd_1976.pdf
  46. https://pubchem.ncbi.nlm.nih.gov/compound/N-Methyl-4-aminophenol
  47. https://www.sigmaaldrich.com/sds/aldrich/320013
  48. https://www.sciencedirect.com/science/article/abs/pii/S0043135424018980
  49. https://eur-lex.europa.eu/EN/legal-content/summary/urban-wastewater-treatment-from-2027.html
  50. https://cinestillfilm.com/pages/photo-waste-management
Add your contribution
Related Hubs
User Avatar
No comments yet.