Catechol

Move hydroquinone’s two hydroxy groups from opposite corners of the ring to adjacent ones and you get a developing agent that stains the gelatin brown, hardens it where it develops, refuses to keep for a second use, and carries a skull-and-crossbones on its label. Catechol is the ortho isomer, and almost nothing transfers.

In photography

A staining, self-limiting developer. Wall’s 1924 formulary prints the plainest version: 1 g of pyrocatechin per litre with a little potassium carbonate, which “gives brownish images, suitable for transparencies” and where “the developer can only be used once, as it spoils rapidly”. Two facts sit inside that sentence. The image is brown because the oxidation product of the agent is coloured and stays in the gelatin with the silver, exactly as Kodak’s 1928 primer describes for pyro; and the solution dies quickly because the same oxidation happens in the tray whether or not there is silver to reduce.

The alkali is the control. Wall’s pyrocatechin formulas run the entire range of alkalis, and they are recognisably different developers. Ellon’s rapid one-solution formula carries 50 g of catechol, 250 g of sodium sulfite and 35 g of caustic soda per litre, used at one part in fifteen. Vogel’s two-solution version puts 20 g of catechol with 50 g of sulfite in one bottle and 14 g of caustic soda in the other, mixed one and one with four of water. Hanneke’s acetone developer drops the alkali altogether — 10 g of catechol and 12 g of sulfite per litre, mixed with a twelfth of its volume of acetone immediately before use. The concentrated cream grinds 100 g of potassium metabisulfite and 150 g of potassium carbonate into 400 cm³ of water in a mortar, takes up 350 g of catechol, is kept in well-corked bottles and is diluted twenty times to work. What changes across that range is how much of the molecule is ionised, and therefore how fast it reduces silver and how fast it destroys itself.

Sulfite decides the colour. The formula that gives brown transparencies has no sulfite in it; the caustic ones carry a quarter to two-fifths of a kilogram per litre. Kodak’s account of the mechanism for pyro applies unchanged: the sulfite is oxidised in place of the agent, so the coloured oxidation product is not laid down in the film, and the image moves from brown towards neutral as the sulfite goes up.

Clean shadows. The 1906 British Journal Photographic Almanac reports Lüppo-Cramer’s comparison of colour fog under a heavy bromide addition: pyro and adurol gave appreciably more fog than the hydroquinone standard, “edinol and glycin, less; pyrocatechin, none; metol, a trace”. In the same tests hydroquinone’s fog was a deep red where pyrocatechin’s, when it appeared at all, was a pale yellow. Wall’s table of Watkins factors gives pyrocatechin 10, between glycin-soda’s 8 and metol’s 30 — a slow agent that finishes late relative to the appearance of the image.

In papers and toners. Wall records that in warm-tone print development pyrocatechin “gives more violet tones” than a hydroquinone-citric acid developer, and it appears again in Valenta’s toning baths, both in a selenium sulfide bath at 20 to 40 cm³ of a one per cent solution per litre and in a three-stock system with Schlippe’s salt for sepia and warm browns. Moersch’s current toning guide uses it in the same spirit: SE20c, described there as “the extremely slow catechol developer”, needs more than one stop of over-exposure to give full gradation used alone, and is therefore used in two-tray development beside a sepia developer, where the pairing produces a two-tone gradation before any toner is reached.

The oddity worth knowing. Wall’s 1912 dictionary gives a developer that fixes at the same time — 40 grains of pyrocatechin with caustic soda, sulfite and hypo in the same bottle — and calls it “perhaps rather an eccentricity in photography than a method of practical value”, because combining the operations loses the control that makes them separate. Wall’s 1924 book prints Hanneke’s version of the same idea. Catechol turns up in monobaths because it works fast in strong caustic alkali and because nothing about it needs protecting from the thiosulfate.

⚗Why the ortho isomer behaves differently

Catechol is a weak acid twice over. The IUPAC dissociation-constant dataset records the first ionisation at pKa₁ 9.23 to 9.45 across determinations between 20 and 31.5 °C at ionic strengths around 0.1 mol/L, and the second, less certainly, between 12.6 and 13.7.

C₆H₆O₂ → C₆H₅O₂⁻ + H⁺

First ionisation: pKa1 about 9.2 to 9.5 near room temperature

Below pH 9 the molecule is essentially undissociated. A carbonate developer sits near the first constant, which is why Wall’s carbonate formulas are slow; a caustic developer runs past it, which is why Ellon’s works at a fifteenth of its strength. The dataset carries one further note against two of the determinations, recorded as a difficulty in the measurement: the dianion oxidises readily in air. That is the same sentence as CAMEO’s “aqueous solutions soon turn brown on exposure to air and light” and as Wall’s “can only be used once”, written by three different kinds of author.

Not sourced here. The course can name neither the oxidation product of catechol nor the mechanism by which that product hardens gelatin, and asserts neither. What Eder records is practice: Warnerke found in the late 1870s that a gelatin film tanned by development was insoluble in warm water where it had been exposed and soluble where it had not, and Koppmann’s Jos-Pe process of 1925 made colour prints from tanned reliefs developed with pyrocatechin. The connection between the molecule and the hardened gelatin is real and undescribed on this page.

Properties

C₆H₆O₂, relative molecular mass 110.11, CAS 120-80-9 — the same formula and the same mass as hydroquinone, and a different substance. Colourless crystals with a faint odour, browning in air and light, and CAMEO adds that the browning is faster when the solid is moist. It melts at 105 °C (221 °F), sublimes rather than boiling cleanly at 245 °C (473 °F), and has a flash point of 127 °C (261 °F); NIOSH classes it a combustible solid with a lower explosive limit of 1.4 per cent, and CAMEO warns that poisonous gases may be produced when it is heated.

Solubility is not the constraint it is for silver halides. The ILO-WHO safety card gives 43 g per 100 mL of water at 20 °C, NIOSH 44 per cent and HSDB 461 g/L at 25 °C — three independent numbers that agree, which is worth noting because the same encyclopaedia’s hydroquinone page has to report two irreconcilable sets. It is very soluble in alcohol and in aqueous alkalis, which is what Wall’s concentrated cream depends on.

Stability is the property that shapes every formula. Dry, dark and closed it keeps; wet it browns; wet and alkaline it browns fast; and the used developer cannot be poured back into the bottle. There are no hydrates in any source the course holds, so no conversion factor to give.

Handling

The aggregated ECHA notifications give Danger, three pictograms including GHS06, the acute toxicity skull and crossbones, and nine statements. The near-unanimous ones are skin irritation above 99.9 per cent and serious eye irritation at 94.6 per cent. The consequential ones are the minority views: H301 and H311 — toxic if swallowed, toxic in contact with skin — at 16.4 per cent, where the majority say “harmful”; H341, suspected of causing genetic defects, at 16.4 per cent; and H350, may cause cancer, at 11.4 per cent. PubChem separately carries the California Office of Environmental Health Hazard Assessment’s statement that catechol can cause cancer according to IARC.

The occupational limits exist and are not generous. NIOSH sets a recommended exposure limit of 5 ppm, 20 mg m⁻³, as an eight-hour time-weighted average, with a skin notation — the Pocket Guide’s mark for a substance whose absorption through intact skin contributes materially to the dose. HSE’s EH40 lists Pyrocatechol at 5 ppm, 23 mg m⁻³, over eight hours with no short-term limit. (The CAS number printed beside it in the course’s mirrored copy of that table reads 102-80-9, which is not the 120-80-9 that twenty-two sources give in PubChem; EH40’s synonym list identifies Pyrocatechol with 1,2-benzenediol and 1,2-dihydroxybenzene, so the course reads the digits as a transcription error rather than a different substance, and says so.) NIOSH’s symptom list is not confined to the skin: irritation of eyes, skin and respiratory system, sensitisation and dermatitis, lacrimation and burns to the eyes, then convulsions, increased blood pressure and kidney injury, with target organs given as eyes, skin, respiratory system, central nervous system and kidneys. Princeton University’s guidance says the same thing from the darkroom end — catechol is among the developers that can be absorbed through the skin to cause severe poisoning, and ingestion of less than a tablespoon of pyrocatechol may be fatal for an adult.

Why Level C, when hydroquinone is Level B. Level A of the course rubric admits substances classified no further than irritant, harmful if swallowed, or corrosive, and an acute-toxicity pictogram is beyond that on its own. The real question is B against C. For hydroquinone the course found that the recognised control is not a fume cupboard: HSE publishes a sheet for manual film development prescribing general ventilation, gloves, eye protection and overalls, and saying respiratory protection is not normally needed. Nothing equivalent exists for catechol. Three things point the other way instead. The carcinogenicity here is a “may cause cancer” from a minority of notifiers plus a regulator’s citation of IARC, not a “suspected”; the skin notation means that the Level B control of gloves and a splash apron is explicitly incomplete; and Florida Atlantic University’s environmental health and safety guidance for photographic chemicals does not describe how to use catechol at all — it tells darkroom workers to avoid pyrogallol and catechol and offers phenidone as the substitute. Level C’s criterion is a reagent classified as acutely toxic, carcinogenic or mutagenic where engineered control and specialist disposal are what answer it, and that is what this is.

⚠Safety

The course teaches catechol as chemistry and history and does not weigh it in a home darkroom. Where it is handled at all, it is handled with engineered extraction, splash goggles, gloves treated as an incomplete barrier because of the skin notation, and eyewash within reach as NIOSH directs. It must be kept away from strong oxidisers and from nitric acid, with which CAMEO records a violent reaction. Once a person is sensitised — H317 is notified by 12.6 per cent — HSE’s HSG262 states the consequence plainly: small amounts then provoke the reaction and preventing further exposure is the only remedy left.

Waste

The whole of the catechol you mixed is still in the tray when you finish, because a developing agent is spent only in proportion to the silver it has reduced, and most of a print’s area reduces none. Bottle it separately, label it with the substance and the date, and route it as household hazardous waste rather than as drain load: the aggregated notifications carry acute oral and dermal toxicity with the precautionary codes for avoiding release to the environment, collecting spillage and disposing to an approved facility. There is no silver in it to recover — Kodak’s J-300 guidance puts developer’s silver content at a negligible level — so combining it with the fixer gains nothing and contaminates the one stream that is worth reclaiming. Check your local regulations; they decide, and for a substance with this classification the reading is better done before the container is opened.

History

Catechol entered photography in 1880, the same year as hydroquinone and by a route that produced a rule. Eder records that he and V. Toth of Vienna found pyrocatechin suitable as an alkaline developer for dry plates and, in the process, gave precise data on the influence of isomerism in the bivalent phenols: the para arrangement acts strongly on silver bromide in alkaline solution, the ortho arrangement has great developing power, and resorcin, with its hydroxy groups meta, has no developing energy at all. Three substances of formula C₆H₆O₂ and only two of them develop. It is one of the cleanest demonstrations in nineteenth-century photographic chemistry that a molecular formula is not a substance.

The commercial names photographers actually met were Kachin and Elconal, both of which Wall records as trade names for pyrocatechin. Eder credits Gustav Koppmann with introducing pyrocatechin in different forms in 1907, and with the Jos-Pe process of 1925, which made colour prints from tanned gelatin reliefs developed in pyrocatechin — the tanning property Warnerke had first seen with pyro in the late 1870s, put to work.

What has changed is not the chemistry but the reading of the hazard. Wall’s 1924 formulary lists catechol among ordinary developers, with no more warning than any other; a century later a university safety office lists it as something to design out of a darkroom, and the aggregated notifications carry a pictogram Wall’s readers had no way to be shown. The substance did not become more dangerous. The evidence did.