Amidol

Every other developing agent in this encyclopaedia needs an alkali. Amidol does not, and that one fact decides the rest of it: what a formula for it looks like, why it will not keep, and why it outlived its contemporaries in one corner of printing.

In photography

The exception in the rule. Kodak’s 1928 primer states rule and exception in a sentence: “Most developing agents cannot develop at all when used by themselves. With the exception of Acrol, developing agents, in order to do their work, must be in an alkaline solution, and the energy depends upon the amount of alkali present.” Acrol was Kodak’s trade name for amidol; Kodak Limited’s 1949 handbook uses another, recording that “‘Dolmi’ is a pure form of diaminophenol hydrochloride, which is also known under the name of ‘Amidol’.” Wall had said it in 1912: “The peculiar characteristic of this substance is that it will develop without the addition of alkali.”

What an alkali-free formula looks like. Kodak’s D-170, the Dolmi developer for bromide papers, is the cleanest demonstration the course can print: a stock of 50 g crystalline sodium sulphite (or 25 g anhydrous) and 1 g potassium bromide in water to 200 cc, the whole of it then diluted to a litre with 4.5 g of Dolmi dissolved in it, two minutes at 65 °F. No carbonate, no borax, no hydroxide — a preservative, a restrainer, water and the agent. Wall’s 1912 formula is the same shape at another strength: 25 grains of amidol and 300 grains of sodium sulphite in 10 ounces of water, with a few drops of 10 per cent potassium bromide “if extra brilliancy is wanted in prints, or if over-exposure is suspected in negatives”.

What it costs. Both sources warn in the same breath as they give the formula. Kodak: “The diluted solution does not keep well and should be made up as required.” Wall: the aqueous solution “does not keep well, not even with sulphite, but soon loses its developing power”, so keep it dry and dissolve as required, and the working bath “will not keep in working order longer than a week”. Wall adds a second failure that is easy to misdiagnose — the sulfite must be fresh, because “an amidol developer made with stale sulphite gives an image lacking in brilliancy and slow in action”. A weak amidol bath is as likely to be the sulfite’s fault as the agent’s.

Where it sits among the agents. Wall’s 1924 table of development factors, the multiple of the appearance time that gives full development, puts amidol at 10, against 30 for metol and rodinal and 5 for hydroquinone with bromide. It starts fast and then needs comparatively little further time, which is why the literature reaches for it for bromide papers, lantern slides and first development in reversal work.

⚗Why no alkali, and what this course will not claim

The other agents here — hydroquinone, metol, glycin, catechol — are phenols, and the reading the course has declined to assert as fact, that the active reducer is the deprotonated anion, is at least consistent with all of them needing alkali. Amidol is the odd molecule out: 2,4-diaminophenol, a phenol carrying two amino groups, sold not as that base but as its dihydrochloride, a salt of a base.

C₆H₈N₂O·2HCl + 2 SO₃²⁻ → C₆H₈N₂O + 2 HSO₃⁻ + 2 Cl⁻

The course's own balancing of what the sulfite does to the salt: sulfite in large excess takes up the two equivalents of acid

Not sourced. No document this course holds explains why the amino groups remove the requirement, or names the species that hands the electron to the grain. The equation above is stoichiometry, not mechanism, and the course does not guess at the mechanism.

Properties

Greyish-white crystals — CAMEO’s word and HSDB’s are the same — of 2,4-diaminophenol dihydrochloride, C₆H₁₀Cl₂N₂O, relative molecular mass 197.06, CAS 137-09-7. Very soluble: HSDB gives 27.5 g per 100 mL of water at 15 °C, and Kodak’s 1928 table, under the name Acrol, 26 ounces of chemical per 100 fluid ounces of saturated solution at 70 °F. Two sources ninety years apart agree that about a quarter of a saturated solution’s weight is amidol, which is what lets a formula dissolve it straight into a finished sulfite bath.

The label is not the substance, and here the difference is 59 per cent. Wall’s 1912 entry gives amidol as “C6H3OH(NH2)2 = 124”, which is the free base at 124.14. The article sold, and the article every one of these formulas weighs, is the dihydrochloride at 197.06. Weighing the base where a formula meant the salt would put 59 per cent more of the reducing molecule in the bath. The historical formulas stand as written, because the historical suppliers sold the hydrochloride and Kodak’s 1949 note confirms it; a modern jar has to be read for which article it holds.

Handling

The aggregated ECHA notifications classify amidol Danger, with three pictograms. The headline is H301, toxic if swallowed, at 96.1 per cent of notifiers — a category worse than the “harmful if swallowed” most darkroom organics carry. H315, H319 and H335 follow at 94.1 per cent each, and the two sensitisation statements, H317 and H334, may cause allergy or asthma symptoms or breathing difficulties if inhaled, at 13.7 per cent each. Japan’s NITE-CMC and Safe Work Australia’s HCIS both add H373, organ damage through prolonged or repeated exposure, while disagreeing over whether the oral hazard is H301 or H302.

There is no workplace exposure limit. Neither EH40 nor the NIOSH Pocket Guide lists amidol or diaminophenol under any name searched, and EH40’s paragraph 6 says that “the absence of a substance from the list of WELs does not indicate that it is safe”: exposure is to be controlled to a level at which nearly all the working population could be exposed day after day without harm, with no figure to work to. HSE’s HSG262 gives the reason the minority sensitisation statements still matter — once a person is sensitised, tiny amounts provoke the reaction and the only remedy left is to prevent further exposure.

Why Level B. Level A of the course rubric admits substances classified “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. H301 is past that, and Level B names both sensitisers and fine powders that must not be inhaled. Level C’s criterion is acutely toxic or sensitising reagents where a fume cupboard or specialist disposal is the recognised control, and neither is: HSE’s COSHH essentials sheet for manual development prescribes general ventilation, nitrile gloves, eye protection and overalls, and says respiratory protection is not normally needed for routine work. The operation that would take it out of Level B is weighing the dry powder in the open — and amidol makes that operation frequent, because a bath that will not keep a week means the jar is opened every session, where a hydroquinone developer’s is opened once a year.

⚠Safety

Weigh over a tray, in still air, container away from the face, with extraction or an enclosure as Princeton University’s guidance for powdered developers directs. Goggles and nitrile gloves while the powder is open; tongs in the tray; hands washed before leaving, because this one is toxic if swallowed rather than merely harmful. Do not try to make an amidol stock keep by adding carbonate: alkali is what this developer was designed not to need, and adding it accelerates the oxidation that already limits the bath to a week.

Waste

Spent developer, bottled separately. No notifier reports an aquatic hazard for amidol, so the reason to keep it out of the fixer bottle is Kodak’s rather than the environment’s: J-300 records that developer holds negligible silver, so mixing the two recovers nothing and spoils a stream that does. ILFORD’s advice to United Kingdom domestic users is to bottle each waste separately, label it, and leave it in the chemical cupboard at a Household Waste and Recycling Centre. A bath made fresh and discarded weekly keeps the volumes small. Check your local regulations; they decide.

History

Eder credits the discovery of metol, amidol and glycin as developers to Dr A. Bogisch, chemist in the photographic department of the chemical factory of J. Hauff at Feuerbach, near Stuttgart — the same works and the same decade that produced metol, which Eder dates into practice about 1893. Three of the developing agents still in use a century later came out of one industrial laboratory in Württemberg.

What kept amidol when its contemporaries went is the property it was named for. A developer needing no alkali is one whose print tone is steered by dilution and bromide alone: Wall records that at full strength his formula “gives fine steely black tones with correct exposures”, that “with diluted solutions clear grey tones are obtained”, and that warmer tones follow from longer exposure and liberal bromide. Kodak was still packing it for bromide papers in 1949. The cost has never changed: mix it fresh, and open the jar to do it.