p-Aminophenol

A Rodinal-type developer is sold as a small bottle of dark syrup and used at one part in twenty-five or one in fifty, and the reason it can exist in that form is a solubility problem solved backwards. Para-aminophenol barely dissolves in water. Put enough sodium hydroxide in with it and it dissolves in its own alkali, and what leaves the factory is a concentrate that needs nothing but water.

In photography

It is the parent of the family, not a variant of it. Kodak’s 1928 primer sets out the relationship: several of the agents introduced after pyro and hydroquinone are built on para-aminophenol, and treating it with methyl alcohol attaches the methyl group to give methyl-para-aminophenol — metol — which the primer calls the more active developing agent of the two. It is equally clear that these substances are bases and that what a photographer buys is a salt: the oxalate of para-aminophenol, the sulfate of methyl-para-aminophenol, the hydrochloride of diaminophenol. Kodak sold the oxalate as Kodelon, noting that many of the so-called new developing agents then on the market consisted entirely or mainly of para-aminophenol under other names.

Its position on the scale. Ranked by reduction potential — the primer’s measure of how much bromide is needed to hold an agent back — hydroquinone is lowest, then Athenon, pyro, Kodelon, and metol highest. Para-aminophenol therefore sits between pyro and metol, and the primer’s practical note follows: Kodelon can be substituted for Elon, but more of it is needed for a developer of the same strength.

Where you meet it: as a liquid concentrate. Wall’s 1924 volume lists the proprietary versions — rodinal, azol, activol, citol, paranol, paramol, certinal, kalogen — as liquid developers consisting of para-aminophenol and an alkali. Foma still sells one: Fomadon R09, a liquid concentrate of a fine-grain, normal-working para-aminophenol negative developer, used at 1+25 or 1+50. It also appears in fixed formulas as a metol substitute — Kodak Ltd’s 1949 formula D-173 pairs 0.375 g of para-aminophenol hydrochloride with 7.5 g of hydroquinone in a litre of stock, diluted with an equal volume of water for use.

⚗The alkali does two jobs, and that is the whole of Rodinal

An alkaline developer normally uses its alkali to set the pH at which the agent will reduce silver. In a para-aminophenol concentrate the alkali is also the solvent.

Wall’s 1924 volume prints the working of it in an Ermen formula of the rodinal type: 50 g of para-aminophenol hydrochloride and 150 g of potassium metabisulfite in about 625 cm³ of water, with a caustic soda solution then added under constant stirring. “At first a precipitate of the paramidophenol base is formed, but, as more caustic soda is added, this dissolves.” The instruction that follows is the interesting part: add enough soda to nearly dissolve the precipitate, and “it is very important to leave some undissolved”. The bottle is deliberately held at saturation, solid base in equilibrium with the solution above it.

Behind that is the acid–base behaviour of an amino-phenol. The salt is soluble, the free base is not, and strong alkali deprotonates the phenol and returns it to solution. HSDB records the free base as soluble in alkalies, and Kodak’s 1924 primer says the same from the bench: a para-aminophenol–carbonate developer is difficult to prepare in concentrated form, but adding a little caustic soda increases the solubility and a stronger solution can be made.

Not verified here: which ionised form reduces the silver, and why leaving some base undissolved should stabilise the concentrate rather than merely saturate it. Wall gives the instruction without the reason, and no source the course holds supplies one.

What more or less of it does. The dilution sets the concentration, and with it contrast and grain: Foma gives 1+25 and 1+50 for R09, with different times against every film in its range. Wall’s warning is worth carrying — the increase in time is not proportionate to the dilution, because it also depends on the air dissolved in the water. Wratten and Wainwright found a plate needing 3 minutes in rodinal at a twentyfold dilution needed, at a two-hundredfold one, 42 minutes with air-free distilled water and 52 with tap water, against the 30 that proportion predicts. It is also more temperature-sensitive than metol: Watkins’s coefficients as Wall prints them give paramidophenol 2.4 against 1.9 for a metol–hydroquinone developer, the factor by which development time changes over 10 °C.

Properties

White or reddish-yellow crystals, or a light-brown powder, turning violet in light. C₆H₇NO, relative molecular mass 109.13, CAS 123-30-8; ChEBI describes it as the amino-phenol isomer with the amino group para to the hydroxy, and lists allergen among its roles. Those numbers describe the free base. Recipes may call for a salt instead, and the salts have different masses: Kodelon was the oxalate on Kodak’s own account, D-173 specifies the hydrochloride, and Wall’s formulas are written for the hydrochloride throughout. Weighing the free base against a formula written for a salt puts more developing agent in the tank than the formula intends.

Solubility is the defining property, and the published figures disagree badly. CAMEO, following NTP, gives less than 1 mg/mL at 23 °C and its air-and-water note says flatly “insoluble in water”; HSDB gives 1.6 g/L at 20 °C in one place and a percentage series — 0.39% at 0 °C, 0.65% at 24 °C, 0.80% at 30 °C — in another, which is four times higher again; HMDB gives 16 mg/mL. That is a spread of more than sixteenfold, and the course prints it rather than picking a favourite. What every figure agrees on is the comparison that matters: para-aminophenol is sparingly soluble, one to two orders of magnitude below hydroquinone’s roughly 70 g/L. Hot water, acetone, ethyl acetate and acetonitrile do better than cold water, and alkali does best of all.

pH behaviour. The IUPAC dissociation-constant dataset gives 4-aminophenol a pKa of 10.46 at 25 °C, from two independent determinations, for the loss of the phenolic proton. A developer buffered at pH 10 has only a minority of the molecules ionised; the caustic concentrate is well past it. Eder notes that the rule he and Toth drew in 1880 about the para arrangement giving strong developing action was later found to hold for other derivatives, para-aminophenol among them — the amino group standing in for one of hydroquinone’s two hydroxy groups.

Stability. The solid darkens to violet in light and belongs in the dark. In alkaline solution it oxidises like every developing agent, and Wall notes that his one-solution formula does not keep as well as the two-solution version.

Handling

The aggregated ECHA notifications classify para-aminophenol as Warning with three pictograms. Three statements are unanimous across 364 reports from 21 notifications: H302 harmful if swallowed, H341 suspected of causing genetic defects, and H410. H332, harmful if inhaled, appears in 99.5% — for a dry powder weighed on an open bench, that is the statement that should change your behaviour. Two more are minority classifications at 34.3% each: H317, skin sensitisation, and H373, organ damage on repeated exposure. This page treats the minority statements as real, because Safe Work Australia’s HCIS puts both in its own classification and because a substance one supplier in three calls a sensitiser is not one to handle bare-handed on the strength of the other two thirds. The five the notifiers do agree on — H302, H332, H341, H400 and H410 — are also harmonised under Regulation (EC) No 1272/2008, so those are settled at law rather than merely reported. HSG262’s point then applies: sensitisation may take weeks, months or years to appear, and once it has, tiny amounts trigger the reaction and the only remedy left is preventing further exposure.

CAMEO’s health-hazard entry adds what the codes do not convey: asthma, skin and eye irritation, dermatitis, and methaemoglobinaemia with cyanosis — haemoglobin converted to a form that cannot carry oxygen, an effect Princeton’s guidance lists for developers as a class. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit for aminophenol, and EH40 states that absence from its list does not indicate a substance is without risk.

Why Level B. Level A of the course rubric admits substances classified no further than irritant, harmful if swallowed, or corrosive at the concentrations handled; a suspected mutagen that is harmful if inhaled is past that. Level B’s criteria name sensitisers and fine powders that must not be inhaled — both apply — and add procedures whose failure mode is a splash of something corrosive rather than merely a spoiled negative, which is exactly what a home-mixed Rodinal-type concentrate is, because of the sodium hydroxide in it. Level C would require a fume cupboard or a licensed waste route to be the recognised control, and neither is: HSE’s COSHH essentials sheet P1 asks for general ventilation of more than five air changes an hour with a through draught, nitrile gloves, eye protection and overalls, and says respiratory protection is not normally needed for routine operations.

⚠Safety

Weigh this powder with extraction or inside an enclosure and never in a draught: H332 is on 99.5% of the notifications and the material is a fine dry powder. Nitrile gloves and eye protection for the solution; tongs, not fingers, in the tray. If you make a Rodinal-type concentrate rather than buying one, the immediate hazard is the sodium hydroxide, and splash goggles, an apron and cold water are the controls for that, not for the aminophenol. Keep the solid dark and away from acids, alkalis, alcohols and amines, with which CAMEO records violent reaction, and away from heat, which decomposes it to hydrogen cyanide, nitrous vapours and carbon monoxide. That warning and the published Rodinal formulas, which dissolve the substance in strong alkali on purpose, both come from sources this course accepts and it cannot reconcile them; read CAMEO as governing bulk dry solid on the shelf, and a published formula as a procedure someone has already run, mixed cold and in the order the source gives.

Waste

Spent developer, bottled separately from fixer and stop bath: Kodak’s J-300 guidance records that developer carries negligible silver, so there is nothing in it to recover and every reason not to contaminate a fixer that does. Two things govern the route. The classification is H410 in every notification and H400 in 98.6%, with P273, P391 and P501 attached; and a used Rodinal-type solution carries the alkali as well as the agent. Ilford asks UK domestic users to bottle each waste chemical separately, label it, and leave it in the chemical cupboard at a Household Waste and Recycling Centre, mixing nothing and using no septic tank. Neutralising the alkali would not touch the aminophenol, so there is no bench treatment this course can offer that would change the stream’s classification. Check your local regulations; they decide what is permitted where you are.

History

Momme Andresen, chemist to the aniline company in Berlin that became Agfa, is the origin of this whole line of developers. Eder credits him with recognising the importance of the amino group NH₂ as an effective group among organic developer substances, and lists his three: para-phenylenediamine (German patent 46495, 8 January 1888), Eikonogen (50265, 2 October 1889), and paramidophenol — rodinal — German patent 60174, dated 27 January 1891. Writing decades later, Eder notes that it was still generally used. It still is.

The compound then spawned its own replacements. Methylating the amine gave metol, which Bogisch at J. Hauff introduced about 1893 and which Kodak rates as the more active of the two. But metol brought dermatitis with it, and para-aminophenol came back the other way: Kodak Ltd’s 1949 handbook prints D-173 as an Elon-free developer for Velox paper, expressly to eliminate the risk of discomfort to persons prone to metol dermatitis, reaching an ordinary paper developer with para-aminophenol hydrochloride and hydroquinone. The parent molecule was the escape route from its own derivative — until phenidone, which is not a phenol at all, replaced both.

One naming trap survives from the period. Wall’s 1924 volume records Kodelon as para-aminophenol hydrochloride; Kodak’s own 1928 primer gives it as the oxalate, and offers a test to tell the families apart — the metol salt dissolves cold in its own weight of strong hydrochloric acid, the oxalate does not. The course follows Kodak on the identity of Kodak’s own product, and records the disagreement because two respected authorities of one decade naming a single trade product as two different salts is exactly why a trade name is never an identification.