Rodinal-type p-aminophenol developer

Every liquid developer concentrate sold since 1891 belongs to one family, and the family exists because of a solubility problem solved backwards. Para-aminophenol is an awkward thing to put in a bottle: as the free base it barely dissolves in water. Dissolve enough sodium hydroxide with it and it goes into solution as its own sodium salt, and what you have is a syrup that carries a litre of working developer in twenty-five millilitres of itself. Momme Andresen, chemist to the Berlin aniline company that became Agfa, took German patent 60174 for that on 27 January 1891, and the product was called Rodinal.

!This page does not publish Agfa's Rodinal

Nobody outside the manufacturer knows what is in a bottle of Rodinal. The ADOX datasheet for the current product is in this course’s research corpus, was fetched successfully, and carries no extractable text layer, so nothing is quoted from it: not a dilution, not a pH, not a keeping claim.

What is published, and what this page prints, is a rodinal-type concentrate: a formula of the same class, printed in full with every weight, by E. J. Wall in Photographic Facts and Formulas (1924), who credits it to Ermen and heads it “One solution, similar to rodinal”. Wall’s own closing sentence on that page is the reason the class needs a name of its own: “Liquid developers consisting of paramidophenol and an alkali are marketed under such names as rodinal, azol, activol, citol, paranol, paramol, certinal, kalogen, etc.” Eight trade names, one composition class, and no disclosure behind any of them.

The paramidophenol solution — the developing agent and the acid sulfite that keeps it alive in the bottle

Ingredient	Quantity	Form the source specifies
p-Aminophenol	50 g	hydrochloride
Potassium metabisulfite	150 g
Water	625 mL, added	"Water 625 ccm (11 oz.)", boiled, allowed to cool for five minutes and given a few crystals of potassium metabisulphite before anything is weighed into it. Wall states no temperature and no reason. Boiling is the standard way of driving dissolved air out of a solvent, and five minutes will not bring 625 mL anywhere near room temperature, so the solids go into water that is still hot. The quantity is water added rather than a volume made up to.
Wall prints the agent above the metabisulphite and says "Stir until dissolved" only after both. His 1912 printing of the same construction reverses the two, dissolving the metabisulphite first; the difference is discussed under Mixing.

The caustic soda solution — the alkali, made up separately so that it can be added by judgement rather than by weight

Ingredient	Quantity	Form the source specifies
Sodium hydroxide	215 g
Water	500 mL, added	"Water 500 ccm (8 and three quarter oz.)". Wall does not write "to make", so this is water added, and the finished solution is more than 500 mL. It is about 430 g of sodium hydroxide per litre of water - a solution that heats itself violently as it forms and must be made with cold water and allowed to cool before it is used.
Only part of this solution goes into the developer. Wall makes up 500 c.c. and uses about 340 to 350 of it, because the end point is judged by eye and not by measure.

Mixed in this order — the finished concentrate

1. Start with the whole of it of The paramidophenol solution — the vessel that receives
2. Then add about 340 to 350 c.c. of The caustic soda solution — added with constant stirring, and only enough to nearly dissolve the precipitate of the paramidophenol base
3. Then add to make 1000 c.c. of water

Add with constant stirring about 340 to 350 ccm (6 oz.) to the paramidophenol solution. At first a precipitate of the paramidophenol base is formed, but, as more caustic soda is added, this dissolves. Enough soda solution should be added to nearly dissolve the precipitate; then add water to make 1000 ccm. Bottle and allow to cool. Should any paramidophenol crystallise out, more soda must be added to nearly dissolve it. It is very important to leave some undissolved.

The 1000 c.c. is the make-up volume of the combined concentrate, which the schema has no field for - docs/FORMULARY-SCHEMA.md records that as open item 9 - so it is stated here and in the Mixing section instead. Every strength quoted on this page is computed against that litre.

Purpose

To carry a complete developer in a small bottle of syrup, and to develop with a bath that contains almost nothing except the developing agent and the alkali. That is the exact opposite of D-76, where ninety-two per cent of the dry weight is sodium sulphite and the sulphite is doing most of the photographic work. A rodinal-type concentrate diluted forty times has about 4 g/L of sulphite, expressed as anhydrous sodium sulphite, where D-76 has 100, and the consequences run all the way to the grain.

Wall’s 1912 Dictionary describes the class in two phrases worth keeping: the developer is “stainless in character” and it “keeps well in single concentrated solution”. Those are the two problems a period photographer had with pyro and with a mixed metol-hydroquinone stock, and this formula answers both.

Recommended uses

Wall gives exactly two, and gives no time for either:

• Plates at 1 part in 10, with potassium bromide added. He does not say how much bromide.
• Papers at 1 part in 40.

Two further uses belong to the class rather than to this formula, and the page marks the difference each time. Wall’s 1912 entry for the proprietary rodinal recommends it for bromide paper and lantern slides, and for stand development; his 1924 chapter on stand development prints a whole time-and-temperature table for rodinal at 1:100. Foma’s current sheet for FOMADON R09, the nearest manufactured equivalent, is for “the manual processing of all sorts of perforated, roll and sheet negative films” at 1+25 or 1+50.

The register files this entry as a film developer because that is what the class became. Wall’s own instruction names plates and papers, so the structured material field names plates and papers, and the mismatch between the two is honest rather than tidy.

When another formula is preferable

• For fine grain, anything with a solvent sulphite level. D-76 at stock strength carries twenty-four times this developer’s sulphite concentration at 1:40, and D-23 carries about the same on two ingredients. Fine grain is the one thing this formula is not designed to give.
• For sharpness without handling caustic soda, the Beutler two-solution developer gets to a high-acutance working bath by the same route - very little sulphite, heavy dilution, one shot - using sodium carbonate as its alkali. If the reason you want a rodinal-type developer is acutance rather than concentration, Beutler is the safer way to it.
• For high contrast, D-19. This developer is energetic but it is a single-agent bath and it was never designed for line work.
• If you want the type and not the hazard, buy it. Mixing a manufactured concentrate is somebody else’s exposure to 215 g of sodium hydroxide rather than yours, and the bought bottle arrives already made to a strength its maker controls, which is more than can be said for a solution whose alkali is added until a precipitate nearly disappears. The course has no product page for any of these, so it names Foma’s FOMADON R09 as the current example and stops there.

Mixing

The procedure is Wall’s, in his order, and every step of it is doing something.

1. Boil 625 c.c. of water and let it cool for five minutes. Boiling a solvent is the standard way of driving dissolved air out of it, and everything that follows is an oxidisable substance. Five minutes will not bring 625 mL anywhere near room temperature, so the solids go into hot water and not cold: warm water is where a sparingly soluble organic salt dissolves. Wall states neither the reason nor a temperature, and both readings above are the course’s.
2. Add a few crystals of potassium metabisulphite. Wall gives no quantity, and none is needed: these are sacrificial, mopping up the oxygen that came back into the water while it cooled, before any of the developing agent meets it.
3. Add 50 g of para-aminophenol hydrochloride, then 150 g of potassium metabisulphite. Stir until dissolved. Both dissolve readily: the agent is a salt at this stage, not the sparingly soluble free base, and 150 g of metabisulphite in 625 mL is 24 g per 100 mL against the 49.5 g per 100 g of water at 25 °C its encyclopaedia page records.
4. Separately, dissolve 215 g of caustic soda in 500 c.c. of water. Cold water, always. Kodak’s 1928 primer gives the reason in the plainest terms it uses anywhere: if hot water is used the solution will boil with explosive violence and may cause serious burns if the hot alkali spatters on the hands or face. Stir as it goes in, or the dense caustic layer sits at the bottom and the solution is not the strength the label says. Let it cool before it is used.
5. Add about 340 to 350 c.c. of the soda solution to the paramidophenol solution, with constant stirring. This is the step the whole formula turns on, and Wall describes what you will see: “At first a precipitate of the paramidophenol base is formed, but, as more caustic soda is added, this dissolves.”
6. Stop just short. “Enough soda solution should be added to nearly dissolve the precipitate.”
7. Water to make 1000 c.c. Bottle and allow to cool.

⚗Why the precipitate appears and then goes away

The three things the caustic does happen in order, and you can watch them.

The agent goes in as the hydrochloride, which is soluble. The first hydroxide to arrive neutralises that hydrochloric acid and sets the free base loose - and the free base is the form that will not dissolve, so it falls out as a precipitate.

C₆H₄(OH)NH₂·HCl + NaOH → C₆H₄(OH)NH₂ + NaCl + H₂O

The salt neutralised, and the base precipitated

More hydroxide takes the proton off the phenol group, giving the phenolate, which is an anion and is soluble again. The encyclopaedia page for the agent records the free base as soluble in alkalies, and the IUPAC dissociation-constant dataset gives the phenolic pK_a as 10.46 at 25 °C, with a second determination at 10.44, so it takes a real excess of alkali - not a carbonate - to push the equilibrium over.

C₆H₄(OH)NH₂ + NaOH → C₆H₄(ONa)NH₂ + H₂O

The base redissolved as its sodium salt

The metabisulphite is being converted at the same time. In water the disulphite ion takes up water and splits into two hydrogen sulphite ions, and each of those needs a hydroxide to become sulphite - the form that actually preserves a developer.

K₂S₂O₅ + H₂O → 2 KHSO₃

The disulphite ion splitting in water

So the caustic soda is neutralising an acid, converting a preservative and dissolving a base, and only what is left over is alkali in the photographic sense.

Why “leave some undissolved” is an instruction and not a caveat. Wall repeats it: “Should any paramidophenol crystallise out, more soda must be added to nearly dissolve it. It is very important to leave some undissolved.” The bottle is deliberately held at saturation, with solid base sitting under a saturated solution of its own phenolate. Wall gives the instruction and not the reason, and no source this course holds supplies one. The obvious guess - that a saturated bottle is buffered against loss of agent - is a guess, and it is written here as one.

Wall’s two printings disagree about the order of the first two solids, and Kodak’s rule prefers the other one. The 1924 formula on this page adds the agent and then the metabisulphite; Wall’s 1912 Dictionary prints the same construction the other way round, metabisulphite into the water first and the agent after. Kodak Limited’s 1949 handbook states the general rule for a formula whose agent is not Elon: “the preservative is dissolved first, then the developing agents, then, after these are completely dissolved, the alkali”, and its reason is that an agent standing in solution before its preservative arrives oxidises in air and forms coloured products. That is the 1912 order, not the 1924 one. Two things soften the disagreement here: Wall’s water already carries the few sacrificial crystals of metabisulphite by the time the agent goes in, and the solution is acid throughout, which is where a developing agent is least reactive. Neither source acknowledges the other, and this page follows the printing whose weights it publishes.

!One column or the other, never half of each

Wall prints every quantity twice, once in grams and c.c. and once in ounces and grains, and the two columns are not exact equivalents. His 50 g of the agent is printed as 385 gr., a conversion at 7.7 grains to the gram; his 150 g of metabisulphite is printed as 2 oz. 192 gr., which works out at 7.1; his 215 g of caustic soda is printed as 3 and a half ounces, which is 7.1 again. The columns disagree by up to eight per cent, so a formula mixed by converting halfway down the list is neither of the two formulas Wall printed. This page prints the metric column throughout.

Behaviour

It is a concentrate and the working bath is thrown away. Wall states no capacity for this formula and none is inferred here. What he does say about the class, in his chapter on stand development, is that a tank of dilute developer is “as a rule, so oxidised at the end of development as to be useless for a second time” - which is a statement about very dilute baths in general and not a published capacity for this one.

The time does not scale with the dilution, and there is a measurement to prove it. Wall records Wratten and Wainwright’s photometric work on rodinal: a plate needing 3 minutes at 1:20 did not need 30 minutes when the developer was diluted to 1:200. It needed 42 minutes with air-free distilled water, 46 with ordinary distilled water and 52 with tap water. Proportion predicts 30; the answer is between 40 and 73 per cent longer than that, and the size of the error depends on the air dissolved in the water you diluted with. This is the single most useful published fact about the class and it is why a dilution series has to be timed rather than calculated.

Munkman’s stand-development table for rodinal at 1:100, as Wall prints it. It is for the proprietary product, not for this formula, and it is reproduced because it is the only published time-and-temperature grid the course holds for any developer of this type. Wall notes that a 2.5 per cent solution of dry sodium sulphite may be used instead of water to dilute the developer.

Temperature		Development in minutes
°C	°F	Portrait	Architecture	Landscape
22	72	14	19½	27⅛
21	70	15	20½	28⅔
20	68	16	21½	30
19	66	17	22½	31⅓
18	64	18	23½	32⅔
17	62	19	24½	34
15.5	60	20	25½	35⅓
14.5	58	21	26½	36⅔
13.5	56	22	27½	38
12.5	54	23	28½	39⅓

The landscape column climbs by exactly 1⅓ minutes a step all the way down, which makes the printed 27⅛ at 22 °C the one figure in the table that breaks its own arithmetic; 27⅓ is what the pattern gives. The course prints what Wall printed and records the discrepancy rather than correcting him.

It is unusually sensitive to temperature. Wall’s table of Watkins’s temperature coefficients - the factor by which development time changes over 10 °C - gives paramidophenol 2.4 against 1.9 for rodinal, azol, victol and certinal and 1.9 for a metol-hydroquinone developer. A coefficient of 2.4 means that a bath five degrees cold takes about half as long again. The two rows are for the same developing agent in different formulations and Wall does not say which formulations, which is the second reason the temperature column of Munkman’s table is not decoration.

The factorial figures show the same split. Watkins’s factors - total development time divided by the time the image first appears - are rodinal 30, azol 30, certinal 30 and metol 30, against paramidophenol 16. Nearly a factor of two for one developing agent. Wall does not name the formulations behind either row. The course’s reading, marked as a reading, is that the 30 belongs to the caustic concentrates, where the image starts almost at once and the appearance time is therefore very short, and the 16 to a carbonate paramidophenol developer of the kind Wall prints on the same page. Nothing in the source says so.

What a manufactured developer of the type publishes now. Foma describes FOMADON R09 as a “liquid concentrate of a fine-grain, normal-working para-aminophenol negative developer”, used at 1+25 or 1+50, in a 250 mL bottle sufficient for 25 films of 135-36 or 120, or up to 62 sheets of 13 by 18 cm. Its published times at 20 °C, with continuous agitation for the first 30 seconds and then 10 seconds a minute:

Film	1+25	1+50
Fomapan 100 Classic	4 min	9 min
Fomapan 200 Creative	5 min	10 min
Fomapan 400 Action	6 min	12 min
Foma Ortho 400	5-6 min	10-12 min

Those are Foma’s numbers for Foma’s product on Foma’s films. They are not times for Wall’s formula, whose concentration of agent at 1:25 is about fourteen per cent higher than D-76’s and whose free alkali nobody has measured, and using them as if they were would be exactly the substitution this course exists to refuse.

Image characteristics

Grain is the contested one, and this page does not settle it.

⁇Is a rodinal-type developer fine-grained or coarse?

Foma says fine. Its own sheet calls FOMADON R09 a “fine-grain, normal-working para-aminophenol negative developer”. That is a manufacturer’s description of its own product, which is evidence, and it is also marketing copy, which is a reason to read it carefully.

The arithmetic points the other way. Fine grain in a developer like D-76 comes from silver-halide solvent action by a high concentration of sulphite, which dissolves a little off every grain so that neighbouring grains stop touching. Wall’s concentrate at 1:40 carries about 0.033 mol/L of sulphite, twenty-four times below D-76’s 0.79 mol/L; taken to 1+50, the dilution Foma publishes for its own product, the same concentrate would be thirty times below. There is not enough sulphite in a working bath of this class to do solvent work, so the course expects the grain of the film itself, sharply rendered, and not a fine-grain negative.

What would settle it: a measured grain comparison on one film between a rodinal-type bath and D-76, which nothing in the corpus supplies. Until then the page reports both positions and marks the second as the course’s inference from concentration, not as a measurement.

Sharpness. Expected to be high, for the same reason and with the same status: an inference from the class. A working bath with almost no silver solvent in it leaves grain boundaries intact, and one this dilute exhausts locally in the heavily exposed areas, which is the mechanism behind adjacency effects. Wall publishes no acutance measurement, and neither does Foma.

Stain. None. Wall’s 1912 entry states plainly that the paramidophenol developer “is stainless in character”, which is a real advantage over the pyro developers it competed with and is a property of the agent rather than of the formulation.

Contrast and tonality. Wall’s word for the class is energetic, and for this formula clean-working. No source the course holds publishes a contrast index, a gamma or a characteristic curve for any developer of this type, so the page states none.

Colour of the image. Not published for this formula. Wall records elsewhere that rodinal gives a brown image on some materials, in a passage about intensification rather than about normal development, and one sentence in another context is not a tonality claim.

The mechanism

Development is reduction, and Part VIII works that through with numbers. What is distinctive here is not how the silver is reduced but why the formula looks the way it does, and the answer is that the alkali was chosen for solubility, not for activity.

Kodak states the problem directly. Its 1928 primer, in the chapter on preparing developers: “A para-aminophenol-carbonate developer is difficult to prepare in concentrated form, though by adding a little caustic soda the solubility of the para-aminophenol is increased and a stronger solution can be thus prepared.” The 1924 edition says the same. That sentence is the whole design argument for the class: you cannot make a concentrated carbonate developer out of this agent, so you make a caustic one.

And the primer says the agent does not need a caustic alkali for activity. Ranking the agents by reduction potential - Kodak’s measure of how much bromide is needed to hold one back - it puts hydroquinone lowest, then Athenon, then pyro, then Kodelon, and Elon highest, and it draws the consequence: “The developers of higher reduction potential, which bring up the image very quickly, require less alkali than those of lower reduction potential.” Kodelon is para-aminophenol oxalate. It sits second from the top. It needs less alkali than hydroquinone, which is the agent that is “often used with caustic alkalis” - and yet the para-aminophenol concentrate is the caustic one. The alkali in this formula is there to keep the agent in the bottle, and it does the pH work as a consequence.

🧮Where the caustic soda goes

Per litre of finished concentrate, from Wall’s published weights.

50 g/L ÷ 145.59 g/mol = 0.343 mol/L

Developing agent

150 g/L ÷ 222.33 g/mol = 0.675 mol/L, giving 1.349 mol/L of bisulphite

Potassium metabisulphite

215 g × (340 mL ÷ 500 mL) = 146 g

Caustic soda taken, at the bottom of Wall's range

146 g ÷ 40.00 g/mol = 3.66 mol/L

Caustic soda, as moles per litre of finished concentrate

Now account for it. Freeing the base from its hydrochloride takes 0.343 mol. Converting 1.349 mol of bisulphite to sulphite takes 1.349 mol. Taking the phenolic proton off all of the agent takes another 0.343 mol. That is 2.04 mol accounted for out of 3.66, leaving roughly 1.6 mol/L of free hydroxide - and at the top of Wall’s range, 350 mL, about 1.7.

This is arithmetic on published weights and an assumption that each of those three reactions goes to completion. It is not a measurement, and the course will not turn it into a pH. At better than 1 mol/L the activity of hydroxide is not its concentration, no source the course holds publishes a pH for any developer of this class, and a calculated figure printed beside measured ones would be read as one of them.

What the number is good for is proportion. At 1:40 the working bath carries about 0.04 mol/L of free hydroxide. That is a strongly alkaline developer by any standard, and it is a fifth of what the same concentrate gives at 1:10 - which is why Wall adds bromide at the stronger dilution and not at the weaker.

The second half of the mechanism is what is missing. Set the working bath beside D-76:

Per litre of working solution	This formula at 1:40	This formula at 1:10	D-76 at stock
Developing agent	0.0084 mol/L	0.031 mol/L	0.0116 mol/L metol, plus 0.045 hydroquinone
Sulphite	0.033 mol/L	0.123 mol/L	0.79 mol/L
Alkali	about 0.04 mol/L free hydroxide	about 0.15 mol/L free hydroxide	0.0052 mol/L borax

Two notes on that table. The agent row counts developing-agent molecules, not weighed salts: D-76’s 2 g/L of metol is 0.0058 mol/L of the sulphate, and each molecule of that salt carries two aminophenol units, so 0.0116 is the comparable figure. And the alkali row compares things that are not alike - free hydroxide against a buffered borate pair - which is the point rather than a flaw in the comparison.

The agent concentration is ordinary; the sulphite concentration is not. At 1:40 there is about seventy per cent as much developing agent per litre as D-76 has metol, which is unremarkable. What has gone is the solvent: D-76’s 0.79 mol/L of sulphite is doing the fine-grain work, and 0.033 mol/L cannot. Everything the class is credited with - sharpness, adjacency effects, visible grain, its usefulness diluted to 1:100 for stand development - follows from that one line of the table rather than from anything exotic about para-aminophenol.

There is no second agent and no superadditivity. D-76 pairs a fast agent with a slow one and gets more than the sum of the two; Part VIII sets out what superadditivity is. This formula has one agent doing everything, which is why the alkali has to be strong enough to run it hard and why the bath is short-lived once diluted.

Function of every ingredient

Para-aminophenol hydrochloride, 50 g. The only developing agent, and the parent of the whole family: Kodak’s primer explains that treating it with methyl alcohol attaches a methyl group to give methyl-para-aminophenol, which is metol, the more active of the two. What you weigh is a salt, not the base - the base is what precipitates during mixing - and the distinction matters on the balance, because the hydrochloride is 145.59 to the base’s 109.13, so a formula written for the salt and mixed with the base puts a third more developing agent in the bottle than intended. Its position on Kodak’s reduction-potential scale, just below metol, means it brings the whole frame up at once rather than building the highlights first, and that it keeps working in the presence of accumulated bromide. More agent gives a faster, more energetic bath and, past a point, chemical fog; less gives a slow one that will not finish. In practice the dilution is the control, not the weight: Wall’s own two dilutions differ fourfold in everything at once. It is also the ingredient that carries the long-term health hazard here: its encyclopaedia page sets out the aggregated ECHA notifications, which are unanimous on harmful if swallowed and suspected of causing genetic defects, all but unanimous on harmful if inhaled, and divided about a third to two thirds on skin sensitisation.

Potassium metabisulphite, 150 g. It arrives as an acid salt and leaves as a preservative, and it is worth being clear that those are two different jobs separated by the caustic soda. In water the disulphite ion splits to two hydrogen sulphite ions, and while those are still acid they keep the whole mixture acid - and Kodak’s 1928 primer states the general fact that makes that useful: developing agents, with one exception, “must be in an alkaline solution” to do their work at all. An acid solution of the agent is a solution that is not yet a developer. Once the caustic soda has converted the bisulphite to sulphite, it is the preservative proper - scavenging dissolved oxygen and intercepting the oxidised agent before it can go on to a coloured product - and it is that 1.35 mol/L of sulphite, not the acidity, that keeps the finished alkaline concentrate alive on the shelf. At 150 g/L it is by far the largest ingredient by mass, and it is still not a solvent concentration once the bottle is diluted forty times. More metabisulphite means a longer-keeping concentrate that consumes more caustic to neutralise, so the free alkali of the finished bottle falls and the developer slows; less means a concentrate that browns sooner. Sodium metabisulphite is not a weight-for-weight substitute. Take the molar masses from the two encyclopaedia pages - 222.33 for the potassium salt, 190.11 for the sodium one - and each gives two bisulphite ions, so gram for gram the sodium salt delivers about 17 per cent more bisulphite because potassium is the heavier passenger. Substituting it changes the preservative level and the caustic budget at once.

Sodium hydroxide, 215 g in 500 c.c., of which about 340 to 350 c.c. is used. Three jobs, in this order: it neutralises the hydrochloride and frees the base; it converts the bisulphite to sulphite; it dissolves the freed base as the phenolate. Only what survives all three is alkali in the developing sense, and the arithmetic above puts that at roughly 1.6 mol/L - which is what makes this a caustic developer rather than a carbonate one. More soda dissolves the last of the precipitate, and Wall says explicitly not to - which is the opposite of what a published formula normally tells you to do, and the reason it is quoted in full above. Less and the agent crystallises out in the bottle. Because it is added by judgement rather than by weight, this is also the ingredient that makes two bottles of the same formula differ. It is the immediate hazard on the page: a strong base that causes severe skin burns and eye damage, and whose dissolution liberates enough heat to spatter.

Water, 625 c.c. boiled and cooled, plus whatever is needed to make 1000 c.c. Not inert. Boiling removes dissolved oxygen, which is the only reason a formula of three solids specifies a treatment for its water at all, and the few sacrificial crystals of metabisulphite that go into the cooled water take out what came back. Kodak Limited’s 1949 handbook adds the warning that applies to any concentrate: the solubility of most chemicals falls as the temperature drops, so a strong solution stored cold tends to throw some of its ingredients out, and “the precipitate which separates under such conditions often contains the most important” constituents of the solution. It is warmed and redissolved, never decanted off.

Potassium bromide, quantity not stated. Wall’s instruction for plates is “1 part with 10 parts water with some bromide”, and he never says how much. It is recorded here because leaving it out would misreport his instruction, and it is not in the structured formula because a quantity nobody published cannot be invented into one. What it is for is clear enough from the restrainer lesson: at 1:10 this bath is close to four times more concentrated in agent and in free alkali than at 1:40, and Kodak’s 1928 rule is that too much alkali tends to produce chemical fog. One practical point comes from Kodak Limited’s 1949 handbook and applies to every developer: since potassium bromide has no action on the developing agents, “it is immaterial at what stage it is added”, so it can go into the diluted bath rather than into the concentrate - which is exactly how Wall uses it.

Interactions

Agent and alkali. The tightest coupling in the formulary. In most developers the alkali sets the pH and nothing else; here it also decides whether the agent is in solution at all, so you cannot lower the alkali to soften the developer without precipitating the thing you are trying to use. The only control the reader is left with is dilution, which moves the agent, the sulphite and the free hydroxide together and in the same proportion. That is why the class is described by its dilution - 1:10, 1:25, 1:50, 1:100 - and not by variants of its composition.

Agent and sulphite. Two separate interactions get confused for one, exactly as they do on the D-76 page. The sulphite protects the agent from aerial oxidation, which is chemistry between the sulphite and the oxidation products; a high sulphite concentration also dissolves silver halide, which is chemistry between the sulphite and the film. In this formula the first is fully present and the second is absent, because 150 g/L in the bottle becomes under 4 g/L in the tank. The bottle is preserved and the bath is not a solvent, and both are the same ingredient at two dilutions.

Sulphite and alkali. The metabisulphite consumes more than a third of the caustic soda before any of it is available as alkali - 1.35 mol out of 3.66. Change one and you have changed the other, which is the practical reason this formula cannot be scaled ingredient by ingredient.

Restrainer. None in the concentrate, and one added at the tank for plates only. Unlike D-76, which acquires its own restrainer as it works because the bromide released by the film stays in a reused bath, a one-shot developer at 1:40 is thrown away before the released bromide matters.

Agent and agent. There is none. A single-agent developer has no superadditive partner, which removes both the benefit and one whole class of failure.

Variants

Wall’s own concentrate of 1912, printed in the Dictionary of Photography as “a good formula for a clean-working energetic developer for plates, films, and papers”, is the same three parts at different weights: 1000 c.c. of boiled water, 300 g of potassium metabisulphite, 100 g of paramidophenol, dissolved in that order, with caustic potash or soda added a little at a time “until the precipitate first formed is dissolved, and the solution is clear”, and used at 1 part in 10 to 40. It is the printing that supplies the description of the class this page quotes throughout, and it is a variant recorded here rather than an entry of its own for three reasons.

The alkali has no quantity, which is the disqualifying one: a formula whose alkali is “a little at a time until” cannot be published as quantities. The final volume is not stated either, so this is not simply twice the 1924 strength - it is twice the weights in a batch of unknown volume, and the caustic that goes in afterwards enlarges it by an unknown amount. And Wall names the solid simply “paramidophenol” here, where his two-solution formula on the same page names the hydrochloride; the instruction to watch for a precipitate implies a salt, but the source does not say which one, and that is a third figure the course cannot supply.

One substantive contradiction is worth keeping in view. Wall 1912 says to add alkali until the solution is clear; Wall 1924 says to stop short of clear and that it is “very important to leave some undissolved”. The two instructions are incompatible, neither source acknowledges the other, and the last of the experiments below is the one that would settle which is better advice.

The composition Rodinal itself “is said to be”. Wall’s 1912 Dictionary has a separate entry for the trade name, and it is careful about its own evidence:

The solution sold as Rodinal is said to be compounded as follows: crystallised sulphite of soda 30 parts, hydrochlorate of paramido-phenol 10 parts, distilled water 100 parts. Add strong caustic soda solution until there is a complete re-dissolving of the precipitate which is first formed.

“Is said to be” is Wall’s hedge and the course keeps it. This is a report of a proprietary composition, not a disclosure, it is given in parts rather than in weights and volumes, its alkali has no quantity, and it differs from the formula on this page in a way worth noticing: it uses crystallised sodium sulphite rather than metabisulphite, so the caustic soda has no bisulphite to neutralise and does less work. Its sulphite-to-agent mole ratio is about 1.7 to 1 where Ermen’s is 3.9 to 1. The course records it as a period claim about a product and uses it for nothing else.

Wall’s carbonate paramidophenol developers, on the same 1924 page, are the same agent without the concentrate idea, and they show what the caustic bought. The one-solution version is para-aminophenol hydrochloride 4 g, sodium sulphite dry 40 g, sodium carbonate dry 40 g, water 1000 c.c., “ready for use” and, in Wall’s words, it “does not keep so well as in separate solutions”. The two-solution version is A of 20 g of the agent per litre and B of 60 g of sulphite and 120 g of potassium carbonate per two litres, mixed 1 part A to 2 parts B, and Wall notes it is “more suitable for papers and transparencies than negatives”. Both are working-strength baths, both keep worse than the concentrate, and neither can be made strong enough to bottle - which is Kodak’s point about the carbonate route exactly.

Kodak’s D-173, printed in the 1944 and 1949 London handbooks, is the other thing the agent was used for: a developer for Velox papers containing no Elon, at 0.375 g of para-aminophenol hydrochloride and 7.5 g of hydroquinone per litre of stock, made expressly for workers with metol dermatitis. It is a paper developer of ordinary construction and belongs to this page only as evidence that the agent had a second career as metol’s substitute.

The manufactured concentrates. Kodak Limited sold Kodinol, listed in its 1944 formulary as “a highly concentrated developer solution which requires only dilution with 16 to 40 parts to make a clean-working developer for general work: the concentrated solution has a long life”, and did not print its composition. Foma sells FOMADON R09 and discloses only the agent class. ADOX sells RODINAL and the datasheet the course holds cannot be read. Wall’s 1924 list adds azol, activol, citol, paranol, paramol, certinal and kalogen. Not one of these is a variant of the formula on this page, because nobody has published what any of them contains; they are members of the same class, which is a weaker statement and the only one the evidence supports.

Safety

Level B, and the reason is the caustic soda rather than the developer.

Sodium hydroxide is classified Danger, causing severe skin burns and eye damage, and this formula asks you to dissolve 215 g of it in 500 mL of water - about 430 g/L, near enough a saturated solution at room temperature. That dissolution is strongly exothermic, and Kodak’s 1928 primer states the consequence twice and in its plainest language: both caustic soda and caustic potash “should be dissolved in cold water … because on mixing considerable heat is evolved and the solution, if too hot, is apt to boil and spatter on the hands or face causing serious burns”, and, against its own caustic developer formula, “if hot water is used, the solution will boil with explosive violence”. Para-aminophenol adds the second criterion: harmful if inhaled as a dry powder, suspected of causing genetic defects, and classified as a skin sensitiser by about a third of the notifiers its encyclopaedia page aggregates.

Under the classification rubric, Level B is the level for concentrated alkalis, for sensitisers and fine powders that must not be inhaled, and for procedures whose failure mode is a burn rather than a spoiled negative. It does not reach Level C: no fume cupboard is the recognised control for any step here, and HSE’s COSHH essentials sheet P1 for manual film and plate development asks for general ventilation of more than five air changes an hour with a through draught, single-use nitrile gloves 0.2 mm thick, eye protection and cotton overalls, and says respiratory protection “is not normally needed for routine operations”.

⚠The caustic solution is the dangerous part of this page

Splash goggles, not glasses, for every step from opening the caustic soda tub to bottling the finished concentrate. Nitrile gloves and an apron. Eyewash within reach and known about before you start - see the first aid quick reference and the alkali spill SOP.

Cold water, and the solid into the water, never water onto the solid. Stir as it goes in so the dense caustic layer does not sit at the bottom. Never in a closed or narrow-necked vessel: the heat can pressurise it. Let the soda solution cool fully before it goes near the paramidophenol solution.

Weigh the aminophenol with extraction running or inside an enclosure, never in a draught. It is a fine dry powder classified harmful if inhaled in all but a fraction of the notifications its encyclopaedia page aggregates. HSE’s HSG262 makes the point that decides how seriously to take the minority sensitisation classification: sensitisation “can develop over time and it may be weeks, months or even years before it becomes apparent”, and once a person is sensitised “tiny amounts of the allergen” will trigger the reaction and “the only remedy is to prevent further exposure”.

If you spill the caustic, COSHH essentials P1 asks for an impervious apron and new 0.4 mm nitrile gloves for the clean-up, not for the 0.2 mm gloves used at the bench, and for the absorbent and the gloves to be discarded as hazardous waste.

The finished concentrate is still strongly caustic. Treat every dilution as a decanting of a corrosive liquid, and label the bottle before you put it down.

What is not a hazard here. Nothing in this formula evolves a gas in normal use provided no acid comes near it - the metabisulphite is the one ingredient that would, and only on contact with acid, which is covered under Incompatibilities. Nothing is heated deliberately; the only heat in the procedure is the caustic soda heating itself, and boiling the water, both of which happen before the two solutions meet. There is no silver in a fresh developer and therefore no recovery question at the mixing bench. And the diluted working bath, at 1:40, is a dilute alkaline solution rather than a corrosive one - which does not make it harmless, but does mean the controls above are for the bench and the bottle rather than for the tray.

Storage

A tightly stoppered bottle, filled full, kept dark and cool but not cold. Three specific things follow from the chemistry rather than from general good practice.

Not a glass stopper. Kodak Limited’s 1949 handbook is explicit: “Glass stoppered bottles are not desirable, as the alkali is apt to make the stopper stick”, and this bottle carries free hydroxide at better than a mole per litre. A plastic screw cap, and a bottle you can still open in a year. The same page asks for a tightly corked bottle with only a small air space, left so that the solution can expand with temperature without loosening the stopper or bursting the glass, and recommends several small bottles over one large one, because the air space in a large bottle grows every time it is opened.

Dark. Solid para-aminophenol turns violet on exposure to light - the property its encyclopaedia page records from CAMEO - so an amber or opaque bottle is not decoration, and neither is a cupboard.

Expect a deposit, and do not throw it away. Wall’s instruction is to leave some of the base undissolved on purpose, so a bottle of this concentrate is meant to have solid at the bottom - and his instruction if more crystallises out is to add more soda solution to nearly dissolve it, not to decant the clear liquid. Kodak Limited’s general warning says the same thing for a different reason: a precipitate formed in the cold often contains the most important constituents of the solution.

Label it with the formula, the date mixed and the fact that it is caustic, using the labelling SOP. Wall’s 1912 note that the class “keeps well in single concentrated solution” is the only keeping claim the course has for it, and it comes without a period, so date the bottle and test it against a control strip rather than trusting a number nobody published.

Incompatibilities

Acid, and this one is not merely a pH problem. Potassium metabisulphite generates gaseous sulphur dioxide on contact with acids - the reactivity its encyclopaedia page records - and this concentrate carries 150 g/L of it alongside a large excess of caustic soda. Acid poured into this bottle would neutralise the alkali, liberate the acid sulphite, and evolve an irritant gas from a vessel that is also boiling itself with the heat of neutralisation. Never put stop bath, acetic acid, citric acid or an acid fixer into a rodinal-type concentrate, and never use the same graduate for both without washing it. See chemical incompatibilities.

Aluminium, zinc, tin and lead. A strong alkali attacks all four with evolution of hydrogen. Mix and store this in glass or in a polyethylene bottle, never in a metal jug or a foil tray.

Fixer, in either direction. Thiosulfate carried back into a developer is a far more aggressive silver solvent than sulphite and will fog and stain. Developer carried into the fixer shortens its life, and a developer carries negligible silver - the figure the p-aminophenol page takes from Kodak’s environmental guidance - so mixing the two spoils a stream that could have gone for recovery and recovers nothing.

Oxidising agents - ferricyanide, dichromate, permanganate, persulfate - must never meet a developing agent in a bottle or a drain.

And its own dilution water. This is a real incompatibility rather than a pedantic one: Wratten and Wainwright’s measurements show development time changing by nearly a quarter between air-free distilled water and tap water at the same dilution, which is a larger effect than most of the variables a careful worker controls.

Waste

Spent working solution from this developer is alkaline and it is aquatically toxic, and the two need separate answers. The p-aminophenol page records the aggregated notifications: very toxic to aquatic life with long-lasting effects, in every report that classifies the substance, with the precautionary statements for avoiding release to the environment, collecting spillage and disposing to an approved facility attached. Neutralising the alkali would do nothing about that half of the problem, so there is no bench treatment this course can offer that would change what the stream is.

Bottle it separately from fixer and stop bath, label it with the substance and the date, and take it to a licensed household hazardous waste route. In England and Wales the government’s hazardous waste service finder will say whether your council offers one; it covers England and Wales only, and the council decides what it will accept.

Unused caustic soda solution is a separate problem from spent developer and should be kept separate, labelled as corrosive, and disposed of through the same hazardous-waste route rather than poured away. The disposal caveat governs and the general chemical waste SOP gives the procedure. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Crystals in the bottle. Expected, and not a fault - Wall asks for them. If the deposit is growing, his instruction is to add more of the caustic soda solution, a little at a time, until it nearly dissolves. Warming the bottle gently and redissolving is the other half of the answer, and decanting the clear liquid off the top is the mistake: the precipitate is the developing agent.

The concentrate has gone dark brown or violet. Two different causes with two different answers. Oxidation of the agent gives the brown, and a bottle that has been half full for months is a slow oxidation cell; light gives the violet, because the solid para-aminophenol darkens in light. Neither colour proves the bath is dead - test it against a control strip before you condemn it, using the aerial oxidation experiment as the method - but a concentrate that has changed colour has lost some agent.

Development is far slower than the dilution predicts. This is the normal behaviour of the class, not a fault, and Wratten and Wainwright measured it: 1:200 needed 42 to 52 minutes where proportion predicted 30, with the spread depending on the air in the water. Time a dilution series; do not calculate one.

Times drift between sessions at the same nominal temperature. Watkins’s coefficient of 2.4 for paramidophenol means a bath two degrees cool needs about 19 per cent longer. Measure the bath, not the room.

Fog and flat, dense negatives at 1:10. At the strong dilution this is an energetic caustic developer with no restrainer, and Wall’s own instruction is to add bromide there. Kodak’s general rule applies: too much alkali tends to produce chemical fog. Dilute further, or restrain, and change one at a time.

Soft, frilling or blistered gelatin. Kodak’s 1928 primer states it as a general property: alkalis soften the gelatin of the coating, and a caustic developer is at the sharp end of that. It shows up most on plates and papers in warm weather, and it is the reason a rinse or a stop bath and a hardening fixer belong in the sequence rather than being optional.

Two bottles of the same formula behave differently. Almost certainly the caustic. Wall’s end point is judged by eye, “enough to nearly dissolve”, and the range he prints for the soda solution - 340 to 350 c.c. - is a three per cent spread in the caustic added before anyone’s judgement is involved. Because the first 2.04 mol of it is spoken for by the hydrochloride, the bisulphite and the phenol, that three per cent at the tap becomes nearly seven per cent in the free alkali that is left. Record the volume you actually added, in the developer laboratory report, or you will not be able to repeat the bottle.

Experiments

Add the sulphite back. Mix two working baths at 1:40, and to one of them add sodium sulphite to 50 g/L. Develop matched strips from one exposure batch. You are testing the claim that the only important difference between this class and D-76 is the solvent concentration, and the prediction is finer grain and softer edges in the sulphited bath. This is the solvent series run from the other end.

Repeat Wratten and Wainwright. Develop matched strips at 1:20 and at 1:200 to the same density, and record the times. The published result is 3 minutes and 42 to 52 minutes rather than 3 and 30. Then run the 1:200 strip again with water that has been boiled and cooled in a stoppered flask; the published difference between air-free and tap water is about ten minutes in fifty, which is large enough to see.

Measure the temperature coefficient. Develop to matched density at 15 °C and at 25 °C and take the ratio of the times. Wall’s table predicts about 2.4 for a paramidophenol developer against 1.9 for a metol-hydroquinone one, so run D-76 alongside as the control. This is one of the few numbers in the historical literature that a home darkroom can falsify in an afternoon.

Find the factorial factor. Time the first appearance of the image and the total development to normal density, and divide. Watkins’s published figures are 30 for rodinal and 16 for a paramidophenol developer, and the course’s reading is that the difference is the alkali rather than the agent. Mixing Wall’s carbonate one-solution formula alongside this concentrate would test that reading directly, and would be the first evidence anyone in this course has for it.

Bracket the caustic. Make two half-litre batches, one stopping the soda addition as soon as a visible deposit remains and one carrying it on until the solution is clear - the 1924 instruction against the 1912 one. Record the volume added in each case, and develop matched strips at 1:40 over several months. This tests the one instruction in the formula that its source gives without a reason.