Kodak D-51

Two chemicals and water, with no alkali of any kind: this is the 1928 version of the idea that D-170 publishes twenty-one years later, and reading the two together shows Kodak changing its mind about how to package an unstable developing agent.

Ingredient	Quantity	Form the source specifies
Sodium sulfite	120.0 g	Kodak’s own tested grade
Amidol	37.5 g	named in the formula as Acrol, diaminophenol hydrochloride
Water to make	1000 mL	750 mL of it at about 52 °C to start

Potassium bromide is added at the tray, to the diluted working solution, and is not part of the stock.

Purpose

To develop a bromide enlarging paper with an alkali-free bath. Kodak’s own primer supplies the reason the formula can exist at all, in the sentence that names its single exception: “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.”

Recommended uses

Bromide papers, at the primer’s working dilution: 180 mL of stock, a small measured addition of 10 per cent potassium bromide solution, and 750 mL of water.

No development time is published for this formula. The primer gives times for its neighbours — 45 seconds for D-72 on Velox, not less than one and three quarter minutes for D-49, not less than one and a quarter for D-52 — and for this one it gives none. The course does not supply one. What it can tell you is that D-170, Kodak’s later amidol formula for the same material at about two thirds the agent concentration, is published at two minutes at 18 °C, and that a developer carrying more agent will not need longer. Treat that as a place to start a test strip, not as a time.

When another formula is preferable

• For an amidol paper developer with a published time and a published capacity, D-170 is the better-documented member of the pair by a wide margin.
• For general printing with a keepable bath, D-72 or D-49, both on the same pages of the same primer.
• For a developer whose restrainer you set per paper, D-52 publishes four doses.

Mixing

1. Start with 750 mL of water at about 52 °C.
2. Dissolve the sodium sulfite completely. It goes first: the sulfite is what the agent needs to be dissolving into.
3. Dissolve the amidol. At 37.5 g/L this is well inside its solubility — the primer’s own table gives a saturated solution at about a quarter of its weight — so it should go into a warm sulfite solution readily.
4. Make up to 1000 mL with cold water.
5. At the tray, take 180 mL of stock, add the 10 per cent potassium bromide, and make up with 750 mL of water. Mix only what you are about to use: the primer’s warning is that the developer “oxidizes quite rapidly when exposed to the air so that only enough developer should be mixed for immediate use”.

!A figure the scan does not settle

The primer’s working dilution prints the bromide addition in both its columns, and the course’s scan of that line reads three quarters of a fluid dram (4 c.c.). Those two do not convert into one another: three quarters of a US fluid dram is about 2.8 mL.

The course prints the metric figure, as it does throughout the formulary, and records the discrepancy instead of resolving it. Two readings are available and neither can be confirmed from the document the course holds: either the avoirdupois figure is one fluid dram, which is 3.7 mL and agrees with 4 c.c., or the metric figure is 3 c.c. and agrees with three quarters of a dram. At 0.4 g against 0.3 g of bromide per litre of working solution the practical difference is small; the reason to write it down is that a reader working from the other column should know why the two do not match.

Whichever it is, this quantity is not critical in the way the agent’s is. It is a restrainer added to taste, and D-52 shows Kodak varying the same addition over a four-fold range across four papers.

Behaviour

It is an alkali-free bath and it behaves like one. Amidol starts fast and then needs comparatively little further time, which is the shape the chemical page records and which is what suits it to bromide papers. With no carbonate there is no pH lever, so a printer’s controls are the dilution and the bromide.

It does not keep, and Kodak says so in the formula itself. The warning is unqualified: the developer oxidises quite rapidly in air, so mix only what you need. Treat every made-up litre as a one-shot bath.

Nothing about its capacity or its life is published. The 1928 primer prints no keeping-properties table for developers, so there is no sheet count and no bottle life to quote. For comparison, Kodak’s 1949 handbook gives its own amidol formula thirty minutes in a dish and 48 sheets of 8 × 10 inches per 160 fluid ounces, which is the nearest published figure and is for a different formula.

⚗Two authorities, one bottle, and Kodak changing its mind

This formula keeps its developing agent in the stock solution. That is a claim about keeping, and the corpus contains a flat contradiction of it.

Kodak, 1928: prints a litre of sulfite and Acrol as a stock solution, from which the working developer is diluted as needed. The implication is that the stock keeps and the diluted bath does not.

Wall, 1912, as recorded on the amidol page: keep it dry and dissolve as required, because the aqueous solution does not keep even with sulfite.

Kodak, 1949: publishes D-170, in which the stock solution contains only the sulfite and the bromide, and the agent is dissolved into the diluted bath at the moment of use. The same company, for the same purpose, has moved the agent out of the bottle.

The course does not declare Wall right and the 1928 primer wrong; it records that a practice was published, that a respected contemporary source disagreed with it, and that the manufacturer’s own later formula adopted the other arrangement. Under Rule 7 that is three pieces of evidence and not one conclusion. The practical advice that follows is unaffected either way: make the stock in small quantities, date it, and test it against a control print before trusting it.

Image characteristics

Nothing is published. The primer gives this formula no description of contrast, tone or image colour, and the course invents none.

What is available, and is labelled as belonging elsewhere, is the amidol page’s account from Wall of a different formula at another strength: at full strength it “gives fine steely black tones with correct exposures”, with diluted solutions “clear grey tones are obtained”, and warmer tones follow from longer exposure and liberal bromide. That is the nearest published description of what an amidol print looks like, and it is a description of Wall’s bath rather than of this one.

Staining. None. Grain, acutance and speed are not meaningful properties of a paper developer.

The mechanism

37.5 g/L ÷ 197.06 g/mol = 0.190 mol/L

Amidol in the stock, as the dihydrochloride

120.0 g/L ÷ 126.05 g/mol = 0.952 mol/L

Sodium sulfite in the stock

0.952 ÷ 0.190 = 5.0 to 1

Sulfite per amidol

37.5 g/L × 180 mL ÷ 934 mL = 7.2 g/L, or 0.0367 mol/L

Amidol in the working bath

Five sulfites to every amidol, in the stock. The amidol page balances what excess sulfite does to the dihydrochloride: it takes up the two equivalents of acid, leaving the free base in solution. Two equivalents is what the stoichiometry needs and five is what Kodak supplies, so the remaining three are doing the preservative’s ordinary job of scavenging oxygen and intercepting oxidation products.

That ratio is the design decision of the page. D-170 uses nearly nine sulfites to every amidol in its finished litre and survives thirty minutes in a dish; this stock has five, and it also has to survive storage. Whether five is enough is exactly the question the callout above says the sources disagree about.

The working bath is 60 per cent stronger in agent than D-170’s. 7.2 g/L against 4.5, at about the same sulfite concentration and less than half the bromide. A stronger, less restrained amidol bath will be faster and, on Wall’s account of the same variables, colder in tone. The course states that as the direction the chemistry points and not as a published property, because the primer publishes nothing about how this bath prints.

And there is still no alkali anywhere. Everything a carbonate does in the other formulas here — setting which agent works, driving fog, swelling the gelatin, exhausting the bath — is simply absent. What the course will not claim is why amidol needs no alkali. No document it holds explains the mechanism or names the reducing species, and the amidol page says so in the same words.

Function of every ingredient

Sodium sulfite, 120.0 g. Doing two jobs, and the first is unusual. It is the acid acceptor that frees the developing agent from its dihydrochloride, at five equivalents to the two the stoichiometry requires. It is also the preservative, scavenging dissolved oxygen and intercepting oxidised amidol — and the primer’s own warning that the developer oxidises rapidly in air is an admission that even 120 g/L is not enough once the bath is diluted and standing. In the working solution it falls to about 23 g/L, which is where the protection actually has to work. More sulfite would extend the working life; less and the stock would not hold the agent at all. It also supplies the whole of the bath’s acid reserve for the stop bath, there being no carbonate.

Amidol, 37.5 g. The only developing agent, and the only one in this formulary that works without an alkali. It starts development quickly and needs comparatively little further time. It is sold as the dihydrochloride at 197.06 g/mol, and the chemical page is emphatic that this is where formulas of the period go wrong for a modern reader: Wall’s 1912 entry quotes the free base at 124.14, and weighing the base where the formula meant the salt puts 59 per cent more reducing molecule into the bath. Kodak’s own naming settles which article this formula means — it writes “Acrol (Diaminophenol Hydrochloride)” in the ingredient line. More amidol raises activity and shortens the life; less and a print will not finish. It carries H301, toxic if swallowed, at 96.1 per cent of notifiers, which is the reason this page’s Safety section is not routine.

Potassium bromide, added at the tray. The restrainer, and not part of the stock. It suppresses development of grains carrying no latent image, keeping the whites of a print white, and in an alkali-free bath it is also one of only two tone controls the printer has. Kodak’s dose here works out at about 0.4 g per litre of working solution, which is less than half what D-170 carries. More bromide cleans the whites, slows the bath and, on Wall’s account, warms the image; less fogs. Keeping a 10 per cent stock solution is the practical way to dose it, and the mixing SOP has the procedure.

Water to 1000 mL. Not inert, and three quarters of it goes in warm. Iron is the classic contaminant. In this formula water is also the timer, because the agent’s life begins when it meets it.

Interactions

Agent and preservative, chemically coupled. The sulfite does not merely protect the amidol; it converts the salt into the species that develops. That is why the sulfite dissolves first and why there is no version of this formula in which the agent goes into plain water.

Agent and air. The dominant interaction, and the one Kodak warns about in the formula itself. It is also the interaction the 1949 formula was redesigned around.

Agent and alkali — absent, and to be kept absent. The amidol page’s warning is explicit: do not try to make an amidol stock keep by adding carbonate, because alkali is what destroys it. Carbonate carried in on a tray or a graduate from another developer will do the same thing.

Restrainer and agent. With a single agent there is no asymmetry to exploit — bromide slows the whole development rather than one half of it — so the bromide here is a tone and cleanliness control rather than the selective instrument it is in a metol–hydroquinone formula. Part VIII covers restrainers.

Variants

D-170 is Kodak’s own later answer to the same problem, and the pair is the most instructive comparison in this corner of the library: same chemistry, same material, the agent moved out of the stock, a published time, a published capacity and a published dish life.

Wall’s 1912 formula, quoted on the amidol page, is the same shape at another strength and is not published as a formulary entry, because the course’s corpus holds it only at second hand through that page’s citation.

The course’s own variants belong in the formula version record. The obvious one here is a stock at D-170’s sulfite-to-agent ratio, to test the keeping question the two sources disagree about.

Safety

Level B, on one ingredient. Amidol is classified Danger with three pictograms: H301, toxic if swallowed, at 96.1 per cent of notifiers — a category worse than the “harmful if swallowed” carried by metol and hydroquinone — with H315, H319 and H335 at 94.1 per cent each, and two sensitisation statements, H317 and H334, may cause allergy or asthma symptoms or breathing difficulties if inhaled, at 13.7 per cent each.

This formula asks you to weigh 37.5 g of it, which is by a long way the largest single weighing of a toxic organic anywhere in this library, and it is a fine powder. Weigh with extraction running or inside an enclosure, never in a draught, with gloves and splash goggles on and a mask rated for fine particulates if the supplier’s safety data sheet asks for one.

In the tray, toxic-if-swallowed is a hand-to-mouth hazard and print developing is where hands are wettest. Print tongs, one pair per tray, never interchanged; hands washed before leaving the darkroom; nothing eaten or drunk in the room.

What is not a hazard here is alkalinity: with no carbonate the working bath is not caustic, and on that one axis it is the mildest paper developer in this library. The classification rubric sets what Level B assumes, the gloves page covers the choice and the first aid page covers a splash.

Storage

Store the dry chemical dry. The amidol page’s instruction is cool, dark, tightly closed, in a labelled container never used for food, away from oxidisers and from alkalis.

The stock solution is the contested part. Kodak publishes it as a stock; Wall says an aqueous amidol solution does not keep even with sulfite; and Kodak’s own 1949 formula moved the agent out of the bottle. No keeping figure is published for this stock at all. The course’s practical advice is therefore to make it in small quantities, fill the bottle to the top, date it per the labelling SOP, and test it against a control print before using it on anything that matters — with the expectation that it will not keep long.

The working solution is not stored. Mix what you are about to use.

Incompatibilities

Alkali of any kind, which for this formula is the important one. Carbonate, borax or hydroxide destroys an amidol bath, and it arrives most often on shared trays, graduates and tongs.

Acid, deliberately, in the next tray — with the note that this bath’s entire acid reserve is its sulfite, so a stop bath meets much less resistance after it than after a carbonate developer, and the sulfur dioxide chemistry Part X sets out is the thing to keep in mind.

Fixer, in either direction. Thiosulfate carried back into a developer fogs and stains; developer carried forward raises the fixer’s pH.

Oxidising agents — ferricyanide, permanganate, persulfate — never meet a developing agent in a bottle or a drain. See chemical incompatibilities.

Waste

A dilute, near-neutral solution carrying amidol and sulfite, and — in the stock — a concentrated one. Amidol’s classification is heavier than the other developing agents here, so neither stream is one to be casual about. It carries essentially no silver and belongs in its own labelled container, never in the fixer bottle. The disposal caveat governs and the general chemical waste SOP gives the procedure. Local regulation decides.

Troubleshooting

Nothing develops, or development is very slow, from a stock that has been standing. The most likely explanation is the one Wall gives: the aqueous solution has not kept. Test a fresh stock before concluding anything about the formula.

A working bath that dies within the session. Expected, and stated by Kodak. Mix only what you will use.

Prints that come up instantly and then stop short of black. Amidol’s characteristic shape when the bath is tiring, or an under-weighed agent. Check which article you weighed: the free base and the dihydrochloride differ by 59 per cent in reducing molecule per gram.

Fog. With no alkali the fogging pressure is low, so suspect the safelight first. Then check whether the bromide addition was made at all — it is not in the stock, so it is easy to forget.

A stock solution that has turned dark. Oxidised amidol. There is no second agent to carry it and no published tolerance to work to, so treat a discoloured stock as spent.

Experiments

Settle the keeping question for yourself. Mix a small stock, seal it full, and print a matched negative from it on the day it is made and at one, two and four weeks, against a fresh control each time. The primer implies it keeps; Wall says it does not; Kodak’s later formula sides with Wall. This is a documented disagreement that a home darkroom can actually resolve, and the result belongs in the lab notebook.

Find the missing development time. Develop identical prints for 30, 60, 120 and 240 seconds at 21 °C and read maximum black. Where the curve flattens is the completion time Kodak did not print. Record it against the developer laboratory report.

This against D-170. One negative, one paper, both baths at their published dilutions, both developed to completion, density matched by exposure. The working baths differ by 60 per cent in agent and by more than half in bromide. Compare contrast, maximum black and tone.

Vary the sulfite-to-agent ratio. Mix the stock at 120, 180 and 240 g/L of sulfite with the agent unchanged, and time how long each working bath survives uncovered in a dish. You are testing whether the difference between Kodak’s two amidol formulas is really the sulfite ratio, which is the hypothesis this page offers and does not test.