Kodak D-170

There is no alkali in this formula. Not a weak one, not a small amount of one — none. That single absence is why it is here, and it is a fact Kodak’s own primer states as a rule with exactly one 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, and the energy depends upon the amount of alkali present.”

Acrol, Dolmi and amidol are three trade and common names for the same substance, and Kodak Limited’s own list of tested chemicals in the same handbook settles the identification: “Dolmi (pure diaminophenol hydrochloride)”.

Ingredient	Quantity per finished litre	Form the source specifies
Sodium sulfite	25.0 g	anhydrous — or 50.0 g crystalline
Potassium bromide	1.0 g
Amidol	4.5 g	Kodak calls it Dolmi
Water to make	1000 mL

Purpose

To develop a bromide enlarging paper without an alkali, and therefore without the fogging tendency, the gelatin swelling and the pH-driven agent balance that every other developer in this library has to manage. Kodak’s product list offers it as “a developer for bromide papers (alternative to D-163)”, which is a striking pairing: the most vigorous carbonate formula in the handbook, and one with no carbonate at all.

Recommended uses

Bromide papers at working strength, two minutes at 18 °C. Made up immediately before use, used, and thrown away.

There is no dilution to choose because there is nothing to dilute: the 200 mL “stock solution” in the handbook holds only the sulfite and the bromide, and the developer is complete the moment the Dolmi goes in. This is a one-shot developer whose published dish life is thirty minutes.

When another formula is preferable

• For anything that has to sit in a dish for an hour, every other formula in this library. Thirty minutes is the shortest working life published anywhere in the handbook.
• For the same job with a carbonate developer, D-163 at 1:3, which is the alternative Kodak itself names.
• For general printing at normal contrast, D-72.
• For low contrast, D-165.
• For the same alkali-free chemistry in the 1928 primer’s version, D-51, which is stronger, uses a keepable stock and publishes no development time.

Mixing

Kodak’s own two-step route, reproduced exactly:

1. Dissolve 50.0 g of crystalline sodium sulfite (or 25.0 g of the anhydrous salt) and 1.0 g of potassium bromide in water and make up to 200 mL. This is the handbook’s stock solution, and it is the part that keeps.
2. When you need developer, dilute the whole 200 mL with water to make 1000 mL.
3. Dissolve 4.5 g of Dolmi in the diluted solution.

The handbook then adds the sentence that governs everything about using it: the diluted solution does not keep well and should be made up as required.

⚗Why the agent goes in last, and what the sulfite is doing to it

In every other formula in this library the order of dissolving is about protecting a developing agent from air while the alkali arrives. Here it is about something else.

Amidol is sold as the dihydrochloride of 2,4-diaminophenol — a salt of a base, not a phenol needing an alkali to ionise it. The chemical page balances what a large excess of sulfite does to that salt:

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

Stoichiometry, from the amidol page: sulfite in excess takes up the two equivalents of acid

At 0.198 mol/L against the agent’s 0.023 mol/L there are nearly nine sulfites for every amidol, which is more than four times what that equation needs. So the bath the agent dissolves into is already prepared to receive it, and the moment it does, the clock starts. That is why it goes in last, and why the developer is finished thirty minutes later.

What the course will not claim is why an amidol molecule can reduce silver without an alkali when a phenol cannot. No document this course holds explains it or names the reducing species, and the equation above is stoichiometry rather than mechanism. The amidol page says so in the same words.

Weighing 4.5 g of amidol is the operation to plan for. It is a fine powder classified toxic if swallowed, and it is going into the tray you are about to put your hands near. See Safety.

Behaviour

It starts fast and finishes early. The amidol page records the shape: it starts fast and then needs comparatively little further time, which is why the literature reaches for it on bromide papers and lantern slides. Two minutes is a generous published time for a bath that will have shown you most of the print in the first thirty seconds.

It has the shortest working life and one of the largest capacities in the handbook, and those two facts are not in conflict. Forty-eight sheets of 8 × 10 inches per 160 fluid ounces is more than D-163’s thirty and more than D-158’s thirty-six. The bath is not short-lived because it exhausts on prints; it is short-lived because it oxidises in air. Put fifty sheets through it inside half an hour and it will do the work. Leave it standing for forty minutes and it will not.

The stock and the developer keep quite differently, and the 1949 table does not label which of its columns applies to which. It gives thirty minutes in a dish, three weeks in a full stoppered bottle, and not recommended for both a tank and a half-filled bottle. The three-week figure can only sensibly belong to the sulfite-and-bromide stock, because the handbook says in the formula itself that the diluted developer does not keep. The course reports both figures and marks the ambiguity rather than choosing.

Image characteristics

Contrast. Normal on bromide paper; Kodak positions it as an alternative to a normal-to-high contrast formula.

Tonality. Full, with the blacks arriving early.

Colour. Not stated by Kodak. What the course can say comes from the amidol page and is Wall’s rather than Kodak’s: at full strength his amidol formula “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 a different formula at a different strength, offered here as the nearest published description and labelled as one.

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

The mechanism

4.5 g/L ÷ 197.06 g/mol = 0.0228 mol/L

Amidol, as the dihydrochloride

25.0 g/L ÷ 126.05 g/mol = 0.198 mol/L

Sodium sulfite

1.0 g/L ÷ 119.00 g/mol = 0.0084 mol/L

Potassium bromide

0.198 ÷ 0.0228 = 8.7 to 1

Sulfite per amidol

No alkali means no pH lever, and everything follows from that. In a carbonate formula the alkali decides which agent is working, how fast, how much fog appears, how much the gelatin swells and how quickly the bath tires. Remove it and all of those become properties of the agent and its concentration alone. Kodak’s own rule — the energy of a developer follows the amount of alkali — simply does not apply to this bath.

The consequences are visible in the published figures. The bromide is 1.0 g/L, a modest dose, and it can be modest because there is no alkali driving fog. The sulfite is 25 g/L, which is low for a formula that must protect a readily oxidised agent, and the thirty-minute dish life is what that buys. There is no second agent and therefore no superadditivity to fall back on, exactly as in D-165 — but where D-165 answers by nearly doubling its metol, this one answers by being mixed fresh.

And the tone controls are different. With no alkali to move, a printer working in amidol has two levers rather than three: the dilution and the bromide. That is what the amidol page means when it says the property amidol was named for is what kept it — a developer needing no alkali is one whose print tone is steered by dilution and bromide alone.

Function of every ingredient

Sodium sulfite, anhydrous, 25.0 g. Doing two jobs, and the second is unique to this family of formulas. It is the preservative, scavenging dissolved oxygen and intercepting the oxidised agent — and it is losing that race, which is what the thirty-minute dish life records. It is also the acid acceptor that frees the developing agent from its dihydrochloride, taking up two equivalents of acid per molecule of amidol, with more than a four-fold excess over what the stoichiometry requires. More sulfite extends the working life and, at much higher concentrations, would begin to dissolve silver; less and the bath is finished before the session is. It also carries the bath’s acid reserve into the stop bath, and it is essentially the only reserve there is, because there is no carbonate.

Potassium bromide, 1.0 g. The restrainer, suppressing development of grains carrying no latent image and keeping the whites of a print white. One gram per litre is modest by the standards of this library, and it can be, because the fogging pressure a carbonate formula applies is absent here. In an amidol bath the bromide is also the printer’s principal tone control, alongside dilution, which is more weight than it carries in any carbonate formula. More bromide cleans the whites and, on Wall’s account, warms the image; less fogs.

Amidol, 4.5 g. The only developing agent, and the only one in this formulary that works without an alkali. It starts development quickly and then needs comparatively little further time, which is what suits it to bromide papers. It is sold as the dihydrochloride at 197.06 g/mol, and the chemical page is emphatic about the trap: Wall’s 1912 entry quotes the free base at 124.14, and weighing the base where a formula meant the salt puts 59 per cent more reducing molecule into the bath. Kodak’s 1949 note that Dolmi is the hydrochloride settles which article this formula means. More amidol raises activity and shortens an already short life; less and two minutes will not produce a black. It carries H301, toxic if swallowed, at 96.1 per cent of notifiers, which is a category worse than most darkroom organics and the reason this page’s safety section is not routine.

Water to 1000 mL. Not inert, and here it is also the timer: the developer’s life begins when the agent meets the water. Iron is the classic contaminant.

Interactions

Agent and preservative, which in this formula are chemically coupled. The sulfite does not merely protect the amidol; it converts the salt into the species that develops. That is why the order cannot be reversed and why there is no version of this formula with the agent dissolved first.

Agent and air. The dominant interaction of the page, and the one the published figures measure. Thirty minutes in a dish, and a half-filled bottle marked not recommended — a keeping instruction so severe that Kodak declines to give it a number.

Agent and alkali — the interaction that is not here. Deliberately. The amidol page warns against supplying it: do not try to make an amidol stock keep by adding carbonate, because alkali is what destroys it.

Restrainer and agent. With one agent there is no asymmetry to exploit, so bromide here slows the whole development rather than one half of it. That is a different instrument from the bromide in D-72, and it is why the amidol literature treats bromide as a tone control rather than merely as an antifoggant.

Variants

D-51 is Kodak’s 1928 amidol paper developer, and the two are worth reading together: D-51 keeps the agent in a stock solution and dilutes it at use, while D-170 keeps only the sulfite and bromide and adds the agent last. The 1949 arrangement is the better one for the same reason the 1928 primer gives for its own — the wet agent is what will not keep.

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

The packed powder. The handbook marks D-170 as available as a Kodak packed developer powder and does not state the powder’s composition. Under Rule 6 the course publishes the formula and makes no claim about the product.

The course’s own variants belong in the formula version record.

Safety

Level B, and unusually the reason is a single ingredient. Amidol is classified Danger with three pictograms: H301, toxic if swallowed, at 96.1 per cent of notifiers — 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. Japan’s and Australia’s schemes add organ damage through prolonged or repeated exposure.

What that changes in practice. Toxic-if-swallowed is a hand-to-mouth hazard, and print developing is the operation in this course where hands are wettest and sessions longest. Print tongs, one pair per tray, never interchanged. Hands washed before leaving the darkroom, every time, and nothing eaten or drunk in the room. Gloves if your hands go in the tray at all. H334 means the powder must not be inhaled: weigh with extraction running or inside an enclosure, and never in a draught.

There is one compensation, and it is real. There is no alkali here, so the working bath is not strongly alkaline and a splash is not a caustic splash — this is the mildest working solution in the paper library on that axis. It is not the mildest on any other.

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 agent dry. The amidol page is unambiguous: dry, cool, dark, tightly closed, in a labelled container never used for food, and away from oxidisers and alkalis. An amidol solution does not keep even with sulfite in it, so there is no stock of the agent to store — only the jar.

The sulfite-and-bromide stock is the part that keeps, at the handbook’s three weeks in a full stoppered bottle. Label it with the formula, the strength and the date, per the labelling SOP, and label it clearly as not developer, because a bottle of sulfite and bromide looks exactly like a bottle of developer and behaves nothing like one.

The made-up developer is not stored. Thirty minutes in a dish, and mix what the session needs.

Incompatibilities

Alkali, of any kind, which is the one this formula cares about most. Carbonate, borax or hydroxide added to an amidol bath destroys it. That includes carbonate carried in on a tray, a graduate or a pair of tongs from another developer.

Acid, deliberately, in the next tray — but note that this bath’s entire acid reserve is its sulfite, so a stop bath meets much less resistance here than after a carbonate developer, and the sulfur dioxide chemistry that Part X sets out is the thing to keep in mind. Tongs never travel backwards along the sequence.

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. It carries essentially no silver and belongs in its own labelled container, never in the fixer bottle. Amidol’s classification is heavier than the other developing agents in this library, so this is not a stream to be casual about. The disposal caveat governs and the general chemical waste SOP gives the procedure. Local regulation decides.

Troubleshooting

A bath that stops working part-way through a session. Almost certainly the clock rather than the sheet count. The published capacity is 48 sheets and the published dish life is thirty minutes; if you have not put fifty prints through it, it has oxidised.

A print that comes up instantly and then goes no further. Amidol’s characteristic shape, and usually correct. If the black never arrives, the bath is old or the agent was under-weighed.

Weak, grey prints from a freshly mixed bath. Check that the Dolmi went into the diluted solution and dissolved completely. Check also which article you weighed: the free base and the dihydrochloride differ by 59 per cent in the amount of reducing molecule per gram.

Fog. With no alkali the fogging pressure is low, so suspect the safelight or the paper before the bromide. If the bromide was left out of the stock, it was left out of every litre you made from it.

A stock bottle that has been standing and now gives a bath that dies in minutes. The three-week figure is for a full bottle. A half-filled one is marked not recommended, which is the handbook declining to guess.

Experiments

Time the death of the bath. Mix a litre, print one identical negative every five minutes for an hour with the tray uncovered, and read the maximum black. You are measuring the thirty-minute figure directly, and the curve is a demonstration of aerial oxidation that no film developer can give you because none dies fast enough to watch. Part VIII’s oxidation experiment has the method.

Cover the tray and repeat. A floating lid, or a sheet of glass, on an otherwise identical bath. If the life extends substantially, the mechanism is air rather than time, and you have shown it rather than assumed it.

Amidol against a carbonate developer, on one paper. Kodak names D-163 as the alternative to this formula. Print one negative in each, developed to completion, matching density by exposure, and compare contrast, maximum black and image colour. Record it against the developer laboratory report.

The bromide-and-dilution tone series. With no alkali there are only two levers. Print the same negative at full strength and at 1:1, each with 1.0 and 2.0 g/L of bromide. Wall’s description predicts steely blacks at full strength, clearer greys diluted, and warmth from liberal bromide and longer exposure. It is a claim about a different formula and this is the fair way to test whether it carries across.