Kodak D-173

Kodak Limited’s heading for this formula names the reason it exists: an “Elon-free developer for Velox paper eliminating risk of discomfort to persons prone to Metol-dermatitis”. That is a manufacturer, in 1949, publishing an alternative formula because one of its own standard ingredients made some people ill. The course prints it in full, because a page that teaches metol as a sensitiser and then offers no route for the reader who is sensitised would be teaching only half of something.

Ingredient	Quantity	Form the source specifies
Sodium sulfite	22.5 g	anhydrous — or 45.0 g crystalline
Sodium carbonate	65.0 g	anhydrous — or 175.0 g of the crystals
p-Aminophenol	0.375 g	as the hydrochloride
Hydroquinone	7.5 g
Potassium bromide	0.15 g
Water to make	1000 mL

The ingredients are listed in the order the handbook prints them, which is also the order it tells you to dissolve them in. That order is unusual and the Mixing section below says why.

Purpose

To develop chloride contact paper to a normal print, at a normal speed, with no metol in the tray. It is a direct substitute for D-158, which is Kodak’s ordinary Velox developer and which contains 3.2 g of metol per litre.

Recommended uses

Velox and other chloride contact papers at 1:1, 35 to 45 seconds at 18 °C. That is the whole of what the handbook publishes, and the time is within a few seconds of D-158’s 30 to 40 — which is the formula’s real claim. It is not a compromise developer that happens to avoid metol; it reaches the same working point by a different route.

⚠What this formula does and does not solve

It removes metol. It does not remove every sensitiser, and the course will not let it be read that way.

p-Aminophenol carries H317, may cause an allergic skin reaction, in 34.3 per cent of the reports behind its aggregated classification — a minority view among suppliers, but a substantial one, and sensitisation is not something you can un-acquire. It also carries H341, suspected of causing genetic defects, in every classifying report, and H332, harmful if inhaled, in 99.5 per cent. Hydroquinone, which is in both formulas, is itself a skin sensitiser at above 99.9 per cent of reports and carries suspected carcinogenicity and mutagenicity.

So the honest statement is narrow and worth making precisely: for a person already sensitised to metol, this formula removes the specific substance they react to, and Kodak published it for exactly that person. For anyone else it is not a safer developer in general — the dose of p-aminophenol is tiny, but the substance is more heavily classified than metol on several counts. If you are choosing a paper developer on hazard grounds and are not metol-sensitised, the comparison to make is the whole classification of both agents, not the presence or absence of one name. The classification rubric sets out how the course assigns levels, and both formulas come out at Level B.

When another formula is preferable

• If metol is not a problem for you, D-158 is the ordinary developer for the same paper, is better documented, keeps longer and has three times the published capacity.
• For bromide enlarging paper, D-72 at 1:4 or D-163 at 1:3 — this formula is published for Velox alone.
• For low contrast, D-165, which contains metol and a great deal of it.
• For a modern metol-free route, the substance the industry actually settled on was phenidone, which does metol’s job at a fifth to a sixth of the weight. No document in this course’s corpus prints a phenidone paper-developer formula, so the course publishes none.

Mixing

The printed order is sulfite, carbonate, agents, bromide — and that argues with the handbook’s own general rule. The rule, given once for all the formulas in the book, is that the preservative is dissolved first, then the developing agents, and then, after those are completely dissolved, the alkali; the reason is that an agent standing in solution without its preservative oxidises in air. This formula’s printed list puts the carbonate second, before either agent.

The course prints it as Kodak printed it. Rule 6 forbids silently changing a published formula, and “the order given” is part of what was published — the handbook’s instruction on this page is literally “dissolve the chemicals in the order given”. What the course will not do is pretend the tension is not there, so:

1. Dissolve the sodium sulfite.
2. Add the sodium carbonate.
3. Add the p-aminophenol hydrochloride.
4. Add the hydroquinone.
5. Add the potassium bromide.
6. Make up to 1000 mL, and dilute with an equal volume of water for use.

⚗Why the alkali might belong second here, and why the course does not assert it

There is a chemical reason this order could be deliberate rather than a slip. p-Aminophenol is supplied as a hydrochloride because the free base is only sparingly soluble; the salt dissolves, and the alkali then liberates the base in solution. The chemical page works this through using Wall’s rodinal-type formula, where caustic soda is added to a p-aminophenol hydrochloride solution and “at first a precipitate of the paramidophenol base is formed, but, as more caustic soda is added, this dissolves”. A formula that puts the alkali in before the salt has the alkali waiting, which is one way to avoid a precipitate appearing at all.

That is a plausible reading and it is not what the source says. The handbook gives the order and gives no reason for it. Under Rule 7 the course separates the two: the order is fact, the explanation is interpretation, and the interpretation is offered here as one rather than smuggled into the procedure.

The handbook prints both salts twice — 45.0 g crystalline sulfite or 22.5 g anhydrous, 175.0 g crystalline carbonate or 65.0 g anhydrous — and warns in capitals that its two columns are not exact equivalents. Use one system throughout.

And weigh the small quantities properly. 0.375 g and 0.15 g are below what most darkroom balances resolve honestly. Check the balance against the balance and thermometer SOP before you trust either number, and consider making the bromide up as 1.5 mL of a 10 per cent stock instead — the handbook does not offer that alternative for this formula, so the arithmetic is yours and belongs in the notebook.

Behaviour

It works at chloride-paper speed: 35 to 45 seconds against D-158’s 30 to 40, at the same dilution on the same paper. The alkali is nearly the same, at 65 g/L against 69, and so is the sulfite. What has changed is the agents.

It does not keep as well as the formula it replaces. Ten hours in a dish against D-158’s twenty four, two months in a full bottle against three, two weeks in a half-filled bottle against one month, and the tank is marked not recommended. The published capacity is 16 sheets of 8 × 10 inches per 160 fluid ounces against D-158’s 36. All of those are worse, and none of them by a factor that makes the formula impractical for a printing session.

!A discrepancy in the source, left as one

The formula’s own instruction is to dilute with an equal volume of water — 1:1. The keeping-properties table’s useful-life column, on a different page, labels this formula’s 16 sheets with (1:2).

Nothing in the handbook reconciles them, and there is no honest way to decide which is the error. The course reproduces both and marks the conflict: the development instruction is on the formula page and is the one to work to, and the capacity figure is reported with the label its own table gives it. A reader who mixes at 1:1 and gets rather more than 16 sheets now knows why.

Image characteristics

Contrast. Normal for a chloride contact paper, which is the material it is published for and the material whose own short scale dominates the result.

Tonality. Comparable to D-158, which is the formula it exists to replace and the fair comparison to make.

Colour. Not stated. D-158’s blue-black claim does not automatically transfer, because the agents are different and it is the rate of silver deposition that sets image colour — see Part V’s chlorobromide project. The alkali and bromide levels here point the same way as D-158’s, so a cold tone would not be surprising; the course records that as a prediction and not as a published property.

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

The mechanism

7.5 g/L ÷ 110.11 g/mol = 0.068 mol/L

Hydroquinone

0.375 g/L ÷ 145.59 g/mol = 0.0026 mol/L

p-Aminophenol hydrochloride

65.0 g/L ÷ 105.99 g/mol = 0.613 mol/L

Sodium carbonate

22.5 g/L ÷ 126.05 g/mol = 0.179 mol/L

Sodium sulfite

0.15 g/L ÷ 119.00 g/mol = 0.0013 mol/L

Potassium bromide

Twenty-six hydroquinones to every p-aminophenol. The substituted agent is present at a quarter of what D-158 uses in metol by moles, and yet the developer runs at the same speed. That is a superadditive pair working at its most lopsided: a trace of a fast agent to start the image, and hydroquinone to build it. Part VIII’s page on superadditivity has the general treatment, and the usual mechanistic explanation — that the second agent regenerates the oxidised first — is not stated by any source this course holds and is not asserted here.

Why a substitute needs a different dose. The 1928 primer ranks the agents by reduction potential: hydroquinone lowest, then Athenon, then pyro, then Kodelon — Kodak’s own p-aminophenol salt — and metol highest. It draws the practical conclusion in the same paragraph: Kodelon can be substituted for Elon, but more of it is needed for a developer of the same strength. That is the general rule, and what makes this formula interesting is that Kodak went the other way and used less: 0.375 g where D-158 has 3.2 g of metol.

The course cannot fully reconcile those two statements from its sources, and says so. Two things are worth noting rather than concluding. The reduction-potential rule is about how much bromide an agent will work against, and this formula carries almost none — 0.15 g/L, the smallest restrainer dose anywhere in the formulary, an eighth of D-158’s already small figure. A bath designed around an agent that tolerates bromide poorly would rationally remove the bromide, and this one has. Whether that is Kodak’s reason is not published.

The alkali is doing the work the missing agent is not. At 0.613 mol/L the carbonate is essentially D-158’s, which keeps the hydroquinone fully active. A metol-free formula that also lowered the alkali would be a slow formula, and slow is the one thing this one is not allowed to be — it has to match a 40-second paper.

Function of every ingredient

Sodium sulfite, anhydrous, 22.5 g. The preservative and the acid reserve, and the first thing into the water in this formula’s printed order. It scavenges dissolved oxygen and intercepts the oxidised agents before they form coloured products. It is half D-158’s dose, which is consistent with a bath whose published dish life is ten hours rather than twenty-four. More sulfite keeps longer and cools the tone slightly; less and an open dish browns within the session. It also carries most of what the stop bath has to neutralise.

Sodium carbonate, anhydrous, 65.0 g (or 175.0 g of the crystals). The alkali, at effectively D-158’s level. It sets the pH region in which hydroquinone is an active agent rather than a reserve, and supplies a carbonate–bicarbonate buffer that holds it there. It is what allows a formula with a trace of its fast agent to work in forty seconds. More carbonate means chemical fog and a swollen gelatin; less means a bath that cannot match the paper it was written for.

p-Aminophenol hydrochloride, 0.375 g. The metol substitute and the starting agent. It sits between pyro and metol on Kodak’s reduction-potential scale, so it starts development in the thinly exposed regions quickly, and at 0.0026 mol/L it is present to trigger the superadditive pair rather than to build density. It is supplied as the hydrochloride because the free base is only sparingly soluble, and the chemical page is emphatic that a formula written for a salt must be weighed as that salt — the free base and the hydrochloride have different masses, and substituting one for the other silently changes the dose. More speeds the appearance of the image and would eventually raise fog in a bath with almost no restrainer; less and the hydroquinone is left to work alone, which at this alkalinity means a slow, contrasty, unreliable developer.

Hydroquinone, 7.5 g. The main developing agent here, and the one doing nearly all of the density building. It gains density steadily and rapidly once the image is started, decides how black the blacks are, and is strongly temperature dependent — which in a formula that finishes in forty seconds makes a cold dish very obvious very quickly. More hydroquinone raises contrast and shortens the bath’s life; less and the print will not finish. It is the most heavily classified substance on the page and it is in the formula this one replaces as well, which is the point the safety callout makes.

Potassium bromide, 0.15 g. The restrainer, at the smallest dose anywhere in this formulary — an eighth of D-158’s 0.9 g/L, which was itself the smallest of the metol–hydroquinone formulas. It suppresses development of grains carrying no latent image, keeping a chloride paper’s whites clean. At this dose that protection is thin, and the practical consequence is that this formula is less forgiving of a marginal safelight than the one it replaces. More bromide cleans the whites, warms the tone and slows the bath; less — there is barely any less to go.

Water to 1000 mL. Not inert. Iron is the classic contaminant, and a stock that has crystallised in the cold is warmed and redissolved rather than decanted, because the precipitate often contains the most important constituents of the solution.

Interactions

Agent and agent, at an extreme ratio. Twenty-six to one by moles is the most lopsided superadditive pair in the formulary, and it is the reason a formula can contain almost none of its fast agent and still work at speed.

Agent and restrainer. The bromide is almost absent, and the mechanism section above sets out the one connection the sources support: the agent that replaced metol is lower on the reduction-potential scale, and lower on that scale means less tolerant of bromide. Whether that is why Kodak cut the bromide to 0.15 g/L is not published. Part VIII covers restrainers generally.

Agent and alkali. Unchanged from D-158, deliberately. Substituting the agent while holding the alkali is what makes this a substitution rather than a redesign.

Salt and base. The alkali does two jobs with p-aminophenol — it sets the working pH and it liberates the free base from its hydrochloride. That is the whole of the rodinal chemistry, worked through on the chemical page, and it is the most likely reason this formula’s printed dissolving order puts the carbonate before the agents.

Variants

D-158 is the formula this one replaces, and has its own entry. Reading the two ingredient lists side by side is the fastest way to see what a metol substitution costs: the agent changes, the bromide falls by a factor of six, the sulfite halves, the keeping figures worsen, and the alkali and the working time stay put.

Phenidone is where this line of development ended. J. D. Kendall filed for Ilford in 1941 a patent whose stated object was a substitute for metol, and 1-phenyl-3-pyrazolidone does the job at a fifth to a sixth of the weight; the phenidone page has the history. No phenidone paper-developer formula is published in this course, because no document in the corpus prints one, and under Rule 6 a formula without a source it can name is not published however obvious its construction looks.

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

Safety

Level B, and the safety callout above is the substance of this section rather than an aside. In brief: p-aminophenol carries H341 in every classifying report, H332 in 99.5 per cent, and H317 — skin sensitisation — in 34.3 per cent, a minority but a large one. Hydroquinone carries a Danger classification with suspected carcinogenicity and mutagenicity, causes serious eye damage, and is a skin sensitiser at above 99.9 per cent of reports. Sodium carbonate at 65 g/L is an eye hazard in the stock.

This formula solves one specific problem for one specific person and should be recommended in those terms. It is the right developer for someone who has become sensitised to metol and wants to keep printing. It is not a low-hazard developer.

The weighing is the operation with the highest exposure, and it is worse here than usual because two of the quantities are small: 0.375 g of a suspected mutagen that is harmful if inhaled, and 0.15 g of a salt. Small quantities tempt people to weigh casually, on paper, in a draught. Weigh with extraction running or inside an enclosure, on a balance you have checked, with splash goggles on and gloves.

In the tray: print tongs, one pair, never interchanged. The gloves page covers the choice, and the first aid page covers what to do about a splash.

Storage

Stock in a tightly closed bottle filled as full as it will go: two months full, two weeks half filled — the shortest stock life of any developer in this library, and a reason to mix in small bottles. Working solution: ten hours in a dish, not recommended in a tank.

Label everything with the formula, the dilution and the date, per the labelling SOP. This one deserves a second line on the label saying what it is for, because a metol-free developer that gets used as an ordinary one by someone else in the darkroom has lost its entire purpose.

A cold stock that has thrown a deposit is warmed and redissolved rather than decanted.

Incompatibilities

Metol, and the point is not chemical. A tray, a graduate or a pair of tongs that has held a metol developer and has not been thoroughly washed will put metol into this bath, which defeats the only reason to mix it. If the darkroom serves someone who is sensitised, dedicate the equipment and mark it.

Acid, deliberately, in the next tray. A splash of stop bath drops the pH out of the region where hydroquinone works, and this is a hydroquinone-dominated bath. Tongs never travel backwards along the sequence.

Fixer, in either direction. A fixer that has drifted alkaline from carried-over developer sludges and stains prints brown, which is the 1928 primer’s finding and the reason the stop bath is not optional; thiosulfate travelling the other way into a developer fogs it.

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

Waste

A dilute alkaline solution carrying hydroquinone and p-aminophenol, both of which are classified very toxic to aquatic life with long-lasting effects. 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

Slow, weak development. With only 0.375 g/L of the starting agent, this formula has less margin than most. Check the temperature first — hydroquinone is the temperature-sensitive agent and it is doing nearly all the work — then the age of the stock, whose published life is only two months.

Grey whites and a general veil. At 0.15 g/L this bath has the thinnest antifoggant protection in the formulary. Suspect the safelight before the formula, and check that the bromide went in at all: it is the ingredient most easily forgotten because it is the smallest.

A print that will not reach black. Exhaustion, and the published capacity is only 16 sheets. Keep a fresh-bath reference print to compare against.

A brown stock solution well before two months. A half-filled bottle. The published figures already distinguish them by a factor of four, which is the largest such gap in this library.

Dermatitis that continues after switching to this formula. The formula removes metol and nothing else. Both remaining agents are classified sensitisers, hydroquinone near-unanimously. This is a matter for a clinician rather than a formula sheet, and the first aid page says what the course can and cannot help with.

Experiments

Against D-158, on the same paper. Print one negative in this and in D-158, both at 1:1, both developed to completion, matching print density by exposure. Kodak’s implicit claim is that the substitution costs nothing visible. Test it on contrast, maximum black and image colour, and record the result in the developer laboratory report — this is one of the few places in the formulary where a manufacturer has effectively published a controlled pair.

Find the real dilution. The source contradicts itself between 1:1 and 1:2. Run both, count sheets to exhaustion in each, and see which one gives 16 sheets per 4.55 litres. You are using the capacity figure to decide which instruction it belongs to, which is a satisfying way to resolve a documentary conflict experimentally.

Restore the bromide. Add 10 per cent potassium bromide to bring the working bath to D-158’s level, about 0.45 g/L. The prediction is cleaner whites, a slower bath and possibly a warmer tone; the question worth asking is why Kodak did not, and whether the answer shows up as a loss of shadow separation.

Weigh the small quantities three times. Not a photographic experiment but a laboratory one: weigh 0.375 g and 0.15 g repeatedly on your balance and record the scatter. If the scatter is a large fraction of the quantity, this formula is at the edge of what your equipment can mix reproducibly, and knowing that is more useful than the prints. The balance and thermometer SOP has the method.