Kodak D-72

Two Kodak publications print this formula twenty-one years apart. They agree on every quantity but one, to a fifth of a gram, and they disagree completely about what it is for: the 1928 American primer files it under Developing Formulas for Paper, and the 1949 London handbook heads it a developer for rapid dish development of plates and films. Both are Kodak. Neither is wrong. Understanding why one formula can be both is most of what this page has to teach.

Ingredient	Quantity	Form the source specifies
Metol	3.1 g	Kodak calls it Elon
Sodium sulfite	45.0 g	anhydrous — or 90.0 g crystalline in the 1949 printing
Hydroquinone	12.2 g	12.0 g in the 1949 printing
Sodium carbonate	67.5 g	anhydrous — or 180.0 g of the crystals
Potassium bromide	1.9 g	or 19 mL of a 10 per cent solution
Water to make	1000 mL	500 mL of it at about 52 °C to start

Purpose

To bring a printing paper to a full black in a dish, in about a minute, at a contrast you set by choosing a dilution. It is the reference print developer of this formulary in the same way that D-76 is the reference film developer: not the best at anything, documented across more materials than anything else, and the bath every other paper formula on this site is described against.

The second purpose is Kodak Limited’s, and it is real rather than a slip. At 1 part in 2 the same stock develops plates and films in about four minutes at 18 °C, and at 1 part in 1 it gives greater contrast on them. A print developer diluted enough is a vigorous negative developer, because paper and film are asking the same chemistry to run for different lengths of time.

Recommended uses

Chloride contact papers at 1:1, 45 seconds at 21 °C. Azo at 1:2, also 45 seconds — and the 1928 primer adds the useful aside that diluting Azo as for Velox, at 1:1, gives colder tones. Bromide enlarging papers at 1:4, one and a half minutes.

Read those times as minima and not as targets. A negative is developed for a time because the time decides its contrast; a print is developed to completion, and the time only has to be long enough for completion to happen. Pulling a print early to make it lighter gives a flat, mottled, under-developed image rather than a lighter version of the same print, and the paper’s own maximum black is never reached. Exposure controls print density; the developer controls whether the black arrives.

When another formula is preferable

• For warm image tone, D-156 and, further along the same axis, D-166 carry three and a half and six and a half times this bromide, which is how Kodak buys warmth.
• For a longer working life in the dish, or for hot weather, D-163 is the formula Kodak Limited recommends for tropical use and offers at two dilutions specifically so that one of them lasts longer.
• For chloride paper alone, with a blue-black tone as the stated aim, D-158 is the developer Kodak built for Velox.
• For low contrast, D-165 removes the hydroquinone entirely.
• If metol is a problem for your skin, D-173 is Kodak’s own metol-free answer, published for exactly that reason.
• For a softer, warmer print from the same four chemicals, D-49 is this formula with a third less carbonate; and D-52 is a weaker version that leaves the bromide out of the stock so that you dose it per paper.
• For film, D-76, which is what a developer looks like when the alkali is chosen to preserve speed and grain rather than to finish quickly.

Mixing

Dissolve in the order printed, and the order is a solubility problem before it is a convention. Kodak Limited’s handbook states the general rule for every formula in it: dissolve the constituents in the order given, because a developing agent dissolved before the preservative oxidises in air and forms coloured products; and where the formula contains Elon, the Elon goes first, since it is readily soluble in warm water but only slightly soluble in a sulfite solution that has no alkali in it. The bromide is the exception the handbook names — it has no action on the developing agents, so it is immaterial at what stage it is added.

1. Start with 500 mL of water at about 52 °C.
2. Dissolve the metol completely.
3. Add the sodium sulfite.
4. Add the hydroquinone.
5. Add the sodium carbonate.
6. Add the potassium bromide, or 19 mL of a 10 per cent stock solution of it.
7. Make up to 1000 mL with cold water.

!Anhydrous or crystalline, and the arithmetic that catches people out

The 1949 handbook prints both forms of both salts and expects you to use one system throughout: sodium sulfite at 45.0 g anhydrous or 90.0 g crystalline, sodium carbonate at 67.5 g anhydrous or 180.0 g crystalline. It recommends the anhydrous forms — greater stability, easier solution, smaller bulk in storage — and its prose gives a rule of thumb for the carbonate, that crystals must be increased to two and a half times.

The printed columns do not obey that rule. 180.0 ÷ 67.5 = 2.67, and the same handbook’s D-163 works out at 175 ÷ 65 = 2.69. The molar ratio of the decahydrate to the anhydrous salt is 286.14 ÷ 105.99 = 2.70, which is what the columns are consistent with and what the course’s separate page for the crystals is about. The two-and-a-half in the prose is a rounding, and using it instead of the printed column costs you seven per cent of your alkali.

The same handbook carries a warning in capitals that applies to all of this: the avoirdupois and metric columns “are not exact equivalents”, so one or the other must be used exclusively. Weigh in grams or weigh in grains, and never convert halfway down the list.

⚗Which sulfite and which carbonate the 1928 primer means

The 1928 primer writes “Sodium Sulphite (E. K. Co.)” and “Sodium Carbonate (E. K. Co.)” — Kodak’s own tested grades — without saying which hydrate. The course reads both as the anhydrous salts, and the reason is arithmetic rather than assumption. The same 1928 book gives D-76 400 g of that sulfite per 4 litres, which is 100 g/L, and the 1949 handbook’s D-76 column gives 100 g anhydrous against 200 g crystalline. The same book gives D-72 67.5 g of that carbonate, and the 1949 column gives exactly 67.5 g anhydrous against 180 g crystalline. Two independent matches on the anhydrous figure and none on the crystalline one is as close to a reading as the documents allow, and it is stated here as a reading.

Behaviour

It is fast, and the speed is the alkali. At 1:1 a chloride paper is finished in 45 seconds. The 1928 primer explains the general case rather than this formula: the quantity of alkali governs the energy of a developer, too much tends to produce chemical fog, too little makes it slow, and alkalis soften the gelatin, so an over-alkaline bath gives frilling and blisters in warm weather. At 67.5 g/L of carbonate this bath sits firmly at the fast end of that trade.

Dilution is the contrast control, and Kodak uses it as one in both directions. More water means less alkali per unit volume, a slower and gentler bath, and — on paper — a slightly softer print. Less water means the opposite, which is why the 1949 instruction for greater contrast on negatives is to dilute 1:1 rather than 1:2. On paper, the three published dilutions track the three paper speeds: the slow chloride contact paper gets the strongest bath and the fast bromide enlarging paper the weakest, so that all three finish in a manageable time.

Capacity is published for negatives and not for prints. The 1949 table gives 24 sheets of 8 × 10 inches in a narrow dish at 1:1 per 160 fluid ounces, 48 in a deep tank, and 18 and 36 at 1:2 — and it leaves the print row empty, although the same table fills in print rows for D-158 and D-163. The honest reading is that Kodak did not publish one, not that the bath has none. If you need a number for prints, measure it: the stop bath capacity experiment is the same method applied to the next tray along.

Keeping is ordinary. A day in a dish, a fortnight in a tank, three months in a full stoppered bottle and one month in a half-filled one. As with every developer, the air space is the variable.

Image characteristics

Contrast. Normal to vigorous, and adjusted by dilution rather than by time. Tonality. A full scale with a strong maximum black, which is why the whole warm-tone family is described as a departure from it. Colour. Blue-black on chloride paper; the 1928 primer’s one explicit colour instruction is that Azo developed at the Velox dilution goes colder, which is a useful demonstration that print colour follows the rate of silver deposition rather than the paper alone. Grain, acutance and speed are not properties of a print developer in any useful sense: the paper’s own grain is far below anything visible, and there is no film speed to preserve. Staining. None; there is no staining agent here and 45 g/L of sulfite intercepts the oxidation products that would colour the gelatin.

The mechanism

Development is reduction, and Part VIII works it through. What this formula is, mechanistically, is D-76’s four decisions taken the other way.

67.5 g/L ÷ 105.99 g/mol = 0.637 mol/L

Sodium carbonate

45.0 g/L ÷ 126.05 g/mol = 0.357 mol/L

Sodium sulfite

12.2 g/L ÷ 110.11 g/mol = 0.111 mol/L

Hydroquinone

3.1 g/L ÷ 344.38 g/mol = 0.0090 mol/L

Metol, as the hemisulfate

1.9 g/L ÷ 119.00 g/mol = 0.016 mol/L

Potassium bromide

The alkali decision is the whole page. D-76 carries 0.0052 mol/L of borax; this carries 0.637 mol/L of carbonate, which is more than a hundred and twenty times as much alkali. At borax pH, hydroquinone with its first pK_a near 9.9 is barely ionised and is effectively held in reserve. At carbonate pH it is a working agent, and it is present here at twelve grams to the metol’s three. That inversion — from a metol developer with hydroquinone in reserve to a hydroquinone developer that metol starts — is what separates a print developer from a film developer, and Part VIII’s page on alkalis has the general argument.

The two agents do different jobs, and the 1928 primer says what they are. An image developed with Elon “comes up very quickly and gains density slowly”, while a hydroquinone image “comes up very slowly but gains density steadily and rapidly”. Elon has the highest reduction potential of the agents Kodak ranked and hydroquinone the lowest, and the consequence is visible in the tray: a high-potential agent makes the image flash up all over at once because it starts even in the lightly exposed regions, while a low-potential one brings up the highlights first and leaves the shadows until the highlights are well advanced. In this bath you watch the metol do the first and the hydroquinone do the second, in sequence, in under a minute.

The sulfite here is a preservative and nothing else. At 0.357 mol/L it is a third of D-76’s 0.79 mol/L, and it is not enough to work as a silver solvent — which is right, because a paper emulsion’s grain is not something anyone is trying to make finer, and dissolving silver out of a print would only cost density.

The restrainer is present rather than absent, which is the other inversion. D-76 has no bromide at all. This has 1.9 g/L, and it needs it: a bath this alkaline would fog a paper that has been under a safelight, and the whites of a print are its subject. The 1928 primer supplies the interaction that makes 1.9 g/L a considered number rather than a round one — bromide affects hydroquinone strongly and Elon much less, and the higher an agent’s reduction potential the more bromide is needed to produce a given effect. So the bromide in this formula lands mostly on the hydroquinone, which is the agent building the highlight densities, which is precisely where fog would show.

Function of every ingredient

Metol, 3.1 g. The starter. It has the highest reduction potential of the agents Kodak ranked, so it works in the thinly exposed regions first and brings the whole image up together; on paper that is what gives you an image you can judge before it is finished. It is also barely restrained by the bromide, so its contribution is nearly constant as the bath ages. More metol raises the speed of appearance and softens the print slightly; less leaves the image slow to arrive and the low tones weak. At 3.1 g/L it is nowhere near its solubility limit, so a metol that has not dissolved is a mixing-order error and not a saturation problem. It is also the ingredient that decides this page’s safety level, being a skin sensitiser.

Sodium sulfite, 45.0 g anhydrous. The preservative, and here that is its whole job. It scavenges dissolved oxygen and intercepts the oxidised forms of both agents before they can go on to coloured products, which is what keeps a working dish of this developer from going brown in an afternoon. It also carries most of the bath’s acid-absorbing reserve, which matters at the next tray: 0.357 mol/L of sulfite is what a stop bath has to neutralise out of a carried-over print. More sulfite keeps better and eventually begins to dissolve silver, which on paper costs density for no gain; less and the bath oxidises within the session.

Hydroquinone, 12.2 g. The contrast agent, at four times the metol by mass and fully in play because the alkali is carbonate. It builds density steadily and rapidly once the metol has started the image, and it is the agent that decides how black the blacks get. It is strongly temperature dependent — the 1928 primer notes that a very little change in temperature affects hydroquinone greatly and Elon very little — which is why a cold dish gives flat prints and why print developers are worth bringing to temperature. More hydroquinone raises contrast and maximum black and shortens the useful life; less gives a grey print that will not finish. It is the most heavily classified substance in the formula.

Sodium carbonate, anhydrous, 67.5 g (or 180.0 g of the crystals). The alkali, and the reason everything else behaves as it does. It sets the pH region in which hydroquinone is an active agent rather than a reserve, and it supplies a genuine carbonate–bicarbonate buffer pair so that the pH does not collapse as the developing agents release acid. More carbonate means a faster bath, more chemical fog and a softened, swollen gelatin that frills in warm weather; less means a slow bath that will not reach maximum black in a reasonable time — which is not a hypothetical, because D-49 is this formula with a third less of it and is published as a softer portrait developer. The crystalline form is the same substance with ten waters attached; weigh 180 g of it or 67.5 g of the anhydrous salt, and never a number between.

Potassium bromide, 1.9 g. The restrainer, and the ingredient a print developer cannot do without. It suppresses development of grains that carry no latent image, which is what keeps unexposed paper white and what makes a safelight survivable. Because it acts far more strongly on hydroquinone than on metol, its effect concentrates where the highlight densities are being built. More bromide gives cleaner whites, a warmer image colour and a slower bath — the whole of the warm-tone family is this one change made larger — and eventually a loss of shadow separation. Less gives a faster, colder, slightly foggier print. The 1949 handbook’s alternative of 19 mL of a 10 per cent solution is the practical form: 1.9 g is an awkward weight and a 10 per cent stock is easy to keep.

Water to 1000 mL. Not inert. The 1949 handbook’s advice on stock solutions applies: iron is the classic contaminant, and a solution that has stood cold enough to crystallise has lost its most important constituents into the precipitate, which is warmed and redissolved rather than decanted off.

Interactions

Agent and alkali. The carbonate decides which agent is working, and the clearest demonstration of it is one dose down the same page: D-49 drops the carbonate from 67.5 to 45 g/L, changes almost nothing else, and is published as a softer developer for portrait bromide paper.

Agent and sulfite. Two separate things share one ingredient. The sulfite protects the agents from aerial oxidation, which is chemistry between the sulfite and the oxidation products; and at high enough concentration sulfite dissolves silver halide, which is chemistry between the sulfite and the emulsion. Only the first is happening here. That is why 45 g/L is a preservative dose and 100 g/L, in D-76, is a fine-grain dose.

Agent and agent. Metol and hydroquinone together develop faster than the sum of their separate contributions, which is superadditivity. The usual explanation — that hydroquinone regenerates oxidised metol — is not stated by any source this course holds, and this page does not assert it.

Restrainer and agent. Unequal, and the inequality is sourced: bromide restrains hydroquinone far more than metol, and the higher the reduction potential the more bromide it takes to make a difference. Adding bromide to this bath therefore does not slow it uniformly; it slows the contrast-building half and leaves the image-starting half nearly alone, which is why extra bromide warms a print instead of merely retarding it. Part VIII’s restrainer page has the general treatment.

Variants

The 1949 Kodak Limited printing is the same formula with hydroquinone at 12.0 g instead of 12.2 g, no paper instructions at all, and a header calling it a plates-and-films developer used at 1:2 for about four minutes at 18 °C, or 1:1 for greater contrast. Two per cent of the hydroquinone is inside anyone’s weighing error at this scale; the difference that matters is the description, and both descriptions are Kodak’s own.

D-49 and D-52 are separate published formulas rather than variants of this one, and they have their own entries. D-49 is the portrait bromide developer — the same agents, a third less alkali, slightly more bromide. D-52 is a weaker stock with no bromide in it at all, which the user then doses per paper.

Packaged products. The 1949 handbook marks several of its paper developers with an asterisk meaning “available as a Kodak packed developer powder”, and D-72 does not carry one. Readers will meet the claim that this formula is sold ready-mixed under one trade name or another. No document in this course’s corpus states the composition of any packaged print developer, so under Rule 6 the course does not make that identification, however widely it is repeated.

The course’s own variants belong in the formula version record, not here. “D-72” names a formula; “D72-EB-002” names a litre you mixed with one changed variable and a prediction attached.

Safety

Level B, and the reason is the same two substances that put D-76 there, in a job that puts your hands closer to them. Metol is a skin sensitiser and sensitisation is not reversible. Hydroquinone carries a Danger classification with suspected carcinogenicity and mutagenicity and causes serious eye damage, and this formula holds two and a half times as much of it as D-76 does. Sodium carbonate at 67.5 g/L is an eye hazard in its own right and is what makes a splash from this tray worse than a splash from a film tank.

Print developing is the part of this course with the highest contact exposure, and that is a formulation problem as much as a habit problem. A film developer lives inside a closed tank; a print developer stands open in a tray under a safelight for an hour at a time, with sheets going in and out of it. Use print tongs, one pair per tray, never interchanged. If you must touch a print, wear nitrile gloves rather than trusting that a dilute solution is a safe one: a sensitiser is a sensitiser at any dilution, and repeated small exposures are exactly how sensitisation happens. The classification rubric sets what Level B assumes, and the gloves page covers the choice.

The powders are the other half of the job. Everything here is weighed dry and two of the five are fine powders. Weigh with the extraction running or inside an enclosure, and never in a draught.

Storage

Stock solution in a tightly closed bottle filled as full as it will go. The 1949 handbook is specific that glass stoppers stick when the solution is alkaline, and that stock keeps better in several small bottles than drawn repeatedly from one large one, because the air space grows every time a large bottle is opened.

Three months full, one month half-filled, a fortnight in a covered tank and 24 hours once it is in a dish — and that last figure is the one that catches printers out, because a tray of paper developer looks perfectly good the morning after a session and is not. Label the bottle with the formula, the dilution and the date mixed, using the labelling SOP.

If a cold bottle has thrown a crystalline deposit, warm it and redissolve it rather than pouring the clear liquid off the top; the 1949 handbook makes the point that the precipitate often contains the most important constituents of the solution.

Incompatibilities

Acid, deliberately, in the next tray. That is what a stop bath is for, and it is why the same tongs must never travel from the stop tray back into the developer. A splash of acetic acid does not merely neutralise a little carbonate — it drops the pH out of the region where hydroquinone works at all, and a bath that has taken one lands somewhere between slow and dead.

Fixer, in either direction. Thiosulfate carried back into a developer is a far more aggressive silver solvent than sulfite and will fog and stain; developer carried forward into the fixer raises its pH, and the 1928 primer records the consequence exactly — a white sludge forms, the bath becomes alkaline, and prints fixed in an alkaline bath are likely to stain brown. That is the argument for the stop bath in one sentence, from the maker.

Oxidising agents — ferricyanide, permanganate, persulfate — must not meet a developing agent in a bottle or in a drain. See chemical incompatibilities.

Waste

Spent D-72 is a dilute alkaline solution whose environmental load is the two developing agents rather than the carbonate: both metol and hydroquinone carry aquatic-toxicity classifications, and both are on the page for a reason. It carries almost no silver — the silver leaves a print in the fixer, not the developer — so it goes into its own labelled container and never into the fixer bottle, which would spoil a solution that is worth recovering.

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

Flat, grey prints that never reach black. Three candidates, in order of likelihood: the bath is too cold, because hydroquinone is the temperature-sensitive agent and it is the one building the blacks; the print was pulled before development finished; or the bath is exhausted. Check the thermometer before you change anything else.

Prints that come up fast and then stop. The metol is working and the hydroquinone is not. A bath that has been standing, a bath mixed with the crystalline carbonate figure taken as anhydrous, or a bath that has taken a splash of stop.

Yellow or brown staining in the highlights. Oxidised developer, or developer carried into an exhausted fixer. The 1928 primer’s account of the sludging fixer is the second case, and the answer is the stop bath rather than a change of developer.

Greyish whites and a general veil. Fog, and the safelight is the first suspect rather than the formula, because a bath at this alkalinity will faithfully develop whatever a safelight has exposed. If the safelight is clean, the bromide is next — and if the bath was mixed from a 10 per cent stock solution, check that stock’s age and strength before blaming the formula. Part VIII on restrainers has the chemistry of what the bromide is doing.

A developer that darkens visibly during the session. Aerial oxidation, accelerated by the large surface of an open tray. It is what the 24-hour dish figure is about.

Experiments

The dilution series, on one negative. Print the same negative at 1:1, 1:2 and 1:4, each developed to completion at 21 °C, matching print density by adjusting exposure rather than time. You are testing whether dilution really changes contrast on paper or only changes the rate. Record it against the developer laboratory report.

Develop to completion, and prove it to yourself. One print at 45 seconds, one at 90, one at three minutes, all at 1:1, all given the same exposure. If print density tracks time, the paper was not being developed to completion at 45 seconds and your working time is wrong.

Add bromide and watch the colour move. Mix a litre at 1:2 and split it. Add 10 mL of a 10 per cent potassium bromide solution to one half — which takes the working bath from about 0.6 g/L to about 1.6 g/L of bromide — and print the same negative in both. The prediction, from D-156 and D-166, is a warmer and slightly slower print with cleaner whites. This is the one-variable version of the entire warm-tone family.

The two printings’ hydroquinone. Mix a litre at 12.0 g and a litre at 12.2 g and try to tell them apart on matched prints. The point of the experiment is the answer you expect: a formula’s published precision is not the same as its meaningful precision, and knowing which digits matter is a laboratory skill rather than a chemical fact.