Kodak DK-50

Most formula pages have to infer what an ingredient does. This one does not, because Kodak printed the answer under the formula. “By increasing or decreasing the quantity of ‘Kodalk’ in the formula it is possible (a) to increase or decrease the contrast obtained in a given time of development; (b) to decrease or increase the time of development without affecting the contrast.” That is a manufacturer stating, in one sentence, that a single ingredient gives you two independent controls depending on which of time and contrast you choose to hold fixed.

Ingredient	Quantity	Form the source specifies
Metol	2.5 g	Kodak calls it Elon
Sodium sulfite	30.0 g	anhydrous — or 60.0 g of the crystalline salt
Hydroquinone	2.5 g
Sodium metaborate	10.0 g	Kodak sold it as Kodalk
Potassium bromide	0.5 g
Water to make	1000 mL

Purpose

Normal-contrast negatives, quickly, by dish or tank. Kodak’s header is an “Elon-hydroquinone-Kodalk developer for normal-contrast negatives by dish or tank development” and the emphasis is on general usefulness rather than on any single quality. This is the workhorse of the group: about ten minutes at 18 °C, the highest deep-tank capacity in the handbook’s table, and no special claim about grain or speed at all.

Recommended uses

Sheet film in a tray or in hangers, which is the job the ten-minute time and the 48-sheet deep-tank capacity were written for.

Anywhere a developer needs to be adjusted rather than accepted. The Kodalk note makes DK-50 the only formula in this section that the manufacturer explicitly invites you to modify, and it tells you which direction each modification takes you.

Where a carbonate developer would cause trouble downstream. Kodak’s reason for using metaborate is partly about the fixer: a film carrying a metaborate developer into an acid fixing bath does not blister, because the alkali evolves no carbon dioxide, and there is less tendency to precipitate aluminium sulfite from an alum hardening bath. The metaborate entry sets out both statements.

When another formula is preferable

• D-76 for finer grain and full emulsion speed, at eight or nine minutes rather than ten but with three times the sulfite doing solvent work. The comparison is the clearest demonstration in this reference that a developer’s grain comes from its sulfite load, not from its agents.
• DK-20 for extra-fine grain. It shares the metaborate and drops everything else that makes DK-50 brisk.
• D-19b for genuinely high contrast. DK-50’s alkali will take contrast up somewhat; it will not take it there.
• D-23 for the opposite end, where highlights need holding back rather than building.

Mixing

Kodak’s printed order is Elon, sulfite, hydroquinone, Kodalk, bromide, with the usual one-line instruction to dissolve in the order given. The reasons are the handbook’s own general rules: the Elon first because it is only slightly soluble in a sulfite solution without alkali; the sulfite next so that the second agent dissolves under protection; the alkali last so no dissolved agent sits in an alkaline solution unpreserved; and the bromide wherever you like, since it has no action on the agents.

Two things about this formula make mixing easier than D-76. The sulfite is 30 g rather than 100, so it goes into solution quickly and in a small volume. And the metaborate is far more soluble than borax at room temperature, so there is no hot-water step for the alkali.

!Ten grams of a hydrate, not ten grams of an alkali

The encyclopaedia entry is for the tetrahydrate, which is what Kodak sold as Kodalk, and the anhydrous salt and the dihydrate have entries of their own. Most of the tetrahydrate’s mass is water of crystallisation, so weighing 10 g of a different hydrate against this formula throws the alkalinity out — and the alkali is the one ingredient this formula is built around adjusting. Read the jar.

Behaviour

About ten minutes at 18 °C, “or as recommended for specific materials”. That last clause is Kodak telling you the figure is nominal. DK-50 was a catalogue developer with published times for particular sheet films, and the handbook’s ten minutes is what you use when you have nothing better.

Its capacity is the highest in the table and it has no replenisher. 48 sheets of 8 × 10 in a deep tank per 160 fluid ounces, against 36 for D-76 and DK-20 — and where those two carry the footnote “life greatly increased by the use of replenisher”, DK-50 carries no footnote and no replenisher formula appears anywhere in the handbook. That is worth noticing rather than explaining: Kodak published DK-15, DK-20 and DK-50 in the same family, gave two of them replenishers, and gave this one the largest unreplenished capacity of the three. No source this course holds says why, and the obvious guess — that ten grams of alkali per litre is enough reserve to make replenishment unnecessary — is a guess.

Keeping is the standard set: 24 hours in a dish, a month in a tank, six months in a full stoppered bottle, two months half-filled. With only 30 g/L of sulfite, that is a slightly more impressive result than D-76’s, and it is the one place where this formula outperforms on the same table.

Image characteristics

Grain: coarser than the fine-grain developers here. Thirty grams of sulfite per litre is a preservative dose, not a solvent one, and this formula does not pretend otherwise — the header claims normal contrast and says nothing about grain. Sharpness: better than the solvent developers, for exactly that reason. Contrast: normal, and adjustable, which is the formula’s distinguishing property. Speed: unpublished. Kodak makes no claim either way, and the course does not turn the silence into one; there is half a gram of bromide in the litre, which is a reason to expect a slightly raised toe rather than full shadow speed. Tonality: brisk and unremarkable, in the way a general-purpose developer is supposed to be.

The mechanism

Set DK-50 beside D-76 and almost every difference points the same way.

	DK-50	D-76
Sulfite	0.24 mol/L	0.79 mol/L
Alkali	0.072 mol/L metaborate	0.0052 mol/L borax
Sulfite as a multiple of the alkali	3.3×	152×
Bromide	0.5 g/L	none
Published time	about 10 min at 18 °C	8 to 9 min at 20 °C on modern films

30 g/L ÷ 126.05 g/mol = 0.238 mol/L

Sulfite

10 g/L ÷ 137.86 g/mol = 0.0725 mol/L

Metaborate

2.5 g/L ÷ 110.11 g/mol = 0.0227 mol/L

Hydroquinone

Fourteen times the alkali and less than a third of the sulfite. Those two changes are the whole formula. The alkali drives the developing agents into their active form, so the bath is far more energetic than D-76 and reaches normal contrast in ten minutes with half the agent load. The reduced sulfite means the bath is not dissolving the grain as it works, so it is not a fine-grain developer — and Kodak, honestly, does not call it one.

The bromide is the price of the energy. Kodak’s 1928 primer states the general rule: the quantity of alkali governs the energy of a developer, too much alkali tends to produce chemical fog, and too little makes it slow. A bath at fourteen times D-76’s alkalinity needs a restrainer where D-76 needs none, and half a gram per litre is what Kodak used. It is the same dose as DK-20’s, which arrives at its fog risk by a completely different route.

And DK-50 has the most alkali reserve of any developer in this batch relative to its sulfite. Part VIII’s buffer table puts the sulfite-to-alkali multiple at 3.3 here against 152 for D-76, which means the borate is a real fraction of the bath’s acid-absorbing capacity rather than a rounding error. That is consistent with a formula that keeps working for 48 sheets without replenishment, though the course states the consistency and not a causal claim.

⚗Kodak's two knobs, and why they are two

The handbook’s sentence about varying the Kodalk names two different operations on the same ingredient.

Hold the time and change the alkali, and the contrast moves. More alkali means faster development, so in a fixed ten minutes the negative runs further up its characteristic curve and the gradient is higher.

Hold the contrast and change the alkali, and the time moves. Aim at the same degree of development and more alkali gets you there sooner.

These are the same fact, read along two different axes, and the reason it is worth stating twice is that darkroom practice usually holds one of them without noticing. A photographer who always develops for ten minutes is using the alkali as a contrast control; one who always develops to a target gradient is using it as a time control. Kodak’s sentence is an instruction to know which you are doing.

What Kodak does not give is a scale. There is no published table of Kodalk quantity against contrast index, and the course does not invent one. What it can say is the direction and that the formula’s own manufacturer expects you to move it.

Function of every ingredient

Metol, 2.5 g. The primary agent, at the same small dose that appears in D-76, D-72 and D-19b — the superadditivity lesson makes the point that across every Kodak MQ formula the primary agent stays between 2.0 and 3.1 g/L while the secondary varies fivefold. It starts the image and fills the shadows. More raises activity and shadow speed; less leaves the shadows thin. It is the ingredient that makes this a Level B page.

Sodium sulfite, 30.0 g anhydrous. Here it is a preservative and a modest buffer contribution and not a silver solvent in any meaningful sense — the concentration is too low. That is a deliberate design decision and the reason DK-50 is sharper and grainier than D-76. More sulfite would move the formula towards fine grain and away from what it is for; less would leave the agents exposed to aerial oxidation, and the six-month keeping figure would go. Note that the sulfite is doing less buffer work here than in any other formula in this batch, which throws that job onto the metaborate.

Hydroquinone, 2.5 g. The secondary agent, at a one-to-one mass ratio with the metol, which is the lowest secondary-to-primary ratio of any MQ developer in this reference — 1.6 : 1 in moles against D-76’s 3.9 : 1 and D-19b’s 6.3 : 1. A developer that gets its energy from alkali needs less fuel behind the front line than one that gets it from a large secondary dose. More hydroquinone would raise contrast and capacity; less would shorten the bath’s working life. At this pH the hydroquinone is far more active than it is in a borax developer, so it contributes real development here rather than sitting in reserve.

Sodium metaborate tetrahydrate, 10.0 g. The alkali, at five times DK-20’s dose and fourteen times D-76’s molar alkalinity, and the ingredient Kodak invites you to change. It sets the energy of the bath, and through the energy the time and the contrast. It also supplies most of the acid-absorbing reserve, since the sulfite is too dilute to carry it. More means faster, more contrasty and more fog-prone development in softer gelatin; less means a slower, flatter bath. It is chosen over carbonate for what it does downstream in the fixer as much as for what it does in the tank.

Potassium bromide, 0.5 g. The restrainer, and here it is doing conventional work: holding the threshold up so that a high-energy bath does not develop unexposed grains. More bromide means cleaner highlights, a raised toe and lost shadow speed — and because bromide restrains hydroquinone much more strongly than it restrains metol, moving it also shifts the balance between the two agents. Less invites chemical fog, which is the specific failure Kodak’s 1928 primer attaches to an over-alkaline bath. It goes into the mixing in whatever position suits you.

Water to 1000 mL. Cold throughout except for the metol step; nothing in this formula needs hot water.

Interactions

Alkali against agent, which is the formula’s whole subject. Raising the metaborate raises the active fraction of both agents, and it raises the hydroquinone’s proportionally more, because hydroquinone’s ionisation is further from complete at these pH values than metol’s is. So increasing the alkali does not simply speed the bath up: it shifts it towards the contrast-giving agent, which is part of why Kodak describes the change as a contrast control.

Alkali against restrainer. Both are set against fog, from opposite directions. If you take Kodak’s invitation and raise the Kodalk, the bromide is the ingredient you may need to raise with it.

Bromide against the two agents. Unequal, and Kodak’s 1928 primer says so: bromide restrains hydroquinone strongly and metol far less. A DK-50 bath that has seasoned on its own bromide has therefore lost proportionally more of its contrast-giving agent than of its shadow-filling one, which is the mechanism behind a seasoned bath going flat rather than merely slow.

Sulfite against grain. Almost absent here, which is the interaction most readers will arrive expecting and will not find.

Variants

No replenisher. Kodak published DK-20R and D-76R and D-19bR and D-25R and no DK-50R, and the handbook’s capacity table gives DK-50 the largest unreplenished deep-tank figure of the group. The course records the absence rather than filling it.

No dilution is published either, and the reason is the same sentence that makes this page interesting: Kodak’s answer to “how do I change this developer” is to change the alkali, not to add water. Diluting would move the agents, the preservative, the alkali and the restrainer together — which is four variables at once, and the opposite of what the handbook’s note offers.

Your own alkali series is the sanctioned variant. This is the one formula in the section where changing an ingredient follows the manufacturer’s own written invitation rather than departing from it. Give each version a code on a formula version record and you have a developer family rather than a set of unlabelled bottles.

DK-15, the tropical developer of the same family, carries 22.5 g/L of the same alkali plus a large sodium sulfate load, and has no entry here yet.

Safety

Level B. Metol is a skin sensitiser; hydroquinone carries a Danger classification with suspected carcinogenicity and mutagenicity and serious eye damage; sodium metaborate tetrahydrate is classified for reproductive toxicity. Sodium sulfite and potassium bromide are Level A materials. The classification rubric sets what Level B assumes.

The metaborate is at 10 g/L here — five times DK-20’s dose — so it is the powder to weigh most carefully, with extraction or inside an enclosure. Gloves and eye protection for mixing and for the tray; a dish developer is handled with tongs, not fingers.

What is not a hazard here. No gas is evolved in use, and the metaborate is specifically the alkali that does not fizz when the film reaches an acid fixer. At 10 g/L of a weak alkali the solution is not corrosive. Nothing is heated. The ventilation requirement is for dust during weighing.

Storage

Tightly corked, filled full, small bottles rather than one large one. Six months full and two months half-filled at 18 to 21 °C.

Two specifics. The dry metaborate melts at 53.5 °C, so the jar does not go on a shelf above a radiator or a print dryer. And if you have taken Kodak’s invitation and mixed a version with a different Kodalk dose, the bottle must say so — a DK-50 at 15 g/L looks exactly like a DK-50 at 10, and the difference is the entire subject of this page. See the labelling SOP.

Incompatibilities

Acid, which is what the stop bath is and why the tongs never travel backwards.

Fixer, in either direction. Note that the metaborate reduces one specific fixer problem — the sludge of aluminium sulfite that an over-carried alkaline developer throws out of an alum hardening bath — but reducing a problem is not removing it, and the carry-over discipline is unchanged.

Oxidising agents — ferricyanide, dichromate, permanganate, persulfate — never with a developing agent. See chemical incompatibilities.

Waste

A dilute alkaline solution carrying metol, hydroquinone, sulfite, borate and bromide. Both agents carry aquatic-toxicity classifications and borate is a registered vegetation control, so this does not go on the garden. Its own labelled bottle, kept out of the fixer so the fixer can go for silver recovery. Unlike DK-20 there is no thiocyanate here, so the ordinary general chemical waste SOP is the right procedure. The disposal caveat governs and local regulation decides.

Troubleshooting

Contrast too high at the published ten minutes. The commonest complaint about this developer, and the handbook has already told you the lever: reduce the Kodalk, or reduce the time. Which of the two you choose depends on whether you are holding the clock or the gradient.

Grain coarser than you expected from a Kodak developer. Expected. Thirty grams of sulfite is not a fine-grain load, and if fine grain is what you want, this is the wrong formula rather than a badly mixed one.

Fog, especially at the edges of sheet film. A high-energy bath with only half a gram of restrainer has less margin than D-76. Check the bromide went in, check the safelight, and check that the metaborate was weighed as the tetrahydrate.

Negatives going flat rather than slow as the bath ages. Accumulated bromide restrains the hydroquinone more than the metol, so a seasoned bath loses contrast before it loses speed. There is no replenisher; count sheets against the 48-per-160-fluid-ounce figure.

Blisters or frilling on the film after the fixer. Not this developer’s failure mode — the metaborate is the alkali chosen to avoid it — so look at temperature, at the hardener and at the wash instead.

Experiments

Run Kodak’s own invitation as an experiment. Four litres at 5, 10, 15 and 20 g/L of Kodalk, everything else held, one exposure batch, all developed for the same ten minutes. Read the strips two ways: as a contrast series at fixed time, which is Kodak’s option (a); then find the time in each that matches the 10 g/L strip’s gradient, which gives you option (b) as a set of numbers. One series, two published claims tested.

DK-50 against D-76 on grain and sharpness. Same film, same exposure, both at their published times and temperatures. The prediction from the sulfite arithmetic is that DK-50 is sharper and grainier, and the two effects should be visible in the same magnification.

Move the bromide with the alkali. At 20 g/L of Kodalk, develop one strip at 0.5 g/L of bromide and one at 1.0. If the fog you gained from the alkali comes back out with the restrainer, you have reproduced the design logic of D-19b at a smaller scale.

Test the missing replenisher. Put 48 sheets’ worth of film through a litre, tracking a control strip every eight sheets, and find out whether Kodak’s unreplenished figure holds for your materials. It is one of the few capacity claims in this reference that is small enough to test in a weekend, and the answer belongs in the developer laboratory report.