Kodak D-25

Kodak published D-23 and D-25 on facing pages, and the difference between them is one ingredient. That makes this the most instructive pair of formulas in the whole reference, because the manufacturer printed the input, the benefit and the price in the same six lines.

Ingredient	Quantity	D-23	Form the source specifies
Metol	7.5 g	7.5 g	Kodak calls it Elon
Sodium sulfite	100.0 g	100.0 g	anhydrous — or 200.0 g crystalline
Sodium bisulfite	15.0 g	—
Water to make	1000 mL	1000 mL

Kodak’s header: an “Elon extra fine-grain developer — a simple formula for minimum grain in films or plates”, followed immediately by a parenthesis that is the honest half of the sentence: “(This developer requires 50 to 100% increase in exposure)”.

Purpose

Minimum grain. Not fine grain, which D-23 and D-76 already give, but the finest this family of chemistry can reach — and Kodak’s own framing tells you it is a specialist bath, because it puts the exposure penalty in the title rather than in a footnote.

Recommended uses

Small negatives that will be enlarged a long way. This is the case that justifies paying a stop of film speed for grain, and in 1949 it meant miniature film enlarged to exhibition size.

Subjects with more range than the film can hold, where the extra softness is wanted anyway, and where an extra stop of exposure is available because the light is not the limiting factor.

Not for anything moving in poor light. Rating a 400-speed film at 200 or 100 because the developer demands it is a decision about shutter speed as much as about grain.

When another formula is preferable

• D-23 is this formula without the bisulfite and without the exposure penalty. If you do not need the last increment of grain, you do not need this bath.
• DK-20 is Kodak’s other extra-fine-grain developer and reaches its grain by a completely different route — a thiocyanate silver solvent at a higher pH, with a proper alkali and a restrainer — in fifteen minutes at 18 °C and with no published exposure penalty. If you want extra-fine grain without losing speed, that is the formula to compare against, and the pair is the best argument in this reference that “fine grain” names an outcome rather than a mechanism.
• D-76 at 1:1 goes the opposite way entirely: less solvent, sharper, grainier, full speed.

Mixing

Metol, then sulfite, then bisulfite — and the third step follows a specific instruction rather than the general rule. Kodak’s 1949 note on making up solutions says that when sodium bisulfite appears in a formula it should be added with the sulfite. In practice: dissolve the metol in warm water first, as always, then add the sulfite and the bisulfite together, then make up to volume cold.

The reason the metol still goes first is unchanged and, if anything, more pressing than in D-23. The bisulfite makes the solution less alkaline, and metol’s poor solubility in a sulfite solution is specifically a problem in the absence of alkali. There is no route back if you get this wrong.

⚠Bisulfite and acid

Sodium bisulfite is the acid salt of sulfurous acid, and a solution containing it will release sulfur dioxide if it meets a stronger acid. Fifteen grams a litre in a mixed developer is not the hazard; the container of dry bisulfite on the bench beside a bottle of glacial acetic acid is. Keep them apart, and read the incompatibilities page before you store either.

Behaviour

Eighteen minutes at 25 °C, and the temperature is the news. Kodak gives D-23 eighteen minutes at 18 °C and gives this bath the same eighteen minutes at 77 °F — seven degrees warmer. The formula lost so much activity to the bisulfite that Kodak recovered the time with temperature rather than printing a longer one, which also means that anyone running it at ordinary room temperature is developing a soft formula in a bath that is slower still.

The exposure penalty is Kodak’s own figure and it is wide: 50 to 100 per cent. That is between half a stop and a full stop, and the width of the range is itself information — the loss depends on the emulsion, so it is a starting point for your own test and not a number to apply blindly.

Capacity is lower than D-23’s, 20 sheets of 8 × 10 in a dish against 24, and 30 in a deep tank against 36. A less active bath reaches the point where it no longer develops to standard sooner.

Keeping is unchanged: 24 hours in a dish, a month in a tank, six months in a full bottle, two months half-filled. The 100 g of sulfite is still doing its preservative work; the bisulfite has lowered the pH without weakening the protection.

Image characteristics

Grain: Kodak’s claim is minimum grain, and this page reports it as the manufacturer’s claim rather than as a measurement, because no source in the corpus puts a granularity number on it. Speed: reduced by half to one stop, published. Contrast: low — lower than D-23, which was already the soft one. Tonality: very long and very gentle, with highlights strongly restrained. Sharpness: the lowest in this section, because you have kept the full solvent load and slowed the development that would otherwise generate edge effects.

The mechanism

One ingredient, one arrow, and the arrow is pH. Sodium bisulfite is the acid partner of sulfite:

Na₂SO₃ + H₂SO₃ → 2 NaHSO₃

Kodak's own 1928 statement of the relation - sodium bisulfite is sulfite plus sulfurous acid

Putting 15 g/L of the acid salt into a solution that already holds 100 g/L of the base salt does two things at once. It lowers the pH, and it installs a proper buffer pair so the pH stays where it was put.

100 g/L ÷ 126.05 g/mol = 0.79 mol/L

Sulfite

15 g/L ÷ 104.06 g/mol = 0.144 mol/L

Bisulfite

(The metol is unchanged from D-23 at 7.5 g/L of the hemisulfate salt, so it plays no part in the difference between the two formulas.)

The ratio of base to acid is about five and a half to one. What the resulting pH actually is, no source this course holds states, and the course does not compute one: the sodium sulfite page records that it has no sourced figure for a working-strength sulfite solution either, so there is no honest starting point to work from. What can be said is the direction and the size of the effect, and Kodak states the size for us — seven degrees of temperature to hold the same time.

Why a lower pH gives finer grain. Metol’s activity falls with pH, so development slows. Slower development gives the sulfite longer to work on each grain before the grain is fully reduced, and it also reduces the tendency for many grains to develop at once in a clump. The consequence is smaller apparent grain and less density for the same exposure, which is the same event described twice — and it is exactly why the speed loss and the grain improvement cannot be separated. You cannot have one without the other, and Kodak’s parenthesis is the manufacturer saying so.

Why a buffer, and not just a lower pH. Without the bisulfite, D-23’s pH drifts down as it works. With it, the bath starts lower and stays there. That makes D-25 more consistent from the first sheet to the twentieth than D-23 is, which is a real advantage that the grain claim tends to hide.

Function of every ingredient

Metol, 7.5 g. The only developing agent, at the same dose as D-23 but working in a less favourable bath. Because the pH has fallen, a smaller fraction of the metol present is in its active form at any moment, so the effective concentration is lower even though the weighed amount is identical. More metol is the obvious way to recover activity, and it works up to a point, but it does not restore the speed: the lost speed comes from the pH, not from a shortage of agent, and Kodak’s own answer to the slowness is temperature rather than a bigger dose. It remains the reason this page is Level B.

Sodium sulfite, 100.0 g anhydrous. The same four jobs it does in D-23 — preservative, alkali, silver solvent, and half of a buffer pair — except that here the buffer is deliberate and present from the start rather than a by-product of use. It is still the source of the fine grain. More would push the pH back up and undo part of what the bisulfite was added to do; less would reduce the solvent action, which is the property the formula was built for.

Sodium bisulfite, 15.0 g. The whole point of the formula. It is an acid salt, so it lowers the pH; it is the conjugate acid of sulfite, so it buffers; and it is a mild reducing agent in its own right, so it contributes a little to preservation. More bisulfite means a lower pH, finer grain, more speed loss and a longer time — the arrow continues in the same direction, and there is no published figure for where it stops being useful. Less moves the formula back towards D-23. The 1928 primer notes that commercial dry bisulfite is largely metabisulfite, which converts to bisulfite on dissolving, and that Eastman Kodak’s dry product could be used for formulas calling for either; that is worth knowing when you read the label on the jar.

Water to 1000 mL. Warm for the metol, cold to make up. A chilled bottle of D-25 will throw down metol just as D-23 does, and the remedy is the same: warm it and redissolve rather than pouring off the clear part.

Interactions

Bisulfite against sulfite is the only interaction that matters here and it is a buffer pair. It sets the pH, it holds the pH, and through the pH it sets the speed, the grain, the contrast and the time. There is no way to move one of those without the others.

Bisulfite against metol. Indirect, through the pH, and it is the reason the metol dose is not the lever it looks like.

Bisulfite against the stop bath. Nothing chemical, but a practical point: a developer that already contains an acid salt still needs stopping. The stop bath is a pH change of two or three units, not a small one, and Part X works out how large it has to be.

Variants

D-23 is the parent formula in every sense except Kodak’s numbering: Kodak printed both as independent formulas, so neither is recorded here as a variant of the other. Read as a pair they are a one-variable experiment the manufacturer performed and published.

D-25R replenishes both D-23 and D-25 and contains no bisulfite at all — it uses sodium metaborate, an alkali, instead. That looks contradictory until you see what a replenisher is for: the working bath needs its pH held down, and a tank in use is generating acid, so what has to be added back is base.

No dilution and no two-bath version is published by Kodak for this formula, and the divided metol-sulfite developers that circulate are not printed here for want of a source.

Safety

Level B, set by metol as a skin sensitiser at 7.5 g/L of fine powder. Sodium sulfite and sodium bisulfite are both Level A materials — bisulfite carries a single statement, harmful if swallowed — but the bisulfite adds a hazard that neither classification captures on its own, which is the sulfur dioxide released when a bisulfite meets a stronger acid.

The classification rubric sets what Level B assumes. For this page: weigh the metol and the bisulfite with extraction or in an enclosure, nitrile gloves and eye protection throughout, and store the dry bisulfite away from every acid in the darkroom.

What is not a hazard here. The mixed developer is not corrosive and is not strongly alkaline — less so than any other developer in this section, since the bisulfite has pushed it down. Nothing is heated beyond the warm water used for the metol. The 25 °C working temperature is a comfort question, not a hazard one; it is below the temperature at which gelatin softening becomes a processing problem.

Storage

Same as D-23: tightly corked, full, small bottles, six months and two months. Add one label the other developers do not need — “D-25, 25 °C” — because the whole difference between a correct negative and a thin one here is seven degrees, and a bottle that looks exactly like D-23 will be used exactly like D-23.

Keep the dry bisulfite jar with the alkalis and the sulfite, not with the acids, and see the labelling SOP.

Incompatibilities

Acids, and more so than for any other developer here, because of the sulfur dioxide. That includes the stop bath in the sense that carry-back on a pair of tongs is worse in this bath than in a carbonate one.

Fixer, in either direction, on the usual dichroic-fog and silver-recovery grounds.

Oxidising agents. A bisulfite is a reducing agent as well as an acid salt, so a bottle of spent D-25 has two independent reasons never to meet a ferricyanide, a dichromate, a permanganate or a persulfate. See chemical incompatibilities.

Waste

Spent D-25 carries metol, a large sulfite load and a bisulfite load, so its biochemical oxygen demand is the highest of the three simple metol developers here. Its own labelled bottle; never into the fixer bottle, which goes for silver recovery. Do not attempt to neutralise it before disposal — adding acid to a sulfite solution is the one thing this page has told you not to do. The disposal caveat governs, the general chemical waste SOP gives the procedure, and local regulation decides.

Troubleshooting

Everything thin, including the shadows. Either the exposure increase was not made — Kodak’s 50 to 100 per cent is not optional — or the bath was run at 18 °C instead of 25 °C. Those two faults look identical on the film and are told apart by asking what the thermometer said.

Contrast lower than D-23 at the same time. Expected. That is what the formula does.

A sharp smell over the tank. Sulfur dioxide, and it means an acid has got into the developer. Stop, ventilate, and find the contamination route before you mix another litre — the usual culprit is tongs or a graduate shared with the stop bath.

Grain no finer than D-23 gave. Check that the bisulfite went in at all, and check the sulfite form: 100 g of the crystalline salt in place of 100 g of the anhydrous halves the solvent action, which is the ingredient the whole formula is built on.

Density falling off across a session faster than expected. Capacity here is 20 sheets per 160 fluid ounces in a dish, four fewer than D-23. Count sheets rather than trusting the look of the bath.

Experiments

The one-variable experiment Kodak already ran. Mix a litre of D-23 and a litre of D-25, expose two identical step wedges, and develop both for eighteen minutes — one at 18 °C and one at 25 °C, as published. Compare speed point, gradient and grain. You have reproduced a manufacturer’s published comparison, and everything you find is attributable to one ingredient.

Measure Kodak’s exposure penalty for your film. Expose a bracket at the nominal speed and at plus a half, plus two thirds and plus a full stop, develop all of them in D-25 together, and find which one matches the shadow density your D-23 negatives have. The answer is a number about your emulsion, not about the developer.

Push the bisulfite further. Mix a third litre at 30 g/L and see whether the arrow keeps pointing the same way. Nothing published says where it stops; that makes it a genuine question, and it belongs on a formula version record with a prediction written down before the strip is developed.

Read the pH nobody has published. Fresh D-23 against fresh D-25, on a meter calibrated with buffers under the pH meter SOP. The difference between the two readings is the whole of this formula’s design, and it is a measurement the course could not make for you.