Kodak D-25R

Kodak wrote one replenisher for two developers whose pH values are deliberately different, and it put an alkali in it that neither of them contains. That is the whole page.

Ingredient	D-25R	D-23	D-25
Metol	10.0 g	7.5 g	7.5 g
Sodium sulfite, anhydrous	100.0 g	100.0 g	100.0 g
Sodium metaborate (Kodalk)	20.0 g	—	—
Sodium bisulfite	—	—	15.0 g
Water to make	1000 mL	1000 mL	1000 mL

The 1949 handbook prints the crystalline sulfite alternative here as everywhere else: 200.0 g of the heptahydrate in place of 100.0 g of the anhydrous salt.

Purpose

To hold a tank of D-23 or D-25 at a constant level and a constant activity. Kodak’s instruction is one sentence: the developer should be maintained at a constant level in the tank by the frequent addition of this replenisher solution. There is no published rate and no published end point, which is a real gap and is stated as one below.

Recommended uses

Tank development of sheet film or roll film in either bath, in a darkroom doing enough work for the level in the tank to fall. The word “frequent” is doing real work in Kodak’s sentence: this is a top-up-as-you-go scheme, not a measured dose after each film.

For anyone processing a few films a month, the better answer is to mix a litre of the developer, use it as a one-shot or for a short session, and discard it. A replenisher whose keeping is six months in a full bottle and two months in a half-empty one is not economical for occasional work.

When another formula is preferable

D-76R replenishes D-76 and is not interchangeable with this: it carries hydroquinone, which neither D-23 nor D-25 contains, and borax rather than metaborate.

DK-20R replenishes DK-20 and carries a thiocyanate as well as the metaborate.

Fresh developer is the alternative Kodak does not print here but which its own D-76 sheet recommends for a different case: where much more solution is lost in processing than replenishment replaces, make the volume up with working-strength developer rather than with replenisher. The same logic applies to a tank of D-23 that is losing solution to carry-out rather than to chemistry.

Mixing

Metol, then sulfite, then the metaborate, which is the order the handbook prints and the order its general note requires: the Elon is dissolved first because it is readily soluble in warm water and only slightly soluble in sulfite solutions without alkali; the preservative follows; the alkali goes in last so that a dissolved developing agent never sits in an alkaline solution without protection.

Ten grams of metol per litre is the largest metol dose anywhere in the film-developer section, and metol dissolves to only about 4.7 g per 100 mL in cold water. Use warm water and give it time.

!Kodalk is a hydrate, and gram-for-gram substitution is wrong

Kodak sold sodium metaborate under the name Kodalk. The encyclopaedia entry is for the tetrahydrate, which is what the photographic product was, and the anhydrous salt and the dihydrate are separate entries for a reason: substituting one for another by weight changes the alkalinity of the solution substantially, because most of the tetrahydrate’s mass is water. Read the label on the jar before you weigh anything called “sodium metaborate”.

Behaviour

It is not a developer and there is no time for it. The keeping table has no dish or tank column for this row, only the two bottle figures, and marks it “used as Replenisher only”.

Kodak publishes no rate. For D-76R and DK-20R the handbook and the later sheets give a volume per roll and a 25 per cent replacement limit. For D-25R they give neither. That is a limit of the evidence, not an indication that the rate does not matter, and the course does not supply a number to fill the gap. What is left is Kodak’s own instruction — maintain the level, frequently — and process control: run a control strip and watch it.

Six months full, two months half-filled. As a solution used a little at a time, the half-filled bottle is its normal state, so decant into small bottles when you mix it.

Image characteristics

None of its own. A replenisher is judged by whether the negatives coming out of a seasoned tank match the negatives that came out of it fresh, and the only honest test is a control strip developed alongside a strip from a freshly mixed litre. Where a replenished D-23 tank drifts, the expected directions are a fall in contrast if the alkali is not keeping up and a rise in fog if it is overshooting; both are readings on film, not on the bath.

The mechanism

A metol-sulfite developer has no alkali reserve at all, and this is Kodak’s answer to that.

In D-23 the alkalinity comes from the sulfite’s own hydrolysis and nothing else. Development produces acid; acid meets sulfite and becomes bisulfite; the pH falls, and each film through the tank leaves the next one a slightly less active bath. There is nothing in the formula whose job is to stop that. In D-25 the pH has been deliberately pushed down and buffered — but a buffer resists change, it does not replace what is consumed, and a working D-25 tank runs down the base half of its own buffer pair.

So the replenisher adds base. Twenty grams of sodium metaborate per litre is a substantial alkali and it is present in neither working bath. Adding it a little at a time lifts the pH of a seasoned tank back towards where it started, while the extra metol replaces the agent that was consumed reducing silver.

Kodak reaches the same conclusion in every replenisher in this section by whichever alkali the parent formula uses: ten times the borax in D-76R, ten times the metaborate in DK-20R, and here an alkali introduced from nothing. What a tank of film takes out of a developer, before it takes anything else, is alkalinity.

⚗Why one replenisher serves two different baths

D-23 and D-25 differ by 15 g/L of an acid salt, so they sit at different pH values, and a single replenisher restores both. That is only puzzling if you think a replenisher sets the pH. It does not: it adds base at a rate, and the working bath’s own composition — 0.79 mol/L of sulfite, plus whatever bisulfite is present — decides what that base does to the pH. In D-25 the metaborate is titrating a buffer that is already there; in D-23 it is titrating one the developer has been making for itself since the first film. The same addition therefore has a smaller effect in D-25, which is exactly what a bath with a deliberate buffer is supposed to do.

Note what this is: a reading of two published formulas, not a statement from Kodak, who print the two formulas and one replenisher without explaining the choice.

Function of every ingredient

Metol, 10.0 g. Replacement for the developing agent consumed. At a third more than the working bath, so that a modest volume of replenisher restores a larger volume of tank. It is the largest metol concentration in this section, close enough to the cold solubility limit that a chilled bottle will throw some of it back out. More would over-restore activity as the tank seasons; less would leave a bath topped up in volume and still slow.

Sodium sulfite, 100.0 g anhydrous. Identical to both working baths, and deliberately so. Its jobs here are to preserve the metol in the replenisher bottle, to preserve it again in the tank, and to keep the tank’s silver-solvent concentration constant as fresh solution goes in. Because the sulfite also is part of the alkalinity of D-23, keeping it at working strength means the replenisher does not disturb the one alkali the developer already had. Changing it would change the grain of a seasoned bath, which is the drift a replenisher exists to prevent.

Sodium metaborate tetrahydrate, 20.0 g. The alkali, and the only ingredient in this formula with no counterpart in the baths it feeds. Kodak’s reason for preferring metaborate over carbonate in this family is given under DK-50: the quantity of Kodalk in a formula moves contrast and time and it does so predictably. Here it is doing something simpler — restoring the base that development destroyed. More would raise a seasoned tank above its original activity and start raising fog; less and the tank slows however much agent is in it. Twenty grams per litre is 0.145 mol/L of a genuine alkali, against the essentially zero the working baths carry.

Water to 1000 mL. Warm to get the metol in, cold to make up. The 1949 handbook’s warning about a cold bottle applies here more than to any other formula in this section, because ten grams of metol is what will come out of it.

Interactions

Metaborate against sulfite and bisulfite. In D-25 the addition is a titration of the bisulfite; in D-23 it is a titration of the bisulfite the developer has generated. Either way the sulfite is the reservoir the reaction happens in, which is why it is at working strength.

Metol against pH. Restoring the alkali restores the effective concentration of the metol as well as the amount, because the fraction of metol in its active form rises with pH. That is why a replenisher that only replaced agent would under-perform.

Replenisher against accumulated bromide. No interaction. Every silver ion reduced puts a bromide ion into the tank, and nothing in this bottle removes it. That is the limit on how long any replenishment scheme can run, and for this formula Kodak does not say where the limit falls.

Variants

None published. Kodak prints one replenisher for these two developers and no modification of it.

The course’s own variants — a D-25R made up at half strength for a tank that is turning over slowly, say — belong on a formula version record with a prediction and a control strip, not on this page. That is the honest place for a change nobody has published a result for.

Safety

Level B. Metol is a skin sensitiser and this formula carries more of it than any other here. Sodium metaborate tetrahydrate carries a Danger classification for reproductive toxicity and serious eye irritation, on a thinner set of notifications than borax’s — the encyclopaedia entry records how thin. Sodium sulfite is a Level A material. See the classification rubric for what Level B assumes.

Two powders here need weighing with extraction or inside an enclosure, and one of them is at 20 g/L. Nitrile gloves and eye protection for mixing and for every addition to the tank.

What is not a hazard here. No gas is evolved: there is no acid in this formula and the metaborate is an alkali, so the sulfur-dioxide caution that belongs to D-25 does not apply to the replenisher. The solution is alkaline but not corrosive at 20 g/L of a weak alkali, and nothing is heated beyond the warm water used to dissolve the metol.

Storage

Small, tightly corked bottles, filled full, six months and two months. Label it REPLENISHER, and label it with which developers it feeds, because a bottle of clear solution that looks like D-23 and contains an alkali D-23 does not have is a mistake with no visible symptom until the negatives arrive. The labelling SOP covers the rest.

Warm and redissolve any bottle that has been cold enough to precipitate; what comes out is metol.

Incompatibilities

Acid. The metaborate is an alkali and the sulfite releases sulfur dioxide on contact with acid; neither belongs anywhere near the stop bath except through a film.

Fixer, in either direction, on the usual grounds — dichroic fog one way, spoiled silver recovery the other.

Oxidising agents. Never in the same bottle or drain as a developing agent. See chemical incompatibilities.

A contaminated tank. Replenishment assumes you know what is in the tank. A bath that has had stop bath or fixer in it no longer satisfies that assumption and should be discarded rather than corrected.

Waste

Spent replenisher and the tank it fed go into the same labelled developer bottle: metol and sulfite with a borate load, kept out of the fixer so the fixer can go for silver recovery. Borate is a registered vegetation control, so this does not go on the garden. The disposal caveat governs, the general chemical waste SOP gives the procedure, and local regulation decides.

The commonest waste here is replenisher that has outlived its keeping date in a half-empty bottle, which is an argument for mixing small.

Troubleshooting

Negatives losing contrast across a session in a replenished tank. Under-replenishment: the alkali is not keeping up. Kodak publishes no rate for this formula, so the correction is empirical — increase the addition, run a control strip, and write down both.

Negatives gaining contrast and fog over weeks. Over-replenishment. You are adding alkali faster than the film is consuming it, and the tank is now more active than the formula.

A replenished tank that behaves correctly and then abruptly does not. Suspect accumulated bromide. Nothing in the bottle addresses it, and no amount of replenisher will.

Undissolved powder in the replenisher bottle. Metol, from cold water or from being made up too close to the final volume. Warm and stir.

A tank whose level falls much faster than its activity. Carry-out, not depletion. Make the volume up with working-strength developer.

Experiments

Find the rate Kodak did not publish. Mix a litre of D-23 and process films one at a time, replenishing at 20 mL a film, and run a control strip after every second film against a strip from fresh developer. Repeat at 30 and 40 mL. The rate that holds the control strip steady longest is your answer for your films and your tank, and it is a measurement rather than a recipe.

Separate the two things the replenisher does. Season a litre of D-23 with four films, split it in two, add metaborate alone to one half and metol alone to the other at the concentrations this formula implies, and develop matched strips. If alkalinity is what a tank loses first, the metaborate half should recover most of the density.

Test the claim that one replenisher suits both baths. Run the same replenishment rate into a D-23 tank and a D-25 tank and track both with control strips. The prediction from the buffer reading above is that D-25 drifts more slowly and tolerates a coarser rate.

Record any of these on a formula version record; the point of the sheet is that a change with a written prediction is an experiment and a change without one is just a different bottle.