Kodak DK-20R

Kodak changed all five ingredients of DK-20 by five different factors to make its replenisher, and the list of factors is the most compact statement in this reference of what a working tank actually uses up.

Ingredient	DK-20R	DK-20	Factor
Metol	7.5 g	5.0 g	1.5×
Sodium sulfite, anhydrous	100.0 g	100.0 g	1×
Sodium metaborate	20.0 g	2.0 g	10×
Potassium thiocyanate	5.0 g	1.0 g	5×
Potassium bromide	1.0 g	0.5 g	2×
Water to make	1000 mL	1000 mL

The 1949 handbook prints the crystalline sulfite alternative, 200.0 g, here as in every other formula in the book.

Purpose

To keep a tank of DK-20 at constant volume and constant activity so that a given highlight density is reached in the same time at the same temperature throughout the working life of the bath. Kodak’s own statement of the goal is that specific — not “keeps the developer fresh”, but a named quantity held constant against a named pair of controls.

Recommended uses

Deep-tank processing of roll film in quantity. Kodak gives one rate and it is a production figure: about 5 gallons of replenisher per 1000 rolls of film, which the handbook glosses as 80,000 square inches. Divide it out and the rate is legible at darkroom scale.

Not for occasional work. A replenisher whose bottle keeps six months full and two months half-filled, feeding a bath that must reach 25 per cent replacement before the scheme retires, is a technique for a tank that turns over. For a few films a month, mix a litre of DK-20 and discard it.

When another formula is preferable

D-76R replenishes D-76 and carries hydroquinone and borax; it will not do this job and this will not do that one.

D-25R replenishes D-23 and D-25 and has no thiocyanate and no bromide in it at all.

D-19bR is the one to read next if this page interests you, because it is the replenisher that goes the other way on bromide: D-19b carries 4.0 g/L and its replenisher carries none.

Fresh developer is the right answer wherever the tank is losing solution to carry-out rather than to chemistry, which is Kodak’s own instruction for the D-76 case.

Mixing

The handbook’s order is Elon, sulfite, Kodalk, thiocyanate, bromide, and its one-line instruction is to dissolve in the order given. The three general rules from the same handbook apply: the Elon first, in warm water, because it is only slightly soluble in a sulfite solution without alkali; the alkali after the agent and its preservative; and the bromide wherever you like, because it has no action on the developing agents.

Two practical notes for this formula specifically. Twenty grams of metaborate is ten times what you weighed for the developer, and it dissolves readily in cold water, so the alkali step is easy. Five grams of thiocyanate is a hygroscopic salt that will have taken up water if the jar has been open, which matters more at 5 g/L than it did at 1 g/L.

⚠The same acid rule, at five times the concentration

Potassium thiocyanate and a strong acid give hydrogen cyanide. The dry jar is stored away from every acid in the darkroom, this solution is never acidified, and its waste is never mixed with an acid stream. See chemical incompatibilities.

Behaviour

It is not a developer. No time, no temperature, and the keeping table marks its row “used as Replenisher only”.

🧮Kodak's production rate, converted to one roll

The handbook is Kodak Limited’s and its conversion tables are British fluid measure, so a gallon here is the imperial gallon of 4.546 litres.

5 gal × 4.546 L/gal = 22.7 L

Replenisher per 1000 rolls

22.7 L ÷ 1000 = 22.7 mL

Per roll

And 80,000 square inches over 1000 rolls is 80 square inches a roll, which is exactly the area Kodak Alaris’s J-78 sheet assigns to one 135-36 roll sixty-eight years later.

Now set that 22.7 mL beside the rate Kodak publishes for a completely different developer: 22.2 to 29.6 mL of D-76R per 135-36 roll. The two land on top of each other.

What that agreement is, and what it is not. It is evidence that the replenishment rate is set largely by how much solution a roll of film physically carries out of a tank, which is a property of the film and the reel rather than of the chemistry. It is not a rule the course can generalise: two figures from one manufacturer are two figures, and neither source explains its number.

Twenty-five per cent is the end of the scheme. Kodak’s instruction is to add replenisher as necessary to keep the volume constant “until 25 per cent of the original developer has been replaced”. The identical limit appears under D-76R. What stops it working past that point is the one thing no replenisher addresses: the bromide released by every developed grain accumulates, and nothing in the bottle takes it out.

Image characteristics

None of its own, and the target is that the tank should have none of its own either. Kodak’s published criterion is unusually testable: a given highlight density, at a constant time and temperature, throughout the life of the developer. That is a control strip and a densitometer, or at this stage of the course a control strip and a step count, run against a strip developed in a freshly mixed litre.

The mechanism

Read the five factors as five separate findings about a working tank.

Sulfite, 1×. At 0.79 mol/L the preservative and bulk-solvent load is in such excess that a tank of film consumes a negligible fraction of it. What is lost is lost by carry-out, and carry-out is replaced by a solution at the same concentration. No change is the correct change.

Metol, 1.5×. Development is stoichiometric in the agent: every silver ion reduced was reduced by a molecule that does not come back. Half again the working strength lets a small volume of replenisher restore a larger volume of tank.

Metaborate, 10×. Development produces acid, and DK-20 carries 2 g/L of alkali — 0.0145 mol/L, almost no reserve at all. The alkalinity is therefore the first thing a tank runs out of, and it needs the largest multiplier. Kodak reaches exactly the same factor of ten with borax in D-76R, which makes it a pattern across two different alkalis rather than a quirk of one formula.

Thiocyanate, 5×. The thiocyanate entry records the stoichiometry that makes this at least unsurprising — the silver complex takes four thiocyanate ions per silver, so a solvent that works by complexation is depleted four times faster than a one-to-one ligand would be. But Kodak gives no reason, and the course does not claim this one. What can be said without inventing anything is that the thiocyanate is the only ingredient here that is consumed by binding to something the film gives up, and that it carries the second-largest multiplier in the formula.

Bromide, 2×. This is the surprise, and it is worth sitting with. Every silver ion reduced releases a bromide ion into the tank, so a working bath is gaining bromide on its own. A replenisher that adds more bromide than the bath contains is therefore adding restrainer to a solution already accumulating it. Kodak does it here and does the opposite in D-19bR, which carries no bromide at all against D-19b’s 4.0 g/L.

Neither choice is explained by any source this course holds. What the two formulas together establish is that a replenisher’s bromide is a design decision rather than a rule, and that the decision runs in opposite directions in a fine-grain solvent developer and in a high-contrast carbonate one.

Function of every ingredient

Metol, 7.5 g. Replacement developing agent. Half again the tank’s concentration, in the same proportion Kodak uses in D-76R, so that the volume added to restore the level also restores the activity. More would push a seasoned tank above its original speed; less would leave it topped up and slow.

Sodium sulfite, 100.0 g anhydrous. Preservative for the replenisher in its bottle and for the tank it enters, and the bulk silver solvent whose concentration must not drift if the grain is not to drift. Deliberately identical to the working bath. Changing it would change the negatives, which is precisely the failure a replenisher exists to prevent.

Sodium metaborate tetrahydrate, 20.0 g. The alkali, restoring what development neutralised, and the largest single change from the working formula. Kodak’s own note on Kodalk — printed under DK-50 and summarised on the metaborate page — is that its quantity moves either the contrast at fixed time or the time at fixed contrast; a replenisher’s job is to hold both, which means holding the alkalinity. More would season the tank upward into rising contrast and fog; less and the tank slows however much agent is in it.

Potassium thiocyanate, 5.0 g. The strong silver solvent, replacing what has been complexed and carried out. It is what keeps the extra-fine grain of a seasoned tank the same as the extra-fine grain of a fresh one. More would progressively increase the solvent action of the tank, dissolving more image silver and losing density; less and the grain coarsens as the bath ages, which would be the drift most easily mistaken for a film problem.

Potassium bromide, 1.0 g. The restrainer, at twice the tank’s concentration. In a bath whose solvent puts silver into solution, fog control is not optional, and Kodak has chosen to keep adding restrainer rather than to rely on what the film releases. More would raise the toe and slow the bath; less risks the dichroic fog a solvent developer is prone to. It is the ingredient whose place in the mixing order does not matter.

Water to 1000 mL. Warm for the metol, cold to make up, and clean. A cold bottle will precipitate, and what precipitates first is the metol.

Interactions

Thiocyanate and bromide, again. The working bath sits at 2.5 : 1 in moles; the replenisher, at 0.051 mol/L of thiocyanate against 0.0084 mol/L of bromide, sits at about 6 : 1. So each addition tilts the tank towards the solvent as well as topping it up, which is a real consequence of the published proportions and is not something Kodak comments on.

Alkali and agent. Restoring the pH restores the effective concentration of the metol as well as its amount, which is why the alkali multiplier can be large without the agent multiplier having to be.

Replenisher and accumulated bromide. No interaction, which is the limit on the whole scheme and the reason for the 25 per cent rule.

Variants

None published. Kodak prints one replenisher for DK-20 and no modification of it, and — unlike D-76R, which has Kodak’s own five-to-one modified version for T-Max films — there is no second formulation here.

Anything you change belongs on a formula version record with a prediction written before the strip is developed. A replenisher is an especially good subject for that discipline, because its effects appear slowly and are easy to attribute to the wrong cause.

Safety

Level B, on metol as a skin sensitiser, potassium thiocyanate with its Danger classification and its cyanide-on-acid hazard, and sodium metaborate tetrahydrate classified for reproductive toxicity. Sodium sulfite and potassium bromide are Level A materials. The classification rubric sets what Level B assumes.

The difference from the developer is concentration, and it runs one way. The thiocyanate is at five times and the metaborate at ten times the doses you weighed for DK-20. Weigh both with extraction or inside an enclosure, and prefer one large batch weighed once to several small ones.

What is not a hazard here. The solution evolves no gas: it is alkaline, and the cyanide hazard belongs to a thiocyanate meeting a strong acid, which cannot happen in this bottle. At 20 g/L of a weak alkali it is not corrosive. Nothing is heated beyond the water used to dissolve the metol.

Storage

Small bottles, tightly corked, filled full. Six months full and two months half-filled, and since a replenisher is drawn on slowly, decant when you mix rather than reopening one large bottle weekly.

Label it DK-20R — REPLENISHER — contains thiocyanate. Two of those three facts change what happens to the bottle: one keeps it out of a developing tank, the other keeps it out of the wrong waste stream. See the labelling SOP.

Incompatibilities

Acids, absolutely and at every concentration, because of the thiocyanate. This governs where the dry salt is stored, what may be poured into the same waste container, and the instruction never to neutralise the spent solution.

Fixer, in either direction. A bath that already contains a strong silver solvent has the shortest possible route to dichroic fog if thiosulfate gets into it.

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

Waste

Like the developer it feeds, this is not ordinary developer waste. It carries thiocyanate, and the tank it has been feeding carries dissolved silver. It goes to the silver-bearing waste route, never into an acid stream, and never down a drain on the assumption that a developer is a developer.

The disposal caveat governs and local regulation decides. Unused replenisher past its keeping date is the commonest waste here, which is the argument for mixing the smallest quantity your throughput justifies.

Troubleshooting

Grain coarsening slowly over weeks in a replenished tank. Under-replenishment of the thiocyanate. Check the rate against the 22.7 mL a roll the deep-tank figure works out at, and check whether the jar of thiocyanate has taken up moisture since you weighed it.

Highlight densities creeping up, or fog appearing. Over-replenishment. You are adding alkali and restrainer faster than the tank is consuming them, and the alkali wins.

A tank that was fine and is now abruptly slow. Count what has gone through it against the 25 per cent rule. Past that limit the accumulated bromide is doing something replenishment cannot undo.

Replenisher that dissolves incompletely. Metol, from cold water. Warm it. If the residue is crystalline and did not dissolve at any temperature, suspect the thiocyanate jar has caked and been weighed as a lump.

A tank whose level falls fast but whose activity holds. Carry-out. Make the volume up with working-strength developer, not with more replenisher.

Experiments

Check Kodak’s rate against your own carry-out. Weigh a loaded reel before and after it leaves the developer. If the difference is close to 22.7 mL, the published rate is mostly replacing liquid; if it is much less, the rest of the rate is chemistry, and you have measured the split.

Test the bromide decision. Two litres of DK-20, seasoned equally, one replenished with DK-20R as published and one with a version of the replenisher carrying no bromide at all. Kodak went one way here and the other way in D-19bR, and no source explains either. The prediction from the accumulation argument is that the bromide-free replenisher holds shadow speed better and fogs sooner, and the prediction is worth writing down before you look.

Hold highlight density, which is Kodak’s own criterion. Run a control strip after every fifth roll and plot the density of one high step against roll number. A flat line is the formula doing exactly what its header promises, and a drifting one tells you which way to move the rate.

Record any of this on a formula version record and bring the plot into the developer laboratory report.