Kodak Hypo Clearing Agent and the washing accelerator Kodak patented

Kodak still sells a hypo clearing agent — its chemical sheets of 2018 list it for both film and paper — and has never printed what is in the packet. It has printed a great deal else: a dilution, two temperature windows, four capacities, a keeping figure and the wash times the step is supposed to buy, and every one of those numbers is on this page. The composition is on none of them, so a formulary that read only the consumer sheets would have to file the most widely used washing aid in the world as a thing whose contents nobody knows.

There is one more document. On 1 April 1954 Richard W. Henn and John I. Crabtree — the second of them the author of the Kodak laboratory’s own published surveys of hypo removal, which this patent cites back to 1940 — filed Photographic washing accelerators for the Eastman Kodak Company. It issued as US 2,860,978 in 1958 with a single claim, and that claim is a recipe.

Ingredient	Quantity	What the patent says
Sodium sulfite	20.0 g	“Grams per liter”, in the preferred example, the claim and Example 10
Sodium bisulfite	5.0 g	the same three places, at the same strength
Water	to make 1000 mL	“per liter of working strength bath”

!What this page publishes, and what it does not

The formula above is the patent’s, not the packet’s. US 2,860,978 nowhere names KODAK Hypo Clearing Agent or any other product. What the course can state as fact is that Kodak’s own washing researchers patented a sulfite–bisulfite washing bath at these strengths, and that everything Kodak publishes about the packaged product — a stock diluted 1+4, which is Kodak’s “1:4” written the way this course writes every dilution, one part of stock plus four of water; a near-neutral bath; one to two minutes for film; a wash cut from twenty-five minutes to five — is consistent with it. That the powder in the packet is this composition is an inference the course does not have the evidence to close. Kodak’s current safety data sheet would decide it, and the course could not obtain one.

The claim carries a third ingredient this page cannot print in its table. Example 7 and the claim both add 0.5 g per litre of the tetrasodium salt of ethylenediaminetetraacetic acid, which the patent names by its trade name, Sequestrene Na-4. The course’s chemical encyclopaedia has entries for the disodium salt and none for the tetrasodium one, and this course does not publish a formula containing an ingredient a reader cannot look up. So the sequestrant is explained in full below and left out of the quantities, and a reader mixing Kodak’s claimed bath adds it.

Purpose

To take thiosulfate out of a fixed film or print in minutes rather than in an hour, and to leave behind something that will not attack the image if the last of it is never removed.

Henn and Crabtree state the problem in one sentence of prior art: the common method of washing a fixed image is running water from one-half hour to an hour. Their objection is not that this is slow, but that it is incomplete and that the previous attempts to speed it up were worse than the disease. The hypo eliminators and washing accelerators the literature had accumulated by 1940, which the patent says Crabtree, Eaton and Muehler had tabulated in that year — oxidising agents, alkalies, and the common salt that Eaton and Crabtree took in 1943 to be the active ingredient in the accelerated washing obtained with sea water — are dismissed on four specific grounds: they are inconvenient to use, they are unstable, they often cause excessive softening of the emulsion, and, most damning, they tend to remove the soluble sodium thiosulfates but not the less soluble silver thiosulfates.

That last clause is the whole reason a wash aid exists. A print carries two problems out of the fixer, and only one of them is hypo.

The patent’s stated objects are therefore double: to remove the residual hypo, and to stabilise the hypo compounds that are not removed, so that the tendency towards fading and staining is greatly reduced. A bath that gets the concentration down and leaves the remainder more dangerous has not helped.

Recommended uses

Fibre-base paper, where the argument is strongest. Paper holds thiosulfate in the base as well as in the image layer, and the patent’s own comparison in Example 3 ran across single-weight, double-weight and extra-thin bases, with and without baryta. Kodak’s paper sheet gives two to three minutes at 10 to 30 °C, two minutes for single weight and three for double and premium weight.

Film, where the saving is largest in proportion. Kodak’s film directions put the wash aid between a thirty-second rinse and a five-minute wash, and state the alternative plainly: five minutes of washing after the clearing agent, or twenty to thirty minutes without it. One to two minutes at 18 to 21 °C, agitated continuously for the first thirty seconds and then at thirty-second intervals.

Microfilm and anything else that has to reach archival levels. The patent singles this out: its preferred solution “has been particularly beneficial in obtaining archival levels when washing microfilms”. That is the one application for which Henn and Crabtree claim the sequestrant-bearing version specifically.

After a wash, never instead of one. Every published sequence in the corpus puts a short running-water wash in front of the aid and a shorter one behind it. Kodak’s film sequence is rinse, aid, wash. ILFORD’s optimum-permanence sequence for fibre paper is five minutes, ten minutes, five minutes. Reilly’s is two to four minutes, three to four minutes, thirty minutes.

Where the wash water is cold. Kodak’s description of the packaged product claims something the patent does not: that it makes “washing at lower wash-water temperatures practical”. The paper sheet’s window runs down to 10 °C, which is well below the temperature at which an unaided wash is normally attempted.

Where the bath sits, and why the wash in front of it is not optional

1. 1Fixerthe material leaves it loaded with thiosulfate and with silver–thiosulfate complexes
2. 2Short running-water washremoves the bulk of the free thiosulfate for nothing, so the exchange bath is not spent on it
3. 3Sulfite–bisulfite baththe exchange: sulfite goes on to the gelatin and the fibre, thiosulfate comes off
4. 4Final washcarries away the displaced thiosulfate and the sulfite that replaced it
5. 5Drywhatever is left is left for the life of the print

Kodak's capacities are the arithmetic of step 2: 12 to 15 sheets of 8 × 10 inches per litre without the pre-rinse, 35 to 50 with it.

When another formula is preferable

• When you want a single-salt bath with a published exhaustion figure and a conservator’s argument behind it, use the one per cent sodium sulfite washing aid. Reilly gives it a time, a place in a sequence, a capacity and a mechanism, and it has one ingredient. It is the bath to reach for on albumen and salted paper, which is what he was writing about.
• When the material is resin-coated paper. Kodak says it twice, on two sheets: the clearing agent is not recommended for water-resistant papers, and the reason it gives is that their wash is four minutes in any case. The course would add a second reason, as its own inference rather than Kodak’s statement — the resin layers keep the wash water out of the paper base, so the reservoir an exchange bath exists to empty is not there — but Kodak states only the first.
• When you would rather buy a liquid than mix a powder, ILFORD WASHAID is the comparable product, and its sheet publishes more than Kodak’s does: 1+4, pH 7.00 to 7.20, specific gravity 1.020 at 20 °C, ten minutes for fibre paper and two to three for film, forty sheets of 20.3 × 25.4 cm or forty 135/36 films per litre, and a concentrate that keeps four years in a full airtight bottle. Its composition is not published either, and the course has no page for it.
• Never in place of a hypo eliminator’s job, because that is a different job. HE-1 destroys thiosulfate by oxidising it; this bath displaces it and leaves it in the tray. The patent is on the displacement side of that argument and says why in its opening paragraphs.
• Not at all, if the fixing was wrong. ILFORD’s sheet is blunt — exceeding the capacity of the fixer and extending the fixing time “both make washing more difficult” — and no wash aid recovers a print whose base has been loaded by over-fixing.

Mixing

Dissolve the sulfite first, then the bisulfite, in water at any ordinary temperature. The patent prints the two in that order in Example 7 and states no mixing temperature anywhere in the specification, so the course states none either. There is no order-of-addition hazard here in the way there is in an acid fixer: both salts are freely soluble, neither generates heat worth the name, and nothing precipitates.

Weigh the anhydrous sulfite. The patent writes only “sodium sulfite”. Sodium sulfite is also sold as the heptahydrate at 252.15 g/mol, and 20 g of that would put half the sulfite in the bath. Every strength on this page is computed on the anhydrous salt at 126.05 g/mol.

Take the bisulfite label seriously. The solid sold as sodium bisulfite is, as the chemical page records, usually largely sodium metabisulfite; and both salts oxidise slowly to the sulfate on standing, so an old caked tub is weaker than it says. Neither fact changes the formula, but both change what 5 g actually delivers.

If you are mixing Kodak’s claimed bath rather than its Example 10 bath, add 0.5 g per litre of the tetrasodium salt of EDTA. It is the third line of the claim. The course cannot print it in the table above for the reason given at the top of this page, and it is the ingredient the patent associates with archival results on microfilm.

🧮What 20 and 5 actually put in the tray

20.0 g ÷ 126.05 g/mol = 0.159 mol/L

Sulfite

5.0 g ÷ 104.06 g/mol = 0.048 mol/L

Bisulfite

0.159 + 0.048 = 0.207 mol/L

Total sulfur(IV)

Two comparisons follow, and both are the course’s arithmetic on published quantities rather than anything a source states.

Against Reilly’s one per cent sulfite bath, which is 0.079 mol/L, this bath is about two and a half times as strong in exchanging ion. That is the practical difference between a bath a conservator mixes for a session of twenty prints and a bath a manufacturer sells with a capacity of fifty.

And the two salts are in a molar ratio of about 3.3 parts sulfite to 1 part bisulfite, which is what makes the mixture a buffer rather than merely a solution. The pH sits where that ratio puts it, not where the water was.

Behaviour

It works fast and its whole effect is invisible. Nothing about a negative or a print looks different coming out of this bath. What differs is the number a residual-thiosulfate test returns afterwards, and what that number does to the image over decades.

Kodak’s numbers for the time it buys. Five minutes of washing for film instead of twenty to thirty. Ten minutes for single-weight paper and twenty for double-weight, against the hour that G-23 gives for an unaided fibre-base wash before toning.

It exhausts, and the rinse in front of it is worth more than another litre of it. This is the most useful pair of figures Kodak publishes, and it publishes it twice. For film: 12 to 15 sheets of 8 × 10 inches per litre without a pre-rinse, 35 to 50 with one. For fibre paper in a tray: 21 per litre without, 50 with. That is a factor of about three on the film sheet and about two and a half on the paper sheet, bought with thirty seconds under a tap — because the rinse leaves the bath spending its ions only on the fraction of the thiosulfate that plain water could not have taken out anyway. Reilly gives the same instruction with the mechanism attached, and calls what the pre-wash prevents an overloading of the ion exchange.

It keeps, but not indefinitely, and not in an open tray. Kodak gives the unused stock three months in a full closed container and the working solution 24 hours in a tray or a month in a tank. The tray figure is the one that matters in a home darkroom, and the reason for the difference is surface area: sulfite in an open dish is oxidising to sulfate the whole time it sits there.

2 SO₃²⁻ + O₂ → 2 SO₄²⁻

Aerial oxidation: the reason a tray of wash aid has a shorter life than a tank of it

Its pH is not the water’s pH. The bisulfite is there to put the bath near neutral and hold it there. The patent’s window is 4 to 10 and its preference is “substantially pH = 7”; ILFORD measures its own comparable product at 7.00 to 7.20. A bath that drifted alkaline would swell the gelatin more and would switch on mechanism (b) below, displacing the hardener out of an alum-fixed emulsion. The patent describes that effect and does not recommend it, and the reason is in its own opening pages: a softer, more swollen layer is one of the four things it holds against the alkaline eliminators it set out to replace.

Image characteristics

None, by design, and that is the specification rather than an omission. A washing aid that changed grain, contrast, tone or density would be doing something to the silver, which is exactly what it must not do.

Two published qualifications belong here, both of them about what the bath prevents rather than what it produces.

It prevents a yellowing that has nothing to do with the bath itself. Kodak Limited prescribes a one per cent sulfite bath for two minutes before the final wash specifically to avoid the slight yellowing of whites that its own hypo eliminator can cause. That is a different formula and a different sequence, but it is evidence that a sulfite bath in this position is doing at least one visible good.

And it prevents a fading that is not the thiosulfate’s fault. Henn and Crabtree found that adding a labile selenium compound to this same bath — 1.0 down to 0.02 g of selenium per litre — protected prints against peroxide vapour so completely that treated prints were “almost completely unaffected” where controls turned brown and lost density. At the highest concentration the image tone changed perceptibly; at the lower ones it did not. That is the ancestor of the modern practice of diluting selenium toner with wash aid, and Kodak’s own toning sheet still prints it as a working sequence.

The mechanism

The patent proposes three mechanisms in one sentence, and they are worth taking one at a time, because only the first is the one everybody quotes.

(a) Displacement. “Upon immersion in the sodium sulfite solution the thiosulfate ions absorbed to the gelatin are displaced by sulfite ions from the solution until equilibrium is reached.” This is ion exchange, and it is an equilibrium, not a reaction. Nothing is consumed and nothing is destroyed: the gelatin and the paper fibre hold a certain quantity of anion, and which anion they hold depends on what is available in overwhelming excess. Flood the material with 0.207 mol/L of sulfur(IV) and the thiosulfate comes off. This is why the bath has a capacity — every ion that goes on to the fibre came out of the tray — and why exhaustion here means the ratio has shifted rather than that a reagent has been used up.

(b) The hardener. “If alkaline, the solution also tends to displace the alum, which has a marked mordanting action for thiosulfate ions.” This is the sentence that explains why a wash aid is particularly useful after a hardening fixer, which is the case ILFORD names on its own sheet. The aluminium from F-5 or any other alum bath does not merely stiffen the gelatin; it acts as a mordant, holding thiosulfate the way a mordant holds a dye. Displacing the alum releases what the alum was holding. Note the qualification, which is the patent’s own: this effect belongs to an alkaline bath, and the bath the claim prefers is neutral. Henn and Crabtree publish the mechanism and then choose not to optimise for it.

(c) Stabilisation. “The sulfite ions, left in the print, stabilize any residual hypo by causing the reaction to shift to the right.” The patent does not print the equation it means by “the reaction”, and the course will not supply one it cannot source. What the sentence does establish is the patent’s second object: the sulfite that stays behind is not merely harmless, it is doing something useful to the thiosulfate that also stayed behind.

⚗Why the substitute ion has to be sulfite, and why the acid salt is in there

An exchange bath works with almost any small, cheap, soluble anion — the patent’s own alternative family is a polyvalent salt (sulfate, phosphate, pyrophosphate, tetraphosphate or citrate) at 1 to 2 per cent with an alkali. What narrows the choice is the residue. Whatever displaces the thiosulfate takes its place on the fibre, and some of it is never washed out either, so the substitute has to be something that does nothing to finely divided silver over fifty years. Sulfite qualifies: it is the preservative already sitting in every developer and every acid fixer in this formulary.

The bisulfite is not a second exchanging ion so much as a pH control. Sulfite in water is alkaline, because it is the salt of a weak acid and a strong base; the acid salt pushes back.

HSO₃⁻ ⇌ H⁺ + SO₃²⁻

The pair that fixes the pH of the bath

Mixing the two gives a buffer whose pH is set by their ratio, and a buffer is what the bath needs, because what arrives in it is alkaline developer carry-over on one side and acid fixer carry-over on the other. A bath that swung with each print would not hold the neutral condition the patent calls particularly useful.

📷What Henn and Crabtree measured, in milligrams per square inch

The patent is unusual among Kodak’s disclosures in printing the numbers rather than the conclusion, and Example 1 is the clearest. Single-weight Velox F-3 paper, fixed in a bath already carrying silver from previous use, bathed for five minutes and then washed for fifteen minutes in cold water:

Treatment	Residual hypo, mg per square inch
Water only	0.22
2 % sodium bisulfite	0.12
2 % sodium sulfite	0.03

Residual silver fell from 0.017 to 0.002 mg per square inch with either salt. Read the hypo column downwards: the bisulfite halves it and the sulfite divides it by seven, on the same paper, the same fixer and the same wash. That is the experimental basis for a bath in which sulfite is the bulk and bisulfite is the minority partner, and it is the strongest single piece of evidence on this page that the sulfite is not simply an inert carrier for a pH.

Example 3 then compares a sulfite bath against the alternatives on every paper base Kodak made, and reports the hypo content “in every case comparable to the lowest hypo content achieved by prior art methods (usually obtained with the HE-2, peroxide-ammonia eliminator), while the stability of the resultant image was in general superior to that obtained by any other method” — and the silver content lower than with either an alkali or peroxide-ammonia. A displacement bath matching an oxidising eliminator on hypo removal and beating it on image stability is the argument the whole patent is built on.

Function of every ingredient

Sodium sulfite, 20.0 g, the anhydrous salt. The exchanging ion and about three-quarters of the sulfur(IV) in the tray. Its job is to be present in such excess that the adsorption equilibrium on the gelatin and on the paper fibre runs towards sulfite and away from thiosulfate, and to be the sort of ion whose residue is of no consequence — the same reasoning that puts it in every fixer in this formulary as a preservative. Example 1 is the evidence that it, and not the acidity, is what does the work: a 2 per cent solution of it left a seventh of the hypo that plain washing did.

More than 20 g per litre is possible and the patent tests it — Example 2 uses a 10 per cent sulfite bath with bicarbonate — but nothing in the specification claims a proportionate gain, and a stronger bath is more sulfite to wash back out of the material afterwards. Less reduces the excess that drives the exchange, and the first thing you would see is not a failure but a shorter capacity: fewer sheets before the tray stops working. That is why Kodak’s capacity figures, not its times, are the number to watch.

Sodium bisulfite, 5.0 g. The acid member of the same family, and the reason the bath is near neutral instead of alkaline. Functionally it is sulfite with one of sulfurous acid’s hydrogens put back, so it contributes exchanging ion as well; but its distinctive job is the pH, and it does that as half of a buffer pair rather than as a dose of acid. The patent’s stated window is 4 to 10, its preference is substantially 7, and 3.3 parts sulfite to 1 part bisulfite is what puts a solution in that region and holds it there against carry-over.

More bisulfite drops the pH. That direction has a hard floor which is a safety limit and not a photographic one: an acid sulfite solution meeting a stronger acid gives sulfur dioxide, and the Incompatibilities section below is where that is dealt with. Less bisulfite lets the bath drift alkaline, which switches on mechanism (b) — displacing the alum out of a hardened emulsion — at the cost of a softer, more swollen gelatin layer, which is one of the four objections the patent makes to the alkaline eliminators it is replacing.

The tetrasodium salt of EDTA, 0.5 g, which the claim carries and this page’s table does not. The patent names it as Sequestrene Na-4 and gives it no explanation at all, which is a fact worth recording rather than papering over: Henn and Crabtree state the quantity and not the reason. What a sequestrant does is not in doubt — it is a chelating agent, and at 0.5 g per litre in a bath fed by tap water the calcium and magnesium of hard water are the obvious targets, since a sulfite solution in hard water can otherwise deposit an insoluble scum on a wet emulsion. The course states that as the chemistry of the class and not as the patent’s claim, because the patent makes none. The encyclopaedia’s nearest entry is the disodium salt, which is a different substance with a different pH in solution and is not a substitute for it; Ware’s alternative-process instructions elsewhere in this course go out of their way to tell readers not to interchange the two.

Water, to make 1000 mL. The patent’s unit is per litre of working strength bath, so this is a make-up volume and the formula is the bath as used, not a stock. No temperature is given. Matching it to the washes either side is ordinary practice for paper and matters more for film than it sounds, because a wash aid is one more tray in a sequence whose temperature differences show up as reticulation and blistering rather than as anything chemical.

Interactions

With the fixer in front of it, through the rinse between them. The whole capacity argument is one of carry-over, and the two capacity pairs under Behaviour are what it costs to get that rinse wrong. There is a second reason for it that the capacity figures do not show: whatever fixer rides into this tray brings dissolved silver with it, so a bath used without a pre-rinse is a more concentrated silver waste as well as a shorter-lived one.

With a hardening fixer, unfavourably in general and usefully in this bath. Alum slows every stage of thiosulfate removal, and the patent explains why in a way no other source in this corpus does: the alum is a mordant for thiosulfate ions. Henn and Crabtree tested exactly that case — Example 8 fixes its prints in Kodak F-5 deliberately loaded with silver to simulate a used bath — and reported prints treated only in water fading severely in two days of moist incubation, prints treated in sodium sulfate fading in seven, and prints treated in sulfite with an alkali fading only slightly in fourteen.

📓An arithmetic check worth doing once

The patent quotes its F-5 from Kodak’s 1936 Book of Formulas as hypo 960 g, acetic acid 190 c.c., boric acid crystals 30 g and potassium alum 60 g. Its sulfite figure and its water volume are both unusable in the text the course read — the first is missing from the optical character recognition altogether and the second reads “6.5 liters”, which the arithmetic below contradicts. Against that, this course’s F-5 page, taken from Kodak Limited’s 1949 London handbook, gives hypo 240 g, boric acid 7.5 g and potassium alum 15 g per litre. Multiply those three by four and all three land exactly on the Rochester ones, which says the two printings are the same formula at two make-up volumes and that the Rochester bath is four litres of finished solution, whatever the “6.5” in the course’s copy is a mis-reading of. None of that is used as a quantity anywhere in this course — it is a reading, not a transcription — and it is here because doing the multiplication yourself is five minutes well spent on any formula you meet in two sources.

With selenium toning, deliberately, on both manufacturers’ instructions. Kodak’s toning sheet allows a working solution of Hypo Clearing Agent to dilute Rapid Selenium Toner 1+20 or 1+40, which eliminates the wash between fixing and toning; the prints go straight from the fixer into the combination bath, and the bath must not afterwards be reused on untoned prints because it carries traces of toner. ILFORD’s optimum-permanence sequence does the same thing with WASHAID. The patent is the reason both work: Examples 9 to 12 are Henn and Crabtree putting selenium into this bath on purpose.

With the final wash, as a shortener and not a substitute. Kodak never publishes the aid without a wash behind it, and the shortest it publishes is five minutes.

With resin-coated paper, not at all. Kodak says so in two separate publications, the processing guide and the toning sheet, and the reason it gives in both is that such paper washes in four minutes without help.

Variants

The claimed bath, with the sequestrant. Sodium sulfite 20.0 g, sodium bisulfite 5.0 g and the tetrasodium salt of EDTA 0.5 g per litre. This is the patent’s own preferred version and the one it associates with archival levels on microfilm. It is not a separate formulary entry because it is not a separate formula — it is this one with its third line restored — and it is not in the table above only because of the encyclopaedia gap described at the top of this page.

The plain sulfite bath. One per cent sodium sulfite, which Reilly publishes for albumen and salted paper with a time, a capacity and a sequence. The patent tests a 2 per cent sulfite solution and it is the treatment that gave 0.03 mg of hypo per square inch in Example 1. A single-salt bath is a real alternative and this course prints it as one.

Sulfite with an alkali instead of an acid salt. Example 2 uses 10 per cent sulfite with 2 per cent sodium bicarbonate; Example 3 uses 2 per cent sulfite with 1 per cent bicarbonate. These are the alkaline versions in which mechanism (b) is switched on, and they are the ones Example 3 compares against the whole prior art. The course records them as the patent’s variants and does not recommend one, because the patent’s own claim went the other way.

Sulfite with selenium. From 1.0 down to 0.02 g of selenium per litre, added as a selenosulfate. Henn and Crabtree’s Example 9 makes potassium selenosulfate by dissolving metallic selenium in potassium sulfite; the resulting powder was stable blended with sodium sulfite, potassium sulfite or potassium metabisulfite but discoloured visibly when sodium bisulfite was present. That last observation is a practical incompatibility inside the patent’s own family and the course records it as such. The modern equivalent of the whole idea is diluting a proprietary selenium toner with working-strength wash aid, which both Kodak and ILFORD publish and which needs no selenium chemistry at the bench.

Not a variant: HE-1. A hypo eliminator oxidises thiosulfate rather than displacing it, and this patent’s opening pages are an argument against that whole class.

Safety

Level A. Two ordinary sulfite salts at a combined 25 g per litre in near-neutral water, handled with the controls a darkroom already has: gloves, eye protection, general ventilation, dedicated utensils, a labelled container that has never held food.

The hazard is in the tub, not in the tray. Both salts are supplied as powders, and the operations that deserve care are weighing and dissolving them. HSE’s COSHH essentials sheet for manual film development takes single-use nitrile gloves 0.2 mm thick as splash protection where a safety data sheet gives no more specific advice, and asks for eye protection, cotton overalls and general ventilation of more than five air changes an hour with a through draught. Weigh without raising dust; wear splash goggles for the weighing and not only for the tray.

The one gas this bath can make, and the one way to make it. An acid sulfite solution meeting a stronger acid releases sulfur dioxide.

HSO₃⁻ + H⁺ → SO₂ + H₂O

Bisulfite plus a stronger acid: the reaction to keep out of a darkroom

HSE’s EH40 puts sulphur dioxide’s workplace exposure limit at 0.5 ppm (1.3 mg/m³) over eight hours and 1 ppm (2.7 mg/m³) over fifteen minutes — among the tightest limits any substance in this course carries — and sodium hydrogen sulphite itself at 5 mg/m³ over eight hours with no short-term limit. The practical consequence is a housekeeping rule rather than a chemistry one: this tray must never meet the stop bath, an acid fixer concentrate, or any acid waste bottle, and a jug shared between the two is the usual way it happens.

What is not a hazard here, and why. The substitute ion was chosen because its residue does nothing to image silver — that is Reilly’s stated criterion for a washing aid and it is the patent’s second object — so this is not a reducer, and a negative forgotten in it does not thin the way one forgotten in a ferricyanide bath does. There is no heavy metal in it at all unless you have deliberately added a selenium compound, which this course does not ask you to do. And the GHS classifications the encyclopaedia records for sodium sulfite and sodium bisulfite are classifications of the substances as supplied, which is a powder being weighed; the course has no classification for a 2.5 per cent solution of them and does not invent one, which is why the controls above are written for the weighing rather than for the tray. Reading a classification of a substance as though it were a classification of a dilute solution, or the reverse, is one of the commonest mistakes made with a safety data sheet, and it goes wrong in both directions.

Storage

The dry salts, separately, airtight, dry, cool, dark and away from acids. Both oxidise slowly to the sulfate in air. A caked or damp tub of either has lost strength, and on this formula weakness shows up as a shorter capacity rather than as an obvious failure — which is the worst way for a darkroom chemical to fail, because nothing tells you.

The made-up bath, in a full stoppered bottle, for as short a time as you can manage. Kodak gives its own product’s stock three months full and closed, and the working bath 24 hours in a tray against a month in a tank. The course has no keeping figure for a bath mixed from the patent’s quantities and does not borrow Kodak’s, but the direction of the difference is chemistry rather than branding: oxygen reaches an open dish freely and a full stoppered bottle hardly at all.

Label it with the formula, the strength and the date, per the labelling SOP, and do not store it next to anything acid.

Incompatibilities

Acids of every kind, which liberate sulfur dioxide from the bisulfite directly and from the sulfite as readily. This is the first line of the page’s safety case and it is a storage rule as much as a mixing rule. See incompatibilities.

Oxidising agents — hydrogen peroxide, persulfate, permanganate, hypochlorite — which convert sulfite to sulfate and destroy the bath’s exchanging ion. That includes a print carrying traces of HE-1, and it is why Kodak Limited’s own sequence puts a sulfite bath after its eliminator rather than before it.

Sodium bisulfite and a selenosulfate, in the same powder. The patent reports it directly: its potassium selenosulfate blend was stable with sodium sulfite, potassium sulfite and potassium metabisulfite, but “visible discoloration occurred when sodium bisulfite was present in the mixture”. Anyone tempted to reconstruct the selenium-bearing version of this bath from Example 9 should read that sentence first.

Toner carry-over, once and then never again. Kodak’s instruction for the combined selenium bath is explicit that it must not be reused afterwards on untoned prints.

Heat and steam, which the sulfite page records as producing corrosive material with this salt.

Waste

What leaves the tray is a dilute sulfite solution carrying the thiosulfate and the silver–thiosulfate complexes it pulled out of the material. It is therefore part of the silver-bearing stream even though it looks like water, and the patent’s own measurement is the proof: the residual silver in its test prints fell from 0.017 to 0.002 mg per square inch, and the difference did not evaporate — it went into the bath.

It is also oxygen-demanding twice over. Sulfite and thiosulfate both consume dissolved oxygen in receiving water, and Kodak’s own disposal guidance sets the scale: sewer codes commonly cap biochemical oxygen demand at 350 mg/L and chemical oxygen demand at 650 mg/L, silver at 1.2 mg/L and pH at 5.6 to 9.4, while the five-day biochemical oxygen demand of photographic effluent typically runs from 100 to 1000 mg/L. A bath of this kind is squarely inside the reason those limits exist.

Collect it with the fixer and the first wash, keep it out of any container that has held acid, label it, and follow the silver-bearing waste SOP and the disposal ruling. The course has no measured silver concentration for a wash-aid tray and prints none. Local regulation decides what may be discharged where you are, and in the United Kingdom the route is your local authority’s hazardous waste service.

Troubleshooting

A residual-hypo test still shows thiosulfate after the full sequence. Work backwards through the sequence before suspecting the bath. Was the fixer inside its capacity and the fixing time short? Was there a rinse between the fixer and the aid? Has the tray done more sheets than its capacity allows — 21 per litre for fibre paper without a pre-rinse, 50 with one? The residual hypo and silver tests SOP is how you find out rather than guess, and the wash testing lab is where the course teaches the measurement.

Prints yellowing in the highlights months or years later. Residual thiosulfate, residual silver–thiosulfate complex, or both, and the patent’s second object is the clue: what stays behind matters as much as what leaves. Check the fixer’s age first — Example 1 was run in a bath deliberately loaded with silver because that is the condition under which the problem appears.

The bath seems to do nothing. Correct, on the day. Its entire effect is a number you cannot see and a change of state fifty years out, which is why this is the step darkrooms skip and why it is not the step to skip.

A sharp smell over the tray. Something acid has reached it. Ventilate, discard the bath, and find the contamination — a stop-bath tray, a shared jug or a splash from a fixer concentrate.

A white scum or bloom on a wet emulsion. Hard water is the first suspect, and it is what the patent’s sequestrant is there to deal with. If you are mixing the two-salt bath, this is the symptom that argues for restoring the claim’s third ingredient, or for mixing in deionised water.

The tub of bisulfite has gone hard, or smells strongly. It has taken up water and been oxidising, and both salts sold under that name behave the same way. Weigh from a fresh tub for anything that matters.

Experiments

Measure the pre-rinse. Kodak publishes two capacities for the same bath and the only difference between them is a rinse. Run two trays of identical volume, one fed by prints straight from the fixer and one by prints given thirty seconds under a tap, and test both baths at intervals until they stop clearing. Kodak’s claim is a factor of roughly two and a half to three; the number you get is a measurement this course does not have.

Reproduce Example 1, in the units you can reach. Henn and Crabtree compared water, 2 per cent bisulfite and 2 per cent sulfite on the same paper, the same silver-loaded fixer and the same wash, and reported 0.22, 0.12 and 0.03 mg of hypo per square inch. You will not match their analytical method, but the ranking is testable with a residual-hypo spot test on three matched prints, and the prediction is unambiguous: the middle tray should be closer to the water tray than to the sulfite one.

Test the buffer. Mix the bath, measure the pH, then add carry-over deliberately — a millilitre of working developer, then a millilitre of working stop bath — and measure after each. Then repeat with the sulfite alone, no bisulfite. The point of the second salt is the difference between those two curves, and it is the cheapest demonstration of what a buffer is anywhere in this course.

Find how much wash time it actually buys you, in your washer. Wash two sets of matched prints, one through the aid and one without, pulling one from each at 5, 10, 20, 30 and 45 minutes, and test each. The horizontal distance between the two curves is Kodak’s claim measured in your own darkroom, and it belongs in a laboratory report with the fixer, its age and the water temperature all recorded.

Weigh the hardening penalty. Fix matched prints in F-5 and in a non-hardening bath such as F-24, put both through the same aid and the same wash, and test. The patent says the alum is a mordant for thiosulfate; this is that claim made into a number, and it is the experiment that decides whether a hardening fixer is worth its convenience in your own work.