One per cent sodium sulfite washing aid

The step most darkrooms skip, and the sentence that explains why they should not: water is such a poor remover of low levels of thiosulfate from prints that an extra step in processing is needed. Not an improvement, not an accelerator — a step, without which a print of optimum stability is not obtainable by washing at all.

Ingredient	Quantity	Form the source specifies
Sodium sulfite	10 g	anhydrous
Water	to make 1000 mL

Purpose

To get thiosulfate out of paper. Fixing leaves the print full of thiosulfate ions and of silver–thiosulfate complexes; both are damaging on long storage, and both are held by the paper fibres and the image layer far more tenaciously than a rinse suggests. A washing aid replaces them with an ion that does not matter.

Reilly’s account of the mechanism is one sentence and it is the whole of the theory: these treatments are effective because they displace the absorbed thiosulfate ions and replace them with less harmful and more soluble ions of various salts. ILFORD’s sheet for its own proprietary product describes it in the same terms — removal of the thiosulphate by-products of fixation by ion exchange.

Recommended uses

Fibre-base paper of every kind, and printing-out papers above all, because their image layers and base stocks hold the most and their permanence depends on the wash more than anything else in the process.

After a hardening fixer, particularly. ILFORD names that case explicitly as one where its wash aid is especially useful, and the reason is mechanical: a hardened gelatin layer releases thiosulfate slowly.

Three to four minutes with constant agitation, after a two-to-four-minute running-water wash, before a thirty-minute final wash. The short wash first is not padding. Reilly explains it: it removes the vast bulk of the thiosulfate and avoids overloading the mechanism of ion-exchange in the sulfite solution that follows. Put a print straight from the fixer into a litre of 1 per cent sulfite and you have spent the bath on the easy nine-tenths.

Longer for thick stocks. Reilly: the thicker the base paper, the longer the wash time required and also the longer the treatment time required for the washing aid.

When another formula is preferable

• When you want a manufacturer’s product with published capacities and a tested sequence, ILFORD WASHAID. The course cannot publish it as a formula because its composition is not disclosed, but the sheet publishes a great deal that is usable: 1+4, pH 7.00 to 7.20, ten minutes for fibre-base paper and two to three for film at 20 °C, and 40 sheets of 8 × 10 inches or 40 films of 135/36 per litre.
• When the print is the kind that a long wash damages. Reilly warns that some papers may be injured by prolonged washing, the image layer beginning to dissolve, in which case a shorter wash is indicated — and a washing aid is what makes a shorter wash defensible.
• Never in place of washing. This bath shortens and improves a wash; it does not replace one, and both the wash before and the wash after are part of the published procedure.
• HE-1 is not an alternative to this and the course does not recommend it. A hypo eliminator destroys thiosulfate chemically rather than displacing it, which is a different and more aggressive idea; that page sets out why the course publishes it as history.

Mixing

Weigh 10 g, make up to a litre, stir. There is no order and no temperature to get wrong, and Reilly notes that the solution may be made in advance — the only bath in his printing sequence of which that is true.

Weigh the anhydrous salt. Reilly specifies it. Sodium sulfite is also sold as the heptahydrate, and 10 g of that would give a 0.5 per cent bath.

Make it in the quantity you will use. It is a one-shot bath: used once and discarded.

🧮Why one per cent is enough, and what the number means

10 g in 1000 mL = 1 % w/v

As a mass concentration

10 g ÷ 126.05 g/mol = 0.079 mol/L of sulfite

As moles

Set that against a fixing bath: F-52 carries 1.01 mol/L of thiosulfate, and Reilly’s alkaline bath 0.60 mol/L. The washing aid is more than ten times weaker than the bath whose residue it is displacing.

That is not a mismatch, because the two solutions are doing different arithmetic. The fixer has to supply two thiosulfate ions per silver ion in the whole emulsion. The washing aid has only to out-number the thiosulfate remaining in a print that has already had most of it washed out, and to keep out-numbering it as the print gives it up. Reilly’s two-to-four-minute pre-wash is what makes 0.079 mol/L sufficient, and his 20-prints-per-litre limit is what stops the ratio from inverting.

Behaviour

It exhausts, and Reilly gives a figure with a puzzle attached. He writes that the rate of exhaustion for this solution is the same as that of the fixer, and then gives no more than 20 prints of approximately 8 × 10 inches per litre — where his own recommendation for the fixer, a page earlier, is 10 to 15. Both numbers are printed on this page as he printed them. The course does not average them, and the practical reading is the conservative one: a litre of a one-shot bath is cheap.

Its effect is invisible on the day and decisive later. Nothing about a print treated in it looks different coming out. What differs is how much thiosulfate the final wash manages to remove, and that shows up as staining and fading over years.

It shortens the wash rather than removing the need for one. Reilly’s sequence still ends in 30 minutes of running water — 40 to 50 for heavy stocks. ILFORD’s published sequences for its own product are more aggressive: five minutes of running water, ten minutes in the wash aid, then five minutes of final wash instead of the usual long one, with the note that this depends on the fixing time being kept very short.

It has a side effect Kodak used on purpose. Kodak Limited prescribes exactly this bath, at exactly this strength, for two minutes before the final wash, to prevent the slight yellowing of whites that its hypo eliminator can cause. A 1 per cent sulfite bath before a final wash is therefore doing at least two useful things, and the second one is why the course would keep it in the sequence even after an oxidising treatment it does not otherwise recommend.

Image characteristics

None, and that is the requirement rather than an absence. A washing aid must displace thiosulfate without touching the image, and sulfite at 1 per cent does not attack metallic silver.

Its only visible effect is a negative one: prints that skip it are more likely to yellow in the highlights over years, which is the thiosulfate and the silver–thiosulfate complexes it was there to remove doing what they do.

Kodak’s yellowing note is the exception that proves the point — there, the sulfite bath is preventing a visible change caused by something else.

The mechanism

Diffusion is not the problem; adsorption is. A print in wash water loses thiosulfate quickly at first and then very slowly, because the last of it is not simply dissolved in the water inside the paper — it is held on the fibres and in the image layer. Reilly’s word is absorbed; ILFORD’s word for the remedy is ion exchange. Washing removes what is free. What is bound stays bound until something displaces it.

The washing aid supplies a competing anion in large excess. Sulfite is small, highly soluble, carries the same charge as thiosulfate, and — crucially — does not attack the image if a trace of it stays behind. So the exchange runs the right way twice over: sulfite displaces thiosulfate because there is a great deal more of it, and the sulfite that takes its place is itself washed out easily and does nothing if some remains.

That is why the requirement is a concentration and not a reagent. Reilly’s own phrase is “less harmful and more soluble ions of various salts” — the identity of the salt matters less than the fact that it is present in excess and does no harm.

⚗Displacement, not destruction, and why the difference matters

Two families of treatment have been sold for this job and they are not the same idea.

A washing aid displaces. Sulfite replaces thiosulfate on the fibre; the thiosulfate goes into the bath and then down the drain with it. Nothing is chemically changed and nothing is left in the print that was not there before except sulfite.

A hypo eliminator destroys. Kodak’s HE-1 uses hydrogen peroxide and ammonia to oxidise thiosulfate to sulfate, which is inert. That works, and it introduces an oxidising agent into a paper carrying a finely divided silver image — and Wall was already sceptical in 1924, writing that the action of such chemicals is probably in most cases to convert these salts into tetrathionates, and that since a print can be practically freed from hypo in half an hour by proper washing their use is only allowable in cases of great pressure of time or shortage of fresh water.

Tetrathionate is precisely the compound the thiosulfate page records as attacking image silver. The course’s position follows Wall’s and Reilly’s: displace, wash, and do not oxidise anything in the presence of the image you are trying to preserve.

Function of every ingredient

Sodium sulfite, 10 g of the anhydrous salt. The whole of the bath. It supplies 0.079 mol/L of a small, doubly charged, freely soluble anion whose only job is to be present in overwhelming excess relative to the thiosulfate remaining in the print, so that the exchange equilibrium on the paper fibres and in the image layer runs towards sulfite. It is chosen rather than some other salt because a residue of it does nothing to the image — it is the same preservative that sits in every developer and every acid fixer in this formulary — and because it is cheap enough to throw away after twenty prints.

More than 1 per cent is what one manufacturer in this course’s corpus recommends, and see Variants; Reilly’s judgement is that 1 per cent is the best for albumen and salted papers, and the course follows the source that has an argument attached. Less fails in the way the bath is designed not to fail: at low enough concentration the exchange stops favouring sulfite, and the print keeps its thiosulfate.

The form matters. Reilly says anhydrous, at 126.05 g/mol. The heptahydrate is 252.15, so the same 10 g would give half the sulfite. A tub of sulfite that has caked or taken up moisture has also been oxidising to the sulfate, and is weaker than its label.

Water, to make 1000 mL. No temperature is specified. Keeping it near the temperature of the washes either side of it is ordinary good practice for paper, and for albumen it is more than that — Reilly’s warning that temperature differences between trays cause blistering applies through the whole sequence.

Interactions

With the fixer before it, through the wash between them. The pre-wash exists to keep the fixer’s bulk thiosulfate out of this bath. Skipping it does not make the aid work harder; it makes it work on the wrong tenth of the problem.

With a hardening fixer, unfavourably in general and usefully here: the alum in F-5 or F-54 slows every stage of thiosulfate removal, and ILFORD names a hardening fixer as one of the conditions under which a wash aid is particularly worth using.

With over-fixing, not at all. Reilly and ILFORD agree on this and it is worth stating baldly: nothing in this bath rescues a print that was left in the fixer too long. Thiosulfate driven into the paper fibres is close to unremovable, and ILFORD’s instruction is not to exceed the fixer’s capacity and not to extend the fixing time, because both make washing harder.

With toning, in ILFORD’s sequence, deliberately. Its optimum-permanence route dilutes selenium toner with working-strength wash aid instead of water. That is a manufacturer’s published sequence for its own product rather than a general rule, and the course records it as such.

With the final wash, as a shortener and not a substitute.

Variants

A ten per cent sulfite rinse, from a manufacturer. BERGGER’s data sheet for its alkaline fixer recommends a washing sequence beginning with a rinse in a sulphite bath at 10 per cent, to eliminate the chemical complexes resulting from the fixing bath and to allow the full dissolution of the anti-halation layer, followed by ten washes in clear water at ten-minute intervals and a final rinse in demineralised water with wetting agent at 1+200.

That is ten times Reilly’s concentration, and the course prints both rather than choosing silently. The course uses Reilly’s 1 per cent, for three reasons, all of which are about evidence rather than about chemistry. Reilly gives a time, an exhaustion figure, a place in a stated sequence and a mechanism; BERGGER gives a concentration inside a paragraph about its own product’s washing. Reilly is writing about the paper this course prints on in Parts XXII to XXIV; BERGGER’s sentence is about film and names the anti-halation layer, which paper does not have. And Kodak Limited independently prescribes a 1 per cent bath at the same point in a print sequence. Where a reader is washing film after a neutral fixer, BERGGER’s figure is the one with the relevant material behind it.

Kodak Hypo Clearing Agent and ILFORD WASHAID cannot be published as formulas. Reilly names the Kodak product as a usable alternative to his own bath and does not disclose its composition; ILFORD describes its own as a hypo eliminator working by ion exchange and publishes dilution, pH, times and capacities but no quantities. Under the course’s rule on proprietary formulations both are taught as behaviour and disclosed components only. What is worth taking from them is the data: ILFORD’s 1+4 working strength at pH 7.00 to 7.20, ten minutes for fibre-base paper, two to three minutes for film, and 40 sheets of 8 × 10 inches per litre.

HE-1 is not a variant of this formula. It is the other family — destruction rather than displacement — and it has its own entry and its own argument against it.

Safety

Level A. One substance at 1 per cent in water, and the controls are the ordinary darkroom ones: gloves, eye protection, ordinary ventilation, dedicated utensils.

The classification belongs to the powder, not to the bath. Sodium sulfite’s aggregated GHS entry is a spread rather than a ruling — about a quarter of reports say it does not meet the criteria at all, and among those that classify it the most common statement is H314, causes severe skin burns and eye damage, followed by serious eye irritation, skin irritation and harmful if swallowed. That describes a substance being weighed and dissolved, not a one per cent solution. Weigh it without raising dust, wear splash goggles for the weighing rather than only for the tray, and keep it off skin.

What is not a hazard here, and why. There is no acid in this bath and none should reach it. Sulfite plus a strong acid gives sulfur dioxide, whose workplace limit is among the tightest in this course, and that is the one way to make a dangerous gas out of a tray that looks like water. Keep it away from the stop bath and from every acid waste bottle.

Storage

The dry salt, airtight, dry, cool and dark, in a labelled container that has never held food. Sulfite oxidises slowly to sulfate in air; a caked or damp tub is weaker than its label says, and on this formula that means a bath that does less than you think it does.

The made-up bath is one-shot. Reilly says it may be made in advance and that it should be used once and discarded, and gives no keeping figure — so the course gives none either. Mix what a session needs.

Label with the formula, the strength and the date, per the labelling SOP.

Incompatibilities

Acids of any kind, which liberate sulfur dioxide from sulfite directly. This is the most important line on the page, because a tray of clear liquid that looks like water invites carelessness. See incompatibilities.

Oxidising agents — hydrogen peroxide, persulfate, permanganate, hypochlorite — which oxidise sulfite to sulfate and destroy the bath. That includes a print carrying traces of HE-1, which is why Kodak’s own sequence puts the sulfite bath after the eliminator and then washes.

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

Nothing in the image, which is the point of choosing sulfite.

Waste

A dilute sulfite solution carrying the thiosulfate and the silver–thiosulfate complexes it has displaced out of the print. It is therefore part of the silver stream, however clear it looks, and it is a substantial part of it after a printing-out session: most of the silver such a process sheds is in the washes rather than in the fixer.

It is also oxygen-demanding twice over — sulfite and thiosulfate both consume oxygen in receiving water — and it must be kept away from acid wastes.

Collect it with the fixer and the first wash, label it, and follow the silver-bearing waste SOP, which cites the disposal ruling. The course could not source a measured silver concentration for a printing-out wash and does not print one. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Prints yellowing in the highlights months or years later. Residual thiosulfate, residual silver–thiosulfate complex, or both. Check the sequence rather than this bath: was the fixer within its capacity, was the fixing time short, was there a pre-wash, was the final wash long enough?

The bath has been used for a whole session’s prints. It is spent. Twenty 8 × 10 sheets per litre is the published limit and it is a one-shot bath; the cost of a fresh litre is a few pence.

A sharp smell over the tray. Something acid has reached it. Ventilate, discard, and find the contamination — most often a stop-bath tray or a jug shared between the two.

No visible effect at all. Correct. This bath’s whole effect is on a timescale of years, which is exactly why it is skipped and exactly why it should not be.

Blisters on an albumen print during the sequence. Temperature differences between trays, per Reilly. Bring the washing aid to the temperature of the washes either side of it.

The tub of sulfite has gone hard. It has taken up water and been oxidising. Weigh from a fresh tub for anything that matters.

Experiments

Measure how much the pre-wash is worth. Treat two matched prints, one straight from the fixer into the sulfite bath and one after Reilly’s two-to-four-minute running wash, then wash both identically and compare what they give up to a final tray of still water, judged by conductivity if you have a meter. Reilly’s claim is that the pre-wash prevents the bath being overloaded; this is that claim made testable.

Compare 1 per cent against 10 per cent. Reilly and BERGGER differ by a factor of ten and neither publishes a comparison. Run one, on the same paper, with everything else held constant. Whatever you find is a measurement the course does not have, and it belongs in a laboratory report with the sequence recorded in full.

Find the wash time the aid buys you. Wash two sets of prints, one with the sulfite bath and one without, pulling a print from each at 5, 10, 20, 30 and 45 minutes, and compare their conductivity in a standard volume of still water. The curves will separate, and the horizontal gap between them is the time this formula saves in your washer.

Test the hardening penalty. Fix matched prints in F-54 and F-52, treat both in the washing aid, and compare. ILFORD says the aid is particularly useful after a hardening fixer; this measures how much of the hardening penalty it recovers.

Weigh what a print carries out. Dry and weigh a sheet, run it through the whole sequence, dry and weigh it again. The difference is small and it is not all thiosulfate — but doing it once makes the quantities concrete, and quantities are what make a permanence argument something other than a slogan.