Kodak bisulphite fixing bath

Kodak’s 1928 primer divides fixing baths into three classes and prints a worked example of the middle one without giving it a number. This is it: an acid fixing bath with no acetic acid, no alum and nothing in it that can sludge — for the darkroom that cannot get acid, and for the negative that must not be hardened.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate pentahydrate	250 g	crystalline hypo
Sodium sulfite (anhydrous)	10.5 g	desiccated
Sodium bisulfite	5.3 g
Water	to make 1000 mL	"Water to make 1.0 liter", so the strengths above are exact for once. No temperature is given for it and there is a tension in the primer's own advice: hypo dissolves better in warm water, and the bisulfite must not meet warm hypo. Dissolve the hypo warm if you must, and let it go cold before the sulfite and bisulfite are added.

Purpose

To fix, and to be acid while doing it, without acetic acid.

The primer’s argument for an acid bath at all is worth quoting in substance, because it is the clearest statement of the case anywhere in the corpus. A plain hypo bath is seldom used, it says, because it gradually becomes alkaline from an accumulation of alkali carried over by prints and films from the developer. That softens the gelatin, and it lets the image continue to develop in the fixing bath — so that if two prints stick together, more development takes place at the point of contact and the print comes out unevenly. An acid bath kills the carried-over alkali and the unevenness with it.

The primer’s argument for this acid bath is equally plain, and is a statement about availability rather than about performance: when acetic acid cannot be obtained for the fixing bath, the only substitute which appears to be generally available is sodium bisulphite. Then it says what that costs, which is the sentence that makes this a formula worth publishing rather than a curiosity.

Recommended uses

Where acetic acid cannot be had, or should not be handled. This is Kodak’s own stated case, and it is a live one for a home darkroom: it removes the single most hazardous operation in mixing a fixer.

Where hardening must not happen. There is no alum here. Before toning, reducing or intensifying, and on a stain-image negative, an unhardened emulsion is what is wanted — and this bath gives the acid protection without the tanning.

Where a bath must not sludge. Aluminium sulfite is the white gelatinous end of every alum fixer. Nothing in this formula can precipitate it, because there is no aluminium in it.

As the American twin of F-52. Kodak Limited’s non-hardening acid fixer is hypo and potassium metabisulphite; this is hypo, sulfite and sodium bisulphite. The primer notes that sodium bisulphite is customarily substituted for potassium metabisulphite weight for weight and that the choice “simmers down to a matter of dollars and cents”, the sodium salt costing a third to a half of the potassium one.

Where a bath is being learnt rather than relied on. Three ingredients, one make-up volume, exact strengths, and no order-of-mixing trap beyond one temperature rule.

When another formula is preferable

• When the emulsion needs hardening, F-1 or, for a tank, F-14. The primer is explicit that bisulfite’s weaker acid reserve matters particularly in connection with the hardening agent, so a bisulfite bath is a poor foundation for one.
• When the acid reserve has to last, any of the acetic-acid baths. This is the formula’s stated weakness and it is stated by its own publisher.
• When speed matters, a stronger hypo bath: 25 per cent is below Kodak’s 30 to 40 per cent optimum, and Wall’s acid bath for negative work carries 400 g of hypo to the litre.
• When the source you are following is Kodak Limited’s, F-52, which is the same idea with metabisulphite and a published keeping table.
• When nothing acid is wanted at all — for a printing-out paper, say — a plain hypo bath or Reilly’s alkaline bath. Wall’s own division puts plain and alkaline baths with printing-out papers and acid baths with plates, film and developed papers.

Mixing

One vessel, one temperature rule.

1. Dissolve 250 g of hypo in something under a litre of water. Warm water dissolves it faster and hypo dissolves endothermically, so the solution arrives colder than the water did.
2. Let it go cold. This is the whole of the formula’s difficulty. The primer’s instruction for class 2 baths is emphatic: do not add the bisulphite or acid sulphite solutions to the warm hypo solution or the hypo will turn milky, and the solutions should be quite cold when mixed.
3. Add 10.5 g of sodium sulfite, then 5.3 g of sodium bisulfite, in that order — the order Kodak prints, and the order in which the alkaline salt goes in before the acid one.
4. Water to make 1,000 mL.

Kodak’s alternative arrangement, for anyone mixing this repeatedly: keep the bisulfite as its own stock solution and add it to a plain hypo stock as required, because an acid hypo solution gradually goes milky on keeping. That is the same architecture as F-1a, one class of bath down.

🧮Two prescriptions on one page, and they are not the same bath

The primer gives a general rule immediately above this formula: for general purposes, 45 c.c. of a 50 per cent sodium bisulphite solution is added to 1 litre of a 35 per cent hypo solution, with the warning that any considerable excess turns the hypo milky through liberation of sulfur, especially in warm weather.

45 mL × 50% = 22.5 g per litre

Bisulfite by the general rule

5.3 g per litre

Bisulfite in the printed bath

The printed bath carries less than a quarter of the bisulfite the general rule prescribes, at 25 per cent hypo rather than 35. The two prescriptions are not reconciled anywhere on the page, and the course does not reconcile them either — but the difference is not arbitrary, and the reason is in the formula: the general rule is for bisulfite added to a plain hypo bath, with nothing else in it, while the printed bath also carries twice its weight of sodium sulfite. Sulfite and bisulfite together are a buffer pair, and a buffered bath needs less acid salt to sit at a given acidity than an unbuffered one needs to get there.

That is the course’s reading, offered as a reading. What is not in doubt is that anyone following the general rule and the printed formula will make two noticeably different baths, and Kodak calls both satisfactory.

Behaviour

It is acid, and mildly so. Sodium bisulfite is, in the primer’s words, intermediate between sodium sulfite and sulfurous acid, and is therefore equal in acidity to a mixture of equal proportions of those two substances. Ten and a half grams of sulfite against 5.3 g of bisulfite puts this bath firmly on the buffered side of that scale rather than at the acid end of it.

Its acid reserve is its weakness, and Kodak says so. It makes a satisfactory acid fixing bath but does not give quite as good a reserve of available acid in the bath as acetic acid does. Every print that enters carrying carbonate spends some of that reserve, and there is less of it here than in an acetic-acid bath of the same size.

It cannot sludge, and it can sulfurise. Those are the two failure modes of a fixing bath, and this formula has traded one away and kept the other. There is no aluminium to precipitate; there is thiosulfate and there is acid, and a considerable excess of the acid liberates sulfur — the primer says so about the general rule, and warm weather makes it likelier.

It goes milky on keeping. An acid hypo solution does, and the primer’s remedy is architectural rather than chemical: keep the two halves apart until the bath is wanted.

It is slower than it needs to be. Twenty-five per cent hypo is below the 30 to 40 per cent the primer calls the fastest range, and this bath has no hardener to protect a swollen emulsion during a long immersion. Time it with a clearing test rather than by habit.

Image characteristics

None of its own while it works, which is what a fixer should have.

A softer emulsion than a hardening bath leaves. Not a fault here — it is the point — but it is a consequence: the film or print comes out of this bath unhardened, and everything after it, especially a warm wash, has to respect that.

Uneven prints if it goes alkaline. The primer’s account of what a plain bath does is what this bath will do once its acid reserve is spent: the image continues to develop in the tray, and prints stuck together develop more at the point of contact.

Brown staining is the same failure a stage further on, and the primer names it about acid baths that have gone alkaline.

Fading, years later, if a sulfurised bath was used and the sulfur penetrated the gelatin.

The mechanism

Fixing is the same reaction as in every bath in this formulary, set out on how fixer works:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

What the hypo does

⚗One salt doing two jobs, and why a second salt is with it

Sodium bisulfite is the acid salt of sulfurous acid. Dissolved, it gives bisulfite ion, which is both a weak acid and a sulfite reservoir - which is exactly the pair of properties a fixing bath’s preservative and its acid have to supply:

HSO₃⁻ → H⁺ + SO₃²⁻

The bisulfite ion, acting as the acid

The primer puts it in one sentence: bisulphite is used very largely as a preservative for fixing baths, supplying both the sulphite and the acid necessary.

Sodium sulfite alone is alkaline, because the strong base predominates over the weak sulfurous acid. So sulfite and bisulfite together are a conjugate pair, and their ratio fixes the pH:

SO₃²⁻ + H⁺ → HSO₃⁻

The buffer, working in the direction that matters here

That is the whole design. Carried-in developer alkali is absorbed by converting bisulfite to sulfite rather than by raising the pH, until the bisulfite is gone. The primer’s complaint about the reserve is a complaint about how much bisulfite there is, not about the mechanism.

And the failure at the other end. Too much acid, or acid meeting warm hypo, decomposes the thiosulfate:

S₂O₃²⁻ + 2 H⁺ → S + SO₂ + H₂O

Why the bisulfite never goes into a warm hypo solution

The course cannot source a quantitative account of why warmth makes this so much worse, and does not offer one; what the primer establishes is the observation and the rule that follows from it.

Function of every ingredient

Sodium thiosulfate pentahydrate, 250 g to the litre. The fixing agent, and 94 per cent of the formula by weight. It complexes undeveloped silver bromide into a soluble thiosulfato-argentate ion so that it can be washed out of the emulsion. At 25 per cent this is a slow bath by Kodak’s own account — the primer puts the fastest range at 30 to 40 per cent — and Wall’s acid bath for negative work carries 400 g/L against 150 g/L for papers, which is the same judgement made differently. More fixes faster; less risks the insoluble intermediate complexes that washing does not remove. The crystalline pentahydrate is meant; 250 g of the anhydrous salt would be a substantially stronger bath.

Sodium sulfite, 10.5 g. The alkaline half of the buffer pair, and the preservative. On its own it is an alkaline salt, because soda is a strong base and sulfurous acid a weak one; with the bisulfite it becomes half of a conjugate pair that holds the bath at a mild acidity and absorbs carried-in alkali by changing its own ratio rather than by letting the pH climb. It is also the sacrificial reserve that acid attacks in preference to the thiosulfate. More of it pushes the bath towards neutrality and eventually towards the alkaline behaviour the acid was meant to prevent; less leaves the bisulfite unbuffered and the bath sharper than Kodak intended. The desiccated salt is meant — Kodak’s tested sulphite was the dried salt at about 92 per cent — and the heptahydrate crystals would need roughly twice the weight. Note that this ingredient is twice the weight of the one the bath is named after.

Sodium bisulfite, 5.3 g. The acid half, the substitute for acetic acid, and the ingredient the formula is named for. It is an acid salt intermediate between sulfite and sulfurous acid, and it does both of the jobs an acetic-acid fixer needs two ingredients for: it supplies the acid that neutralises developer carried in, and it supplies sulfite that protects the hypo. More of it makes a sharper bath with a larger reserve, up to the point the primer marks clearly — a considerable excess turns the hypo milky through liberation of sulfur, especially in warm weather. Less gives a bath that goes alkaline early and develops prints in the fixing tray. Two practical notes from the primer’s own chemical entry: commercial dry sodium bisulfite consists chiefly of metabisulphite which converts to bisulfite when it dissolves, so the two are customarily substituted weight for weight; and bisulfite is difficult to prepare free from iron, which matters if the same tub is ever used to make up a pyro developer.

Water, to make one litre. For once in this family of baths, a make-up volume rather than an addition, so the strengths above are exact and can be compared with any other formula in the formulary. Its temperature is the one thing to get right: warm to dissolve the hypo, and cold before the bisulfite goes in. There is no alum in this bath and no carbonate either, which is what makes it a two-salt buffer in a hypo solution and nothing more complicated.

Interactions

With carried-over developer, which is what it exists to neutralise and what will eventually exhaust it. Because the reserve is smaller than an acetic bath’s, a rinse before the tray matters more here, not less.

With alum, if anyone is tempted to add some. The primer warns against exactly this combination in a different place: a solution of alum in water is weakly acid, so alum added to plain hypo without sufficient sulfite turns the bath turbid and precipitates sulfur. And its remark about bisulfite’s weaker reserve is made specifically about the hardening agent. This is not a bath to convert into a hardening one.

With warmth, which is the one operational rule: not while mixing, and not much while working.

With a pyro-stained negative, favourably, in that there is no alum to tan the gelatin further and no strong acid to attack a stain image. The staining developers lesson is where that requirement comes from.

With a second bath of itself, usefully — see the two-bath rotation. A bath with a modest acid reserve is a good argument for two of them.

Variants

F-52, Kodak Limited’s non-hardening acid fixing bath. Hypo and potassium metabisulphite, at the same 25 per cent hypo. The same idea with the potassium salt instead of the sodium one and no separate sulfite; it has a published keeping table, which this formula has not.

Wall’s acid bath of 1924. Hypo 150 g and potassium metabisulphite 25 g per litre, which Wall calls about the correct strength for papers, with the hypo raised to 400 g for negative work. Far more metabisulphite than this bath’s bisulfite and a very different balance; it is a different formula rather than another printing of this one.

The primer’s own general rule, on the same page: 45 mL of a 50 per cent bisulfite solution added to a litre of 35 per cent hypo. It carries more than four times this formula’s bisulfite and no sulfite at all, and the arithmetic is set out under Mixing.

F-1 and the hardening baths, which are what this formula is the alternative to and which are on their own pages.

No course variant is offered. There is nothing to make safer: this bath is already the version of an acid fixer that removes the hazardous ingredient, which is why it earns its place beside the acetic-acid formulas rather than behind them.

Safety

Level A, and it is the mildest fixing bath in this batch by some distance.

The formula’s whole safety argument is what is missing from it. No acetic acid, glacial or diluted; no alum. What remains is hypo, which carries no agreed GHS classification, sodium sulfite and sodium bisulfite, both of which are ordinary darkroom solids handled with gloves, eye protection and the dust down.

The hazard that remains is sulfur dioxide, and it is real. Sulfite and bisulfite both release it in acid, and a bisulfite solution can smell of it on its own in a warm room. The controls are on the sodium bisulfite page; the practical rules are to weigh it in moving air, to keep the tub closed, and never to put a stronger acid anywhere near this tray or its waste.

Anyone with asthma should read the bisulfite page before weighing it. Sulfite sensitivity is the one place where a Level A bath deserves a second thought, and that judgement belongs on the chemical page rather than here.

Ventilation as for any darkroom.

Storage

Better as two bottles than one. The primer’s own advice for this class of bath is to keep a bisulfite stock and add it to the plain hypo stock as required, because an acid hypo solution gradually becomes milky on keeping. That is the whole storage instruction the source gives, and it is a recommendation to avoid storing the bath at all.

No keeping figure and no capacity figure is published. Both are absent from the source and neither is invented here. What is left is a count and a clearing time: the clearing-time test and the batch record.

Glass or plastic, stoppered, cool and dark. Label it per the labelling SOP — and label it clearly as a non-hardening bath, because two colourless fixers on a shelf are indistinguishable and using the wrong one before a toning session is the mistake this formula exists to make possible.

Keep the sodium bisulfite tub dry and closed. It loses sulfur dioxide on standing, which means an old tub is weaker than the label says — and the primer records that bisulfite is difficult to prepare free from iron, so the same tub should not be trusted for a pyro developer.

Incompatibilities

Strong acids, which liberate sulfur dioxide from every one of the three ingredients at once. The incompatibilities page has the general rule.

Alum, and any hardening stock. Not a hazard so much as a design failure: the primer warns that alum in a hypo bath without sufficient sulfite turns it turbid and precipitates sulfur, and its own note on bisulfite’s smaller acid reserve is made about hardening baths specifically.

Developer, in either direction. One pair of tongs per tray.

Ferricyanide baths and their waste, because acid on ferricyanide is the documented route to hydrogen cyanide, and this bath is acid.

Alkalis, which neutralise the reserve at a stroke and turn it into a plain hypo bath.

Oxidising agents, which attack thiosulfate and sulfite alike.

Waste

Silver-bearing, like every spent fixer, and simpler than most: hypo, sulfite, bisulfite and dissolved silver, with no aluminium and no acetate in it.

Collection rather than treatment at darkroom scale. Follow the silver-bearing waste SOP and Part XII.

Keep it away from acid wastes, because acidifying a thiosulfate-sulfite solution is how sulfur dioxide gets made by accident. The disposal ruling governs and local regulation decides; this course cannot tell you what it says where you are.

Troubleshooting

The bath turned milky as it was mixed. The hypo was warm when the bisulfite went in. That is the one mistake this formula makes available, and the primer names it in terms. The precipitate is sulfur and does not redissolve; mix fresh.

The bath turned milky after standing. Expected of an acid hypo solution, per the primer. Next time keep the bisulfite as a separate stock.

Prints coming out unevenly, worse where two were stuck together. The acid reserve has gone and the bath is behaving as a plain one — development is continuing in the tray. This is the primer’s own description of what an exhausted or unacidified bath does.

Prints staining brown. The same failure further on.

Fixing slower than expected from the start. It is a 25 per cent bath and Kodak’s fastest range is 30 to 40. Time it rather than trusting a habit formed on a stronger fixer.

Fixing slower than it was. Exhaustion. Time a clearing test and discard past 12 to 15 minutes for a slow material.

A smell of sulfur dioxide over the tray. Either the bath is decomposing, or an acid has got into it. Ventilate first and diagnose afterwards.

Emulsion soft, or damaged in the wash. Expected — there is no hardener in this bath. Wash cooler, handle wet film less, or use a hardening formula if hardening is actually wanted.

Experiments

Measure the acid reserve the primer complains about. Titrate this bath and an equal volume of F-1 against a standard carbonate solution to the point where each stops being acid. Kodak’s claim is comparative and unquantified — not quite as good a reserve of available acid — and this turns it into a number.

Test the two prescriptions against each other. Mix the printed formula and the primer’s general rule (45 mL of 50 per cent bisulfite in a litre of 35 per cent hypo), and compare pH, clearing time and the number of prints each will take before it goes alkaline to litmus. They are printed within a paragraph of one another and they are not the same bath.

Find where the sulfur comes out. Add bisulfite to a small volume of this bath in measured increments until it clouds, at 18 °C and again at 28 °C. The primer says a considerable excess liberates sulfur, especially in warm weather; how considerable is not stated anywhere in the corpus.

Compare it with the metabisulphite baths. This formula, F-52 and Wall’s acid bath, all fixing identical strips: clearing time, pH, and prints to exhaustion. The primer says sodium bisulfite and potassium metabisulphite are interchangeable weight for weight and that the choice is a matter of cost. That is a testable claim.

Fix a pyro negative in this and in an alum bath, then read the stain densities. The claim that a non-hardening, mildly acid bath preserves a stain image better is made about modern pyro developers by their makers and not by Kodak in 1928; the two baths and one step wedge are all it takes to look.