Kodak F-52

The shortest useful fixer in this formulary, and the one that answers the question F-5 raises: what if you do not want the emulsion hardened? Two ingredients, and one of them does three jobs at once.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate	250 g	crystalline hypo
Potassium metabisulfite	25 g
Water	to make 1000 mL

Purpose

To fix, and to arrest development while doing it, without hardening the gelatin. Kodak Limited’s own header states the case negatively — a non-hardening acid fixing bath “for use when hardening is not desired, or must be avoided” — and gives two period examples, Transferotype and Bromoil papers, both of which depend on the gelatin layer remaining able to swell, absorb and release.

The modern cases are the same case. A hardened emulsion resists everything that follows it: toner, selenium, dye, bleach, redevelopment, and the wash. Where any of those matters more than the mechanical toughness of the wet layer, hardening is a cost rather than a benefit.

Recommended uses

Anything that will be toned. Sepia, selenium, gold and the bleach-and-redevelop processes all work by getting a reagent into the layer and out again. Hardening slows every stage of that and makes it uneven.

Anything whose washing matters more than its handling. A non-hardened layer releases thiosulfate faster. Where a print is being made for permanence rather than for the trade counter, that is the trade you want.

Prints, generally, in a careful darkroom. Modern papers and films are hardened at manufacture far more than the materials Kodak was writing for, and the mechanical argument for a hardening fixer is correspondingly weaker than it was in 1949.

Kodak publishes no fixing time for this formula. The handbook gives one for F-5 and none here, and the course does not invent one. Use the clearing-time rule under Behaviour, which is the method the number would have been derived from anyway.

When another formula is preferable

• Where the wet emulsion has to survive warm handling, F-5, which is the same handbook’s answer and hardens to a measured melting point.
• For paper, if you want Kodak’s own paper bath, F-54 — but that one hardens, so it is the opposite choice rather than a neighbouring one.
• For a printing-out process, the alkaline bath. Acid of any kind attacks the finely divided silver of a printing-out image, and this bath’s acidity is not buffered by anything except its own sulfite.
• Where the smell of sulfur dioxide is the problem, a bath acidified with a solid organic acid and a separate sulfite. Wall published one and the course could not print it; see Variants.
• Where fixing speed is the problem, a rapid ammonium thiosulfate concentrate. The course publishes no such formula, because no maker in its corpus discloses one; what can honestly be said about those products is under Variants.

Mixing

Hypo first, dissolved, and cold before the acid salt goes in. The 1949 handbook’s general rule for acid fixing baths is hypo, then sulfite, then acid, then alum; with only two ingredients that reduces to hypo, then metabisulfite. The 1928 primer gives the reason with unusual directness: 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.

Do not make a stock of the mixed bath and keep it for months. The same primer: on keeping, an acid hypo solution gradually becomes milky. Its own recommendation is to keep the acid salt as a separate concentrated stock and add it to a plain hypo stock as required — for general purposes, 45 mL of a 50 per cent sodium bisulfite solution to a litre of 35 per cent hypo — and to note that any considerable excess over that turns the hypo milky, especially in warm weather, through the liberation of sulfur. Kodak Limited’s own keeping table gives F-52 three months in a stoppered bottle, which is a long time by fixer standards and still not indefinite.

🧮Twenty-five grams of one salt doing the work of two

250 g ÷ 248.19 g/mol = 1.01 mol/L, a 25 % w/v bath

Thiosulfate

25 g ÷ 222.33 g/mol = 0.112 mol/L

Metabisulfite

Metabisulfite is a disulfite: K₂S₂O₅. In water it takes up a molecule of water and becomes two bisulfite ions, so 0.112 mol/L of the salt delivers 0.225 mol/L of hydrogensulfite — which is simultaneously the acid that stops development and the sulfite reservoir that protects the thiosulfate from it.

S₂O₅²⁻ + H₂O → 2 HSO₃⁻

Metabisulfite in water

Compare the two other constructions in this formulary. F-5 reaches the same end with three separate ingredients — 0.295 mol/L of acetic acid, 0.12 mol/L of sulfite and 0.12 mol/L of boric acid — because it also has to hold an alum in solution. This bath has no alum to protect, so one salt suffices.

Behaviour

Time it by clearing, because Kodak gives you nothing else. Drop a scrap of undeveloped film or an unexposed corner of the paper into the bath, time the disappearance of the milky opalescence, and fix for twice that. The 1928 primer establishes the rule and lists what the clearing time depends on: hypo strength, the material, the temperature, and the degree of exhaustion.

It exhausts at half the rate of F-5, on Kodak’s own table. 60 sheets of 8 × 10 inches per 160 fluid ounces after a water rinse, against F-5’s 120. Two things are going on and only one of them is chemistry: this bath carries a quarter less hypo, and it carries far less acid reserve — 0.225 mol/L of hydrogensulfite against F-5’s acetic acid plus boric acid, both of which are there to absorb carried-over alkali for a long time.

The SB-3 row is a puzzle the course leaves open. The same table gives 90 sheets where the chrome alum hardening bath SB-3 has been used between development and fixing, half as much again as the water-rinse figure. Kodak prints both numbers and explains neither, and no source in this course’s corpus supplies the mechanism. It is recorded here because it is data, and not turned into a recommendation.

A bath that goes milky is finished. With no alum in it, this formula cannot produce the white aluminium sulfite sludge that ends an F-5 bath. The only precipitate it can make is sulfur, pale yellow and slow to settle, and it means the acid has got ahead of the sulfite — through warmth, through mixing error, or through contamination.

It keeps unusually well for a fixer. Three months in a stoppered bottle whether full or half full, which is the same pattern as SB-1: a solution destroyed by warmth and by standing open, but not much bothered by the air in a half-empty bottle.

Image characteristics

None of its own, and one large negative one it does not have.

A hardening fixer changes the emulsion’s future: what a toner can reach, how fast a wash proceeds, how evenly a bleach works. This bath leaves that future open. In a print sequence that ends in selenium or sulfide, the choice between this formula and F-5 shows up in the toned print as evenness — a hardened layer takes a toner in a way that can be visibly patchy — even though neither bath leaves any mark of its own.

The one visible fault it can produce is the same as any acid fixer’s: a bath that has gone alkaline through carried-over developer stains prints brown, which the 1928 primer states directly.

The mechanism

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the bis(thiosulfato)argentate complex

The fixing chemistry is common to every bath in this section. What is particular here is the acid system, and it is the most economical one in the formulary.

Why an acid fixer at all. Two reasons, both in the 1928 primer. Development continues in a plain bath, so prints that touch develop unevenly where they touch; and, for the hardening baths, alum will not tan gelatin unless the solution is acid. Only the first reason applies to F-52, and it is sufficient on its own.

Why the acid must be a sulfite salt. Acid and thiosulfate are enemies:

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Acid attacking thiosulfate

Note what appears on the right: hydrogensulfite. The decomposition’s own product is the ion that metabisulfite floods the bath with, so the reaction is suppressed by the very thing that made the bath acid. That is why the 1928 primer can say that all acid fixing baths contain either sodium bisulfite, potassium metabisulfite, or a mixture of sodium sulfite and a weak acid. A fixer acidified with hydrochloric acid would fizz itself to sulfur in minutes.

And why the quantity has a ceiling. The primer’s warning against excess is specific: add much more than its stated proportion of bisulfite and the hypo turns milky, especially in warm weather, owing to the liberation of sulphur. More acid salt is not a stronger bath; past a point it is a bath that destroys itself.

⚗One salt, three jobs

Metabisulfite in this formula is the whole of the non-thiosulfate chemistry.

It is the acid. Hydrogensulfite is weakly acidic, so the bath sits acid enough to neutralise carried-over developer alkali and stop development on contact.

It is the preservative for the thiosulfate. Because hydrogensulfite is the product of the reaction that would decompose the thiosulfate, its presence pushes that reaction back.

It is the preservative for the developer carried in. The primer’s own account of sodium bisulfite is that it acts in two ways — it prevents the formation of sulfur by the action of the acid on the hypo, and it prevents the developer carried into the fixing bath from oxidising and turning brown.

The cost of that economy is that the three jobs cannot be tuned separately. F-5 can have more acid without more sulfite, or more reserve without more acidity. F-52 has one dial.

Function of every ingredient

Sodium thiosulfate, 250 g of the crystals. The fixing agent, at 25 per cent w/v — a little over 1 mol/L, and at the lower end of the 30-to-40-per-cent range the 1928 primer gives as fastest. It complexes silver out of the undeveloped halide, two thiosulfate ions to each silver ion. More fixes a little faster and puts more thiosulfate into the emulsion to be washed out afterwards; less slows fixing and moves the bath towards the region where the insoluble silver–thiosulfate complex is the product, which is the invisible failure that ruins prints years later. The crystals are the pentahydrate at 248.19 g/mol; 250 g of the anhydrous salt would be a 39 per cent bath, not a 25 per cent one.

Potassium metabisulfite, 25 g. Acid, preservative and antioxidant in a single salt, as the callout above sets out. In water it hydrolyses to two hydrogensulfite ions per formula unit, giving 0.225 mol/L of a weakly acidic, reducing species. More does not make a better bath: past the proportion Kodak’s primer gives, the excess acidity liberates sulfur and the bath goes milky, faster in warm weather. Less gives a bath that goes alkaline early, allows development to continue, and lets carried-over developer oxidise and brown it. Its own page records the classification that matters when you weigh it — skin and eye irritation and possible respiratory irritation — and the respiratory statement is the one that distinguishes this salt: it is a sulfur dioxide reservoir, and a scoopful raises a little of that gas over the balance.

Sodium bisulfite would do the same job, which is why the 1928 primer names both, but not gram for gram: sodium bisulfite at 104.06 g/mol delivers one hydrogensulfite per formula unit against metabisulfite’s two per 222.33, so 25 g of the potassium salt is matched by about 23 g of the sodium one. The course prints Kodak’s formula rather than the substitution, and this paragraph is the arithmetic if you have the other tub.

Water, to make 1000 mL. No temperature is published for this formula. Cold when the acid salt goes in is the rule that matters, and it is a rule about the order and the temperature rather than about the water.

Interactions

With the developer, by design. The bath’s acid is spent on alkali carried over on wet sheets, and its capacity is therefore a property of your developer and your drain time as much as of the formula. A stop bath between the trays spends its own acid instead, and the 1928 primer measures the gain for a hardening fixer at about a quarter more prints per gallon.

With warmth, badly. Every failure mode of this formula is accelerated by temperature: the milkiness on mixing, the milkiness on keeping, and the two-week and two-day footnotes on Kodak’s own keeping table.

With toners, favourably, which is the point. No alum means an open layer.

With hardening baths, deliberately. Where a non-hardening fixer is wanted but the wet emulsion still needs to survive handling, the period answer was a separate hardening bath before fixing — Kodak Limited’s SB-3 and SB-4 — rather than a hardening fixer. Those are chrome alum baths and the course does not publish them; the chromium ruling permits chromium(III) only where a lesson genuinely requires an in-emulsion hardener, and this is not that case.

With an ammonium-based rapid fixer, not at all. They are different chemistries with different pH windows and different waste behaviour. Do not top one up with the other.

Variants

Wall’s acid bath, 1924, is the same construction at a different strength. Hypo 150 g and potassium metabisulfite 25 g per litre, which Wall calls about the correct strength for papers, adding that for negative work the hypo should be increased to 400 g. Kodak’s F-52 sits between those two at 250 g with the same 25 g of metabisulfite. Three published points, twenty-five years apart, on the same line: the acid salt stays constant and the hypo is what moves with the material.

Wall’s citric-acid fixing bath could not be published. On the same page he offers, as “an equally efficient bath”, one built on a solid acid instead of a bisulfite salt: hypo 400 g, citric acid 30 g, dry sodium sulfite, and hot water to 1000 ccm, with the mixing instruction to dissolve the hypo in half the water and the acid and the sulfite in a quarter of it, then mix and make up to bulk. It is the odour-free acid fixer this formulary would otherwise have — citric acid is a solid and does not evaporate — and it pairs with the course’s citric stop bath.

The sulfite quantity is not legible. The copy the course holds renders the line as 35,2, which is either 35 g or 35.2 g, and Wall’s avoirdupois column cannot decide between them because the two columns in that table are not exact equivalents — his own hypo line differs between them by twelve per cent. The difference between the two readings is under one per cent and would change nothing in the tray, and the formula is still not printed, because a published formula’s quantities are either read from the source or they are not, and “probably 35” is the beginning of a habit this formulary cannot afford. What is recorded instead is the construction, which is the part that teaches: a fixer can be acidified with an organic acid plus a separate sulfite rather than with a single bisulfite salt, and the 1928 primer names that third route explicitly among the three that acid fixing baths use.

Kodak’s own Bisulphite Fixing Bath, 1928, reaches the same place by a different route: hypo 250 g, sodium sulfite 10.5 g and sodium bisulfite 5.3 g per litre. Note how much less acid salt that is — about 0.05 mol/L of hydrogensulfite against F-52’s 0.225 — with separate sulfite carrying the preservative load. It is a genuinely different design, not a restatement of this one, and the course has not published it as its own entry.

The proprietary powders are not variants and cannot be published. The 1949 handbook’s own footnote against F-52 offers Kodak Rapid Acid Fixer in powder form as an alternative to mixing your own, and names Kodak Acid Fixing Salt with Hardener elsewhere. It discloses the composition of none of them. Under the course’s ruling on proprietary formulations, a product whose full composition its maker does not disclose is taught as behaviour and as disclosed components only, never reconstructed and never assembled from a patent or a forum.

The rapid ammonium thiosulfate concentrates, as far as their makers disclose

This is the honest limit of what the formulary can hold about the fixers most readers will actually buy. None of the three makers in this course’s corpus publishes a composition, so none of them has an entry. What they do publish is a great deal, and it is set out here because a formulary that simply omitted the modern fixers would be less useful and no more honest.

ILFORD RAPID FIXER and HYPAM are the only ones that name their fixing agent. ILFORD states that the fixing agent in RAPID FIXER is ammonium thiosulphate and that it contains no sodium thiosulphate; the HYPAM sheet says the same of that product. Both are described as non-hardening, and RAPID FIXER must not be used with fix hardeners; HYPAM’s sheet adds that a fix hardener is no longer generally recommended because modern camera films are sufficiently hardened at manufacture. Beyond that, ILFORD publishes working data of a kind Kodak’s 1949 handbook never did: 1+4 for film and 1+4 or 1+9 for paper, pH 5.0 to 5.5 at both dilutions with specific gravity 1.070 to 1.080 at 1+4 and 1.030 to 1.040 at 1+9, minimum fixing times at 20 °C of 2 to 5 minutes for general-purpose film at 1+4 and half a minute to two minutes for papers, and capacities per litre of 24 films of 135/36, 80 sheets of 8 × 10 inches of resin-coated paper or 40 of fibre-base. It also publishes the two things that matter most for permanence and that no period source gives in these terms: the clearing-time test — fix film for twice the time the emulsion takes to clear, and discard the bath when the clearing time in used fixer exceeds twice that in fresh — and silver limits in the bath, 8 to 10 g/L for film, below 2 g/L for commercially permanent fibre-base prints and below 0.5 g/L, about ten 8 × 10 prints per litre, for maximum long-term stability.

FOMAFIX discloses less: a liquid concentrate rapid fixer with, in Foma’s words, high efficiency, strong buffer ability and long-term stability, at 1+5 for manual work and 1+4 for automatic, with a capacity per litre at 1+5 of 17 films of 135-36 or 120, up to 40 sheets of 13 × 18 cm, 2 m² of baryta-base paper or 4 m² of resin-coated, and a fixing table giving 3 minutes for the Fomapan films against 10 for the two-component powder fixer FOMAFIX P. It does not name its fixing agent at all.

BERGGER Berfix Neutral is described as an alkaline-based, non-acid universal fixer supplied as a liquid concentrate, with a stated pH of 7, dilutions of 1+4 for film and 1+4 or 1+9 for paper, and fixing times of 5 to 7 minutes for film and 1 to 3 minutes for papers. Its composition is not disclosed either. Its washing recommendation — a 10 per cent sulfite rinse — is discussed on the washing aid page.

What follows from all this. A rapid fixer is faster because ammonium thiosulfate is a more effective solvent for silver halide than sodium thiosulfate, and the ammonium thiosulfate page carries what is established about the substance, including the reason its baths must not be alkalised past about pH 8 — ammonia is evolved. The course teaches the behaviour and the published working data, and prints no quantities, because there are none to print.

Safety

Level A, and the criteria that applied are worth naming because this is the acid fixer that is not Level B.

No substance in this formula carries a corrosive classification at the concentration used. The thiosulfate has no aggregated hazard statements at all; potassium metabisulfite carries skin irritation, serious eye damage and possible respiratory irritation, and its own page assesses it at Level A for ordinary darkroom use. The controls are the standard ones: gloves, eye protection, ordinary darkroom ventilation, dedicated utensils.

Compare what is absent. F-5 is Level B because it contains boric acid, whose aggregated classification is H360, may damage fertility or the unborn child, and glacial acetic acid, which is corrosive and flammable. Neither is in this bath. If the reason you were looking for a non-hardening fixer was the reproductive-hazard classification rather than the hardening, F-52 is the answer to that question too.

What is not absent is sulfur dioxide. Metabisulfite is an SO₂ reservoir by design, and the whole protective mechanism of the bath consists of holding that gas as an ion in solution. Weigh it without raising dust, keep the bath covered, and never let anything more strongly acid than the formula reach it or its waste bottle. Sulfur dioxide’s workplace limit is among the tightest in this course.

Eye protection is not optional here even though the level is A. H318, causes serious eye damage, appears in a minority of the notifications for metabisulfite but it appears; a splash of a saturated solution is not a thing to find out about.

Storage

Three months in a stoppered bottle, full or half full; one month in a covered tank; one week standing in a dish — with the caveat that both bottle cells in Kodak’s table carry a footnote mark, and the table’s two footnotes warn of two weeks and of two days respectively above 24 °C. Which mark belongs to which cell cannot be read in the copy the course holds, so both are recorded and neither is assigned.

Keep the dry metabisulfite dry and closed. It loses sulfur dioxide slowly in air, and a tub that has been open for a year is weaker than the label says — which on this formula means less acid and less preservative at once.

Glass or plastic. Label with the formula, the date and the sheet count, per the labelling SOP.

Incompatibilities

Strong acids, which liberate sulfur dioxide from the metabisulfite directly and sulfur from the thiosulfate at the same time. This bath contains two SO₂ sources rather than one. See incompatibilities.

Alkalis and carbonates, which neutralise it and end it.

Alum and hardener stocks. Adding F-53 to this bath does not upgrade it; it makes a different formula, and F-54 is what that formula is when Kodak does it.

Oxidising agents — peroxide, persulfate, permanganate, hypochlorite — which oxidise both the sulfite and the thiosulfate.

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

Waste

Silver-bearing, and simpler than most: thiosulfate, sulfite, sulfate, potassium and dissolved silver, with no aluminium and no borate in it.

That simplicity does not make it dischargeable. It is oxygen-demanding, it carries silver as a soluble complex, and sulfite is itself a substance that consumes oxygen in receiving water. Collect it, label it, keep it away from acid wastes — the reaction with acid is exactly the one this bath is designed to prevent — and follow the silver-bearing waste SOP, which cites the disposal ruling. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

The bath went milky the moment it was mixed. The hypo was warm, or the metabisulfite went in before the hypo had dissolved. It is sulfur, and it does not redissolve. Remake it cold.

The bath goes milky after weeks in the bottle. The 1928 primer’s own observation about acid hypo solutions on keeping. Kodak’s three months is an outside figure; store cool, and keep the acid salt as a separate stock if you mix in quantity.

Prints staining brown. The bath has gone alkaline through carried-over developer. Rinse or stop before fixing, and check the sheet count against the 60-per-160-fluid-ounces figure.

Uneven density where two sheets touched. The bath is spent enough that development is continuing in it. Same cause, earlier symptom.

Fixing time creeping up. Clear a test strip. The 1928 primer’s rule is to discard when a slow film’s clearing time passes 12 to 15 minutes.

A sharp smell over the tray. Sulfur dioxide, from the metabisulfite. Either the bath has been contaminated with something acid or the ventilation has stopped. Do not diagnose this one by leaning closer.

The negative feels soft and marks easily. That is the formula working as intended. If it is a problem, you wanted a hardening bath and should read F-5 or F-54.

Experiments

Fix two prints and tone them. One in F-52, one in F-5, otherwise identical, then tone both. The difference — in speed of toning and in evenness — is the whole argument for a non-hardening fixer, and it is visible in an afternoon.

Measure the wash. Fix matched prints in the two baths, wash both under the same schedule, and compare how long each goes on giving something up to a fresh tray of water — judged by conductivity if you have a meter. Kodak’s own residual-hypo test solution is a permanganate one, and the course cannot publish it as a formula because potassium permanganate has no entry in the chemical encyclopaedia; that gap is recorded rather than worked around.

Titrate the exhaustion. Measure the pH of a fresh litre and of the same litre after every ten sheets, with the pH SOP. The point where the curve turns over is the point Kodak’s 60-sheet figure is describing, and yours will not be Kodak’s because your developer and your drain time are not theirs.

Compare the three published strengths. Wall’s 150 g, Kodak’s 250 g and Wall’s 400 g, all with 25 g of metabisulfite, clearing the same film at the same temperature. The prediction is that clearing time falls and then stops falling.

Substitute sodium bisulfite on a molar basis — about 23 g in place of the 25 g — and see whether anything changes at all. The prediction is that nothing does, and a negative result honestly obtained is worth having in a laboratory report.