Fixer Formulations: Plain, Acid, Hardening, Rapid, Neutral, Alkaline

Only one ingredient in a fixing bath dissolves silver halide. Everything else is there because of a problem the thiosulfate does not solve, and each of those additions charges a price that is paid later — in the wash, in the toner, or in the archive. Reading a fixer formula is mostly a matter of tracing which problem each line answers and what it costs.

The quantities live in the formulary. This page is about function.

Plain hypo: the bath with nothing to go wrong except everything

Plain hypo is sodium thiosulfate in water and nothing else. It is what Herschel used, and Wall’s 1924 formulary still gives it as a working bath, with 40 to 45 per cent named as the strongest and most rapid strength worth using.

Its virtues are real. It has no acid to decompose it, no alum to sludge, no ingredient whose failure is a different failure from exhaustion. Reilly’s print fixer for printing-out papers is deliberately of this family, and slightly alkaline rather than merely neutral — sodium thiosulfate with a little sodium carbonate — for two reasons that are worth having in mind before the acid baths arrive. It stops acid carried in from decomposing the thiosulfate, and it keeps the bath from attacking the very finely divided silver of a printing-out image, which an acid fixer bleaches out of the highlights and mid-tones.

Its two weaknesses are the reason the rest of this page exists.

It has no reserve against acid. A plain bath meeting the carryover from an acid stop bath, or a sulfite-free acid rinse, is a plain bath being acidified, and acidified thiosulfate does not simply become less effective — it decomposes.

H₂S₂O₃ → H₂SO₃ + S

Kodak, 1928: thiosulfuric acid is unstable and throws sulfur out of solution

Kodak’s 1928 primer gives that as the observation and the equation together: a few drops of a strong acid in a weak hypo solution turn it milky, because the acid frees thiosulfuric acid and that breaks down to sulfurous acid and sulfur. This is sulfurisation, and it is the failure the lab in this part reproduces deliberately at a scale of a few millilitres so that you have seen the milkiness once, in a beaker, rather than for the first time in a litre you have just mixed.

It does not keep indefinitely in solution either. Reilly records that dissolved sodium thiosulfate decomposes in part to sodium sulfite and elemental sulfur, and mixes his print fixer shortly before use. That is the same equation read from the other end, without any acid to drive it.

The acid fixer, and the problem it was invented to solve

Kodak’s 1928 primer sets out the difficulty better than any modern sheet, and it is worth following because the answer is not obvious.

Wet film comes out of the developer carrying alkali. That alkali lands in the fixer, where it oxidises, turns the bath brown and stains prints. Sulfite prevents the oxidation, and it works better in a slightly acid bath. So the bath wants acid. But a great deal of alkali arrives over a working life, so the bath needs a great deal of acid-neutralising capacity — and yet, from the reaction above, it must not actually be strongly acid, or the thiosulfate falls apart.

The primer states the escape route in one sentence, and it is the same insight Part X built a whole page on: the quantity of alkali an acid can neutralise depends on the total hydrogen it carries, while the acidity of the solution depends only on the fraction that is dissociated. So use a large quantity of a weak acid. Acetic acid is the best available compromise, and that is why it is in nearly every acid fixer ever published.

⚗Sulfite is not a preservative here in the sense Part VIII means

In a developer, sulfite is an antioxidant — it competes with oxygen for the developing agent, as Part VIII sets out. In a fixer it does that job too, on the carried-in developer, but it has a second and more specific one.

The decomposition of thiosulfate into sulfite and sulfur is reversible, and Kodak’s primer says so explicitly: boil sulfite with sulfur and you get thiosulfate back. Manufacture works the same way.

Na₂SO₃ + S → Na₂S₂O₃

The reverse of sulfurisation, which is also how hypo is made

So sulfite in an acid fixer is not merely mopping up oxidation products. It is sitting on the right-hand side of the sulfurisation equilibrium and pushing it back. Acid drives sulfur out; sulfite pulls it in. A bath carrying both can hold far more acid than one carrying acid alone, and the amount of sulfite is what sets how much acid is survivable. That is why the mixing order puts sulfite before acid, and why the lab’s deliberate wrong-order demonstration works: add acid to hypo that has no sulfite in it yet and there is nothing to pull the sulfur back.

The hardener, and the window it has to live in

A hardening fixer adds an aluminium salt, almost always potassium alum. Aluminium(III) cross-links the gelatin, raising its melting point and making the wet layer far less easily damaged — Kodak’s primer measured that as a melting temperature and compounded its F-1 bath to give 54 to 77 °C.

The alum is also the ingredient that turns a fixer into a system with a pH window rather than a pH target, and Kodak’s 1928 troubleshooting section is the clearest statement of the trap in the corpus. Three findings, all from the same page:

• Hardening increases with the quantity of alum. Straightforward.
• Hardening increases with acetic acid to a maximum, then decreases, until at high enough acid the bath does not harden at all.
• The minimum acid needed for a long sludging life is usually greater than the quantity that gives maximum hardening.

Read those together and the design problem is exact: the amount of acid that hardens best is less than the amount that keeps the bath from sludging, so a formula has to choose, and either choice is a compromise. The primer even quantifies which way the compromise runs in use: with use, hardening first increases as developer is added, reaches a maximum, and then falls off rapidly.

The pH window an alum fixer has to live inside

1. 1Too alkaline: aluminium sulfite sludge — a white gelatinous precipitate; the acid has been spent by carried-over developer and alum meets sulfite. Also where dichroic fog appears
2. 2The working window — hardening, a protected thiosulfate and a long sludging life. ILFORD publishes pH 5.0 to 5.5 for its own rapid fixer
3. 3Too acid: sulfur precipitates — a pale yellow precipitate that settles slowly; too much acid, too little sulfite, or too high a temperature
4. 4The trade-off that has no clean answer — more acid buys sludging life and costs hardening; less acid buys hardening and costs sludging life

Regions, not thresholds. No measured curve for hardening against pH, or for the onset of sulfurisation, exists in any document the course holds; the only number on the drawing is ILFORD's published working pH for its own product.

Boric acid is the ingredient that widens the window, and it is the one line of F-5 that is not obvious from anything above. Boric acid is a very weak acid; it therefore does very little to the pH of the bath as mixed, and a great deal to how far the pH moves as alkali arrives. It is a buffer in the sense Part III defines, placed to hold the bath inside the window for longer than the acetic acid alone would.

That reading is the course’s, and the course says so. Neither Kodak Limited’s 1949 handbook nor Kodak’s G-23, both of which print the ingredient, states what it is for. The 1928 primer explains the problem completely and never names the solution. So the argument here is: boric acid is present, it is a weak acid, a weak acid at that concentration can only be a buffer, and the fault it prevents is exactly the fault the primer describes. If a source turns up that says otherwise, this paragraph is what should change.

⚠What boric acid costs, and why the course keeps it off the bench

Boric acid’s aggregated classification is Danger, H360 — may damage fertility or the unborn child — in 88.7 per cent of the 2,123 ECHA company reports that classify it, with the more specific H360FD in a further 11.2. Reproductive toxicity is the only hazard class in the aggregated classification: there is no acute toxicity, corrosion or eye damage entry, which is why the encyclopaedia page puts the controls at dust suppression, gloves, eye protection and hygiene rather than at anything more elaborate.

Against the course rubric that puts the substance at Level B, because Level A is capped at classifications that go no further than irritant, harmful if swallowed or corrosive. Weighing 7.5 grams of it is therefore a Level B operation, and the mixing lab is a Level A session. The lab resolves that by mixing the bath without the boric acid and giving the complete F-5 as a clearly labelled Level B extension. This is not a claim that the ingredient is unusable; it is the rubric doing what it exists to do, which is to make the reader’s controls the thing that decides.

By contrast, potassium alum carries no hazard statement in any of the 43 ECHA reports across its two notifications. What it does carry is dust: HSE’s EH40 gives soluble aluminium salts 2 mg/m³ over eight hours, a control that no GHS statement mentions.

Chrome alum, and why this course does not use it

Kodak’s F-16 is the chrome alum version, mixed as two solutions and combined on the day. It is instructive about hardeners generally and unattractive as a bath: the 1949 handbook says it “rapidly loses its hardening properties, with or without use”, and the 1928 primer explains why — in the presence of sulfite a chrome alum solution loses hardening power within one or two days at ordinary concentrations. The primer also records a trap that belongs on this page for its own sake: use the ammonium chrome alum rather than the potassium salt to save a penny a pound, let the bath go alkaline, and the ammonia evolved produces a greenish sheen on the emulsion — dichroic fog.

Chromium is a subject the course settles in one place. Chrome alum is chromium(III), not the chromium(VI) of dichromate, and the chromium policy permits it at Level B where a lesson genuinely requires an in-emulsion hardener. A fixer is not that case: potassium alum does the job, keeps better, and carries no classification at all.

The rapid fixer

Substitute ammonium for sodium and the bath fixes in roughly a third of the time. The previous lesson gives the evidence and is candid that no open source the course holds explains the mechanism.

What matters for formulation is that the ammonium salt is sold as a liquid concentrate, not a powder, and that the modern liquid concentrates are non-hardening. ILFORD’s RAPID FIXER sheet says so and adds an instruction: it “must not be used with fix hardeners”. The HYPAM sheet gives the reason in the sentence that ended the hardening fixer’s dominance — a fix hardener “is no longer generally recommended because modern camera films are sufficiently hardened at manufacture”.

That is a single sentence with a very long tail. If the film is already hardened, the alum is buying nothing and still costing everything the rest of this page lists.

The class is not uniform, and the differences between products are worth reading rather than skipping. Foma’s FOMAFIX is also a liquid rapid fixer but is diluted 1+5 for manual work rather than 1+4, gives 3 minutes for its own films rather than 2 to 5, and is described by its maker as having “strong buffer ability” — a claim about acid reserve rather than about speed. Its capacity is published as 17 films of 135-36 per litre against ILFORD’s 24, and its two-component powder fixer, FOMAFIX P, takes 10 minutes on the same films. None of those numbers transfers between products, which is the practical point: a dilution, a time and a capacity belong together and to one bottle, and the commonest way to under-fix is to carry a figure across from a fixer you used to buy.

The ammonium chloride conversion is the historical bridge between the two families and Wall documented it in 1924 as a controlled comparison: add 2.5 to 5 per cent of ammonium chloride to a 20 per cent hypo bath and fixing speeds up; 10 per cent ammonia water does the same; and neither has any effect at all on a 40 per cent hypo bath. It is a real route to a faster bath from a sodium one, it identifies the ammonium ion rather than the chloride as the active part, and it comes with the warning that goes with every ammonium fixer: pushed alkaline, an ammonium bath gives off ammonia.

Neutral and alkaline fixers

The acid in a fixer exists to deal with carried-over developer. Remove the reason and you can remove the acid, and several manufacturers have.

Bergger’s Berfix Neutral is the one whose sheet the course holds: an alkaline-based non-acid universal fixer at pH 7, diluted 1+4 for film, and claiming to require “a shorter washing time than other fixers would require to achieve the same archival level”. Note the price: its published film time is 5 to 7 minutes against ILFORD’s 2 to 5 for an acid-side rapid fixer at the same dilution. Removing the acid costs speed.

The washing advantage is the argument for the class, and it is the same one Reilly’s alkaline print fixer rests on: a thiosulfate ion in a neutral or alkaline bath is not being attacked, so there is less decomposition chemistry to remove afterwards, and the paper base is not being loaded with an acid that has to be washed out as well.

The toning advantage is more concrete, and two independent sources make it. Kodak’s G-23 tells you not to use a hardening fixer for prints you intend to tone at all, because it makes the emulsion less receptive to the toner. Moersch’s toning instructions go further and turn it into a workflow: with an alkaline fixer no rinse is needed between fixer and selenium toner, whereas an acid fixer must be rinsed out thoroughly first.

!TF-2, and why this page names it without printing it

The alkaline fixer most often discussed by name among practitioners is TF-2, which is documented by respected specialist authors rather than by a manufacturer. The course’s standing rule on provenance — the one that produced its bibliography and its tiers — is that a formulation documented mainly by specialists needs either a primary source or restriction to behaviour discussion, and that question is not yet settled for this family. So this page discusses the behaviour of the alkaline class, using the manufacturer sheet it does hold, and prints no TF-2 composition. TF-4 and TF-5 are proprietary and their compositions are not disclosed at all, which is a different reason for the same outcome.

⁇Does hardening lengthen the wash? Kodak in 1924 says no; ILFORD in 2015 says yes

Kodak’s 1924 primer says it is a myth, and gives a mechanism. “It is sometimes stated that material which has been hardened in the fixing bath washes more slowly than material which has not been hardened. This, too, is incorrect.” Gelatin is a sponge; hardening contracts the network of the sponge without contracting the gelatin as a whole, so diffusion through it is unchanged — unless the gelatin has been dried after hardening, in which case it will not swell properly on re-soaking and diffusion really is impeded.

ILFORD’s 2015 washing sheet says the opposite, flatly. Its minimum-water instructions are declared suitable for its own non-hardening fixers, and it warns that “using hardening fixers can significantly increase the required wash time”.

What the course uses. Both, because they may not be in conflict. Kodak’s claim is about a wet layer going straight from fixer to wash and is argued from a physical model; ILFORD’s is a practical instruction covering fibre-base paper, where what has to be washed out is held in a paper core rather than in the emulsion, and covering readers who will not necessarily wash before the layer has dried anywhere. The course’s working position is ILFORD’s, because it is the more recent, it is a manufacturer instruction rather than a correction of folklore, and it errs in the direction that costs water rather than permanence. The reason for setting the disagreement out rather than picking silently is that a reader who meets Kodak’s sentence elsewhere should know the course has seen it.

Mixing order, and why it is not a stylistic preference

Kodak Limited’s 1949 handbook states the rule for the whole family in two sentences, and every one of its clauses answers a specific failure above.

The order for an acid hardening fixing bath, and what each step prevents

dissolve in this order

1. 1 · Sodium thiosulfate, in warm water, completelyDissolving the pentahydrate takes heat out of the solution, so Reilly instructs starting slightly warmer than the working temperature. Adding a hardener to hypo that has not fully dissolved is what the 1928 primer names as a way to precipitate sulfur.endothermic
2. 2 · Sodium sulfiteIt must be in solution before any acid arrives, because it is the species that pushes the sulfurisation equilibrium back. This is the step the lab deliberately skips on a small sample.the protection
3. 3 · The acidAcetic acid, in the quantity the formula gives, added to a solution that is already protected. Never the reverse: acid meeting concentrated thiosulfate with no sulfite present is the milky beaker.the reserve
4. 4 · Boric acid, where the formula has itKodak G-23 notes that crystals rather than powder should be used because the powder does not dissolve easily — a practical detail that costs an hour of stirring if ignored.the buffer
5. 5 · Potassium alum, lastWhere the hardener is a separate stock, the 1949 handbook requires it be added to the hypo solution slowly, with vigorous stirring, and that both solutions be cold. Warm or fast, and the alum precipitates.the fragile one

Order and reasons from Kodak Limited's 1949 handbook and Kodak's 1928 primer. The sequence is not a convention: each step exists because the step after it would otherwise destroy something.

The chrome alum baths are, in the handbook’s words, “even more critical”, and the directions on stirring and temperature are to be followed exactly — which is another reason a course that has an alternative does not teach one.

Choosing

The job	What to use	Why
Roll and sheet film	Non-hardening rapid fixer at the maker’s film dilution	Modern camera films are hardened at manufacture; ILFORD says a fix hardener is no longer generally recommended, and the alum buys nothing while costing wash time and toning receptivity
Resin-coated prints	Non-hardening rapid fixer, short times	The base never wets, so only the emulsion has to give anything back. ILFORD’s figure is half a minute at 1+4
Fibre-base prints for permanence	Non-hardening, short times in fresh fixer, two baths	Long fixing drives thiosulfate into the paper core, which is the thing that later yellows. ILFORD’s archival sequence is one minute at 1+4
Prints you intend to tone	Non-hardening, and an alkaline bath if you have one	Kodak G-23: a hardening fixer makes the emulsion less receptive to toner. Moersch: an alkaline fixer needs no rinse before selenium
A hand-coated emulsion that frills	An alum hardening fixer, knowingly	This is the case the hardener was invented for and it has not gone away; Part V’s coating pages reach for it when a layer will not stay on its support
A printing-out image	Plain or slightly alkaline hypo	Reilly: an acid fixer bleaches the finely divided silver of a print-out image out of the highlights and mid-tones

Two things do not appear in that table and are worth stating because their absence is a decision. There is no row for “general purpose”, because the choice that is right for film is wrong for a fibre print destined for selenium. And there is no row where the answer is a hardening fixer for a modern film, because the manufacturer of the film says so.

Recap

A fixing bath is thiosulfate plus a set of answers to problems thiosulfate creates. Sulfite sits on the sulfurisation equilibrium and pushes it back, which is what lets an acid fixer carry enough acid to survive the alkali arriving from the developer. Acetic acid is chosen because a large quantity of a weak acid neutralises a great deal of alkali without making the bath strongly acid. Alum hardens the gelatin and imposes a pH window, because hardening rises with acid to a maximum while sludging life goes on rising past it; boric acid widens the window, on the course’s own reading of an ingredient nobody explains. Ammonium in place of sodium fixes three times faster and arrives as a non-hardening liquid; neutral and alkaline baths trade speed for a shorter wash and for compatibility with toners. The mixing order — thiosulfate, sulfite, acid, buffer, alum — is a list of failures avoided, in the order in which they would otherwise happen.

Check your understanding

Question 1. Kodak's 1928 primer reports three things about an alum fixer: hardening rises with the amount of alum; hardening rises with acetic acid to a maximum and then falls; and the minimum acid needed for a long sludging life is usually more than the amount that gives maximum hardening. What does that combination imply for anyone designing such a bath?

• That the bath should carry as much acid as possible, since acid is what preserves the thiosulfate
• That no single acid concentration optimises both hardening and sludging life, so a published formula is a chosen compromise rather than an optimum — and a buffer that holds the pH steady as alkali arrives is worth more than any particular starting value
• That alum should be omitted, since it is what creates the conflict
• That the bath should be mixed fresh each session, which removes the sludging question

Show the answer and why

Answer: That no single acid concentration optimises both hardening and sludging life, so a published formula is a chosen compromise rather than an optimum — and a buffer that holds the pH steady as alkali arrives is worth more than any particular starting value

The two curves peak in different places, so a formula has to sit between them and accept less of both than it could have of either. That is exactly the situation a buffer exists for, and it is the argument for boric acid in F-5 — note that the argument is the course's own reading, since no Kodak document that prints the ingredient says what it is for. The primer also gives the in-use direction: hardening first rises as developer is added and then falls off rapidly, so a bath in the middle of its life hardens better than a fresh one and a tired one hardly at all.

Question 2. A litre of plain hypo has been used for a printing session with an acetic stop bath before it, and no sulfite anywhere. Halfway through, the bath turns faintly milky. What has happened, what will it do to the prints, and what would have prevented it?

• Silver has precipitated as the metal; the prints will be unaffected; nothing prevents it
• Thiosulfate has been acidified by carryover and is throwing out free sulfur; sulfur that penetrates the gelatin can later fade the image; sulfite in the bath would have pushed the equilibrium back
• The bath has absorbed carbon dioxide from the air; the prints will be stained yellow; a floating lid would have prevented it
• Aluminium sulfite has precipitated; the prints will carry a white scum; more acid would have prevented it

Show the answer and why

Answer: Thiosulfate has been acidified by carryover and is throwing out free sulfur; sulfur that penetrates the gelatin can later fade the image; sulfite in the bath would have pushed the equilibrium back

Acid converts thiosulfate to thiosulfuric acid, which is unstable and decomposes to sulfurous acid and sulfur; Kodak's 1928 primer gives both the observation and the equation, and adds the consequence that matters: if a sulfurised bath is used, the sulfur is apt to penetrate the gelatin and may later cause fading. Sulfite works the equilibrium in the opposite direction, which is why it is in every acid fixer and why a plain bath must be kept away from acid. Aluminium sulfite is the other sludge and it is white and gelatinous rather than pale yellow, and it needs an alum bath to occur at all.

Question 3. Berfix Neutral publishes pH 7 and a film time of 5 to 7 minutes at 1+4; ILFORD publishes pH 5.0 to 5.5 and 2 to 5 minutes at the same dilution. What is the trade being made, and who should take it?

• The neutral bath is simply weaker and should not be used
• Speed is being traded for a shorter wash and for toner compatibility; a printer making fibre-base prints for selenium toning gains most, a photographer processing film in a tank gains least
• The neutral bath is faster in practice because pH 7 favours the complex
• The difference is only in how the two makers measure time, so the comparison means nothing

Show the answer and why

Answer: Speed is being traded for a shorter wash and for toner compatibility; a printer making fibre-base prints for selenium toning gains most, a photographer processing film in a tank gains least

Both figures are the makers' own for their own products, so the comparison is fair as far as it goes. Removing the acid removes the decomposition chemistry that has to be washed out afterwards, which is the basis of Bergger's shorter-wash claim, and it removes the acid that Moersch says has to be rinsed out before a selenium toner. It costs about a factor of one and a half in fixing time. For film in a tank there is nothing to gain and a slower step to accept; for a fibre print heading for a toner, the gain is in the two places permanence is actually decided.

Question 4. You have mixed an acid hardening fixer by dissolving the hypo, then adding the alum, then the sulfite, then the acid. What is the most likely outcome and which step is the error?

• No difference at all, since the final composition is the same
• The alum will precipitate or the bath will sludge, because alum was added before the acid and sulfite that hold it in solution — the alum must go in last, cold, and slowly
• The bath will fix faster, because the alum is dispersed earlier
• The hypo will not dissolve, because alum reduces its solubility

Show the answer and why

Answer: The alum will precipitate or the bath will sludge, because alum was added before the acid and sulfite that hold it in solution — the alum must go in last, cold, and slowly

Kodak Limited's 1949 handbook states the order and the reason together: hypo, then sulfite, then acid, and finally the alum, "if decomposition of the hypo and precipitation of the alum are to be avoided". Where the hardener is a separate stock it goes into the hypo solution slowly, with vigorous stirring, and both solutions must be cold. Every clause of that is a specific failure: a hot or fast addition throws the alum out; acid before sulfite throws sulfur out. A composition is not a formula — the order is part of it.

Question 5. ILFORD's HYPAM sheet says a fix hardener "is no longer generally recommended because modern camera films are sufficiently hardened at manufacture". List the consequences of taking that advice.

• Only one: fixing becomes slightly faster
• Shorter wash times, better toner receptivity, no aluminium sulfite sludge as a failure mode, and no pH window to hold — at the cost of a wet emulsion with no additional protection, which matters for hand-coated layers and not for commercial film
• The film will frill in the wash, so the advice applies only to paper
• None; a hardening fixer and a non-hardening one behave identically once the film is dry

Show the answer and why

Answer: Shorter wash times, better toner receptivity, no aluminium sulfite sludge as a failure mode, and no pH window to hold — at the cost of a wet emulsion with no additional protection, which matters for hand-coated layers and not for commercial film

Removing the alum removes the ingredient that created most of the acid fixer's complications. ILFORD's washing sheet says hardening fixers can significantly increase the required wash time; Kodak's G-23 says a hardening fixer makes the emulsion less receptive to toner; the aluminium sulfite sludge and the acid-against-hardening trade-off both disappear with the alum that caused them. What is genuinely lost is physical protection of a soft wet layer, which is why Part V's hand-coated emulsions still reach for an alum fixer when a layer frills. The advice is specific to films hardened at manufacture and does not generalise to everything you might coat yourself.

Check my answersClear