ILFORD RAPID FIXER

This is a product page, not a formula page. Every fixing bath elsewhere in this library arrives as a weight with a source beside it — two hundred and forty grams of hypo, twenty-five of bisulphite — and an account of why that weight and not another. Nobody outside Harman knows the weight in this bottle. What follows is what its maker publishes, which for a fixer is more than for any other chemical ILFORD sells; what its maker’s safety data sheet discloses, which is three substances and no more; what the two documents together still leave unknown, including whether they describe the same product; and the open non-hardening bath that does the same job with its arithmetic on the page.

HARMAN technology Limited (ILFORD Photo) — sold as liquid concentrate

Components the maker discloses — from the safety data sheet, which classifies hazards and does not state a formula

Component	Concentration as the sheet gives it	Hazard codes
Ammonium thiosulfatenamed on the sheet as Ammonium thiosulphateCAS 7783-18-8The fixing agent, and the one component whose presence ILFORD states outside the hazard sheet — the technical sheet says the fixing agent is ammonium thiosulphate and that the product contains no sodium thiosulphate (hypo)	35-50%
Sodium bisulfitenamed on the sheet as sodium hydrogensulphite … % sodium bisulphite … %CAS 7631-90-5	1-5%	H302
Acetic acid (glacial)named on the sheet as acetic acid … %CAS 64-19-7	<2%	H226, H314

Not disclosed. HARMAN publishes no composition for RAPID FIXER. One sentence of the technical sheet names the fixing agent as ammonium thiosulphate and rules out sodium thiosulphate, and that is the whole of the composition stated outside the hazard paperwork. The safety data sheet names three components and gives each the concentration band it needs in order to classify a mixture, which is not an assay: section 11 records that every endpoint on the sheet was reached by calculation from those bands. It names nothing else at all — not the water, and not whatever else is in the bottle — and this page will not add the bands up and call the remainder a discovery. What is published nowhere in ILFORD's literature is the thiosulphate concentration of the concentrate or of either working dilution, the molarity of anything, the identity of the buffer that holds the working solution at pH 5.0 to 5.5, whether any sulphite is present beyond the bisulphite the sheet names, the concentrate's own pH, density or colour — section 9 records all three as not known — and any statement that the product described by the 2010 technical sheet is the product described by the 2024 safety data sheet. A reader who wants to know how many moles of thiosulphate are in the tank, or to move that number and watch the fixing time and the capacity move with it, has to mix an open formula rather than buy this one.

Nearest open formula. Kodak F-24 — Both are non-hardening acid fixing baths built the same way — a thiosulphate to dissolve the halide, a bisulphite to supply sulphite and acid at once — and the difference between them is which half of the page you can read. F-24 is 240 g of sodium thiosulphate, 10 g of sodium sulphite and 25 g of sodium bisulphite in a litre, published by Kodak with every quantity on the page and no fixing time at all; RAPID FIXER is an ammonium bath at a concentration nobody outside Harman knows, published with two dilutions, five fixing times, three capacities, a pH and a specific gravity window for each dilution, four replenishment rates and four keeping figures. Swapping sodium for ammonium is what buys the speed, and it is the reason the bought bath fixes film in two to five minutes where the mixed one carries no published time to compare it against. One is a known composition with unmeasured performance; the other is a measured performance from an unmeasurable composition, and Part XI's clearing-time experiment is the bench where a reader turns one into the other.

Purpose

To make the developed image permanent, by removing the silver halide that development did not reduce. ILFORD’s own sentence in the beginner sheet is exactly that short: “ILFORD RAPID FIXER makes the developed image permanent.”

Two words in the product name carry the rest of it. Rapid: the fixing agent is ammonium thiosulphate rather than hypo, and the sheet’s film time is two to five minutes. Fixer, unqualified: this one is non-hardening, and the sheet says so in the first paragraph and then forbids the obvious addition — “ILFORD RAPID FIXER must not be used with fix hardeners.”

The third thing the product is, it says by exclusion. “The fixing agent in ILFORD RAPID FIXER is ammonium thiosulphate, it contains no sodium thiosulphate (hypo).” That sentence is the only statement of composition HARMAN makes anywhere outside the hazard paperwork, and it does two jobs at once: it names the working substance, and it forecloses the assumption a reader coming from the plain hypo bath would otherwise make.

What it does not say is how much of it there is, which is where this page starts and Part XI continues.

Recommended uses

Film in a spiral tank, at 1+4, for two to five minutes at 20 °C. This is the application the sheet is built around, and 1+4 is the only dilution offered for film. The agitation is the same as for development — four inversions in the first ten seconds, and four more in the first ten seconds of every subsequent minute — which is a convenience worth noticing: nothing about the rhythm of the process changes when the developer comes out and the fixer goes in.

Resin-coated paper in a dish, at 1+4, for thirty seconds. The technical sheet writes it as half a minute and the beginner printing sheet writes it out in seconds; they are the same instruction. Agitation is intermittent rocking of the tray.

Fibre-based paper in a dish, at 1+4 for one minute or at 1+9 for two. The fibre times are exactly double the resin-coated ones at both dilutions, and the sheet gives no reason for it here. It does give one for the parallel gap in the silver limits — the resin-coated base “is protected on both sides by an impervious polythene coating” — and the course reads the two as the same fact about the base rather than about the emulsion, which is an inference of its own under Rule 7.

In a processing machine, at 1+4, anywhere between 18 and 40 °C. The published temperature range is enormous by the standards of anything else in this library — a developer is specified to a degree, this to twenty-two of them — and ILFORD adds that a machine will often give a shorter time than the table, which should cause no problems because the bath is usually running at 25 to 40 °C. The one machine instruction that matters at installation: after filling with fresh solution, bring it to temperature and let it recirculate for at least ten minutes before anything is processed.

Across many sessions, in a bottle. Both beginner sheets say it outright — the fixer can be made up and used for more than one session, and the film sheet ends its fixing step with “then pour the fixer into a storage bottle”. A working-strength solution keeps six months in a full capped bottle. That is a genuine difference from a print developer, and it is why the whole apparatus of clearing tests and silver limits exists at all: a bath you keep is a bath whose state you have to know.

In two baths rather than one, when the work is worth it. ILFORD publishes the classic method: two baths of the same volume, half the recommended time in each, the first discarded at capacity and the second promoted to first with a fresh second made up. The rotation SOP is the procedure; the reason is on this page under silver limits.

When another formula is preferable

• When you want to understand a fixer rather than buy one, mix Kodak’s F-24. It is the same kind of bath — non-hardening, acid, sulphite and bisulphite — with every quantity published, and it is the comparison this page is built around.
• When the emulsion must be hardened. RAPID FIXER is prohibited from taking a hardener, so this is not a choice you can make inside the bottle. ILFORD’s own answer is its sibling: the HYPAM sheet says a fixer hardener may be added to working strength HYPAM, and lists the four cases where it is still recommended — a film process above 30 °C, poor drying performance, a need for shorter drying, or a risk of physical damage such as a roller transport processor. For paper it is not recommended at all, and hardener must never be mixed into concentrated fixer. The course has no page for HYPAM yet, because HARMAN publishes no composition for that one either. Where you want a hardening bath whose composition is published, F-5 and F-6 are the classical answers and F-52 is the non-hardening one Kodak Limited printed beside them.
• For a printing-out image — salted paper, albumen, Van Dyke, a kallitype left to print out. Part XI’s survey of fixer types records, on Reilly’s evidence, that an acid bath attacks the finely divided silver of a printing-out image, which is why Reilly’s alkaline bath and plain hypo exist. That page sets out which process wants which, and this product is on the wrong side of the line for all of them.
• When the wash water is short or the wash time must be. The argument that an alkaline fixer washes out of negatives and prints more rapidly than an acid one is TF-2’s, made there by its own authors and with its own evidence. ILFORD’s answer within its own range is the opposite one: keep the acid fixer and add a washing aid afterwards, which is exactly the fibre-paper sequence printed on this sheet.
• When you need a fixing time you can cite for a film this sheet does not name. ILFORD publishes “general purpose film, 2 to 5 minutes” and leaves the reader to place their own film in that range. The honest way to close the gap is not another manufacturer’s number but the clearing-time test below, which measures it for your film in your bath.
• When the budget is the constraint and the volume is small. A 500 mL bottle of concentrate makes 2.5 litres at 1+4, which is 60 films or 200 resin-coated prints. If you print six times a year, the keeping figures will beat you to the bottom of the bottle, and 240 g of hypo in a litre of water is cheaper per print than any of it.

What the maker publishes

This is the fullest sheet HARMAN publishes for anything in its chemical range, and the quantity of it is exactly what makes a page about a secret composition useful rather than merely careful.

Two dilutions, and a rule about which. 1+4 for all film fixing applications and for paper in a processing machine; 1+4 or 1+9 for manual paper work. The sheet states each of those separately, on page one and again in the tables, and it never offers 1+9 for film.

Five fixing times. General purpose film at 1+4, two to five minutes. Resin-coated paper, half a minute at 1+4 and one minute at 1+9. Fibre-based paper, one minute at 1+4 and two at 1+9. All are at 20 °C, all are described as average minimum times in fresh fixer, and each is tied to an agitation regime: inversions for film, rocking for paper.

A temperature range rather than a temperature. 18 to 40 °C (66 to 104 °F), for manual and machine work alike, with the working rule that every process solution should be at the same temperature or at least within 5 °C of the developer.

pH and specific gravity, for both working dilutions. pH 5.0 to 5.5 at 1+4 and at 1+9 alike; specific gravity 1.070 to 1.080 at 1+4 and 1.030 to 1.040 at 1+9, both at 20 °C. ILFORD prints the caveat with the numbers, and the caveat is better advice than the numbers: they were obtained under carefully controlled laboratory conditions, may differ slightly from measurements made by users in their own areas, and the right use of them is to measure your own fresh solution and compare later ones against it. The sheet even specifies the instruments — pH sticks covering pH 4 to 6 are sufficient if a meter is not available, and a hydrometer covering 1.000 to 1.200 will serve a wide range of process solutions.

Three capacities, per litre of working solution, unreplenished. 24 films of 135-36 at 1+4. 80 sheets of 8 × 10 inch resin-coated paper, given also as 4 m² (44 ft²). 40 sheets of 8 × 10 fibre, given also as 2 m² (22 ft²). The paper rows carry one dilution column covering both 1+4 and 1+9, and one sentence of permission that appears nowhere else in ILFORD’s chemical literature: the figures for paper “may be exceeded whenever print stability is not critically important”.

What exhausts a bath, in the maker’s own list. An unreplenished fixer bath is exhausted by the build-up of silver and halides in it, and by the action of solutions carried over from the preceding baths, which can cause some dilution and the pH to be raised. Three mechanisms, and only one of them is the silver everybody thinks of first.

Four replenishment rates. Film at 1+4: 45 mL of working strength fixer per 135-36 film, or 855 mL/m² (78 mL/ft²). Resin-coated paper at either dilution: 250 mL/m² (23 mL/ft²). Fibre paper at either dilution: 500 mL/m² (46 mL/ft²). For machine processing of resin-coated paper the suggested rate is 300 to 450 mL/m² (27 to 41 mL/ft²), and where a properly set up silver recovery system is in use the rate can come down by 50 to 75 per cent. The deep-tank method is given too, and it is the part people get wrong: work out the replenisher volume, remove more than that volume of used fixer, add the replenisher, then top back up to level with some of the fixer you removed, stirring thoroughly.

A clearing-time test, described well enough to perform. Put a drop of working strength fixer on the emulsion side of a piece of scrap unprocessed film and leave it until the emulsion under the drop is a clear spot, which should take about 30 to 60 seconds. Immerse the piece in the bath and time how long the rest of it takes to match that spot. That time is the clearing time; the fixing time needed is double it; and the bath is discarded when the clearing time in used fixer exceeds twice the clearing time in fresh. It can all be done in normal lighting.

Three silver limits, and a test to check the tightest of them. A film bath may rise to 8 to 10 g/L without serious effect. A bath fixing fibre paper for commercial permanence stays below 2 g/L, about 40 8 × 10 prints. A bath fixing prints for maximum long-term stability stays below 0.5 g/L, about ten 8 × 10 prints per litre — and ILFORD adds that silver estimator papers are usually not sensitive enough to read levels that low. Resin-coated paper is the exception in the other direction: because the base is protected on both sides by an impervious polythene coating, 4 to 6 g/L is tolerated. The test ILFORD gives instead of an estimator paper is a sulphide one: 2 g of sodium sulphide in 125 mL of water, diluted 1+9 for use, one drop on a white area of a print known to be well fixed and well washed, blotted, leaving a barely visible cream tint as the reference for that paper. Any later print whose test spot yellows is not properly fixed; soak it for five minutes and repeat the fixing and washing with fresh fixer. The test only works on a print that has already been washed.

A whole page on silver recovery, which is unusual in a consumer product sheet and is the part of this document that reads as though it were written for a laboratory. Any method may be used; electrolytic is recommended because the treated fixer can be recycled or reused; the unit may be off-line or in the processor’s recirculation loop. Too large a current when the silver level has fallen too low breaks down the fixer’s active ingredients and deposits silver sulphide on the cathode — sulphiding, which reduces the efficiency of both the fixer and the recovery — and vigorous electrolysis may release hydrogen sulphide, which the sheet names as hazardous and describes by its smell. A properly set up system reaches 50 to 100 ppm of silver; secondary and tertiary ion exchange and metal exchange on the fix and wash overflows can reach about 3 ppm.

Four keeping figures for the working solution and two for the concentrate. Working strength: six months in a full, tightly capped bottle; two months in a tank or dish with a floating lid; one month in a half-full tightly capped bottle; seven days in an open dish or tray. Concentrate: two years in a full, unopened bottle at 5 to 20 °C, and used up within six months once opened. A properly replenished tank solution has a very long life but should be replaced after twelve months.

The washing instructions that go with it, because a fixer sheet that stopped at the fixer would be half a document. Films: 5 to 10 minutes in running water within 5 °C of the process temperature, or ILFORD’s water-saving alternative of three fills of the spiral tank with 5, 10 and 20 inversions. Resin-coated paper: two minutes in running water above 5 °C, or thirty seconds vigorously when speed matters more. Fibre paper: sixty minutes — or five minutes’ wash, ten minutes in WASHAID at 1+4 and 18 to 24 °C, then five minutes’ wash.

🧮The availability paragraph checks out, which is not true of every sheet

A 5 litre bottle at 1+4 makes 25 litres of working solution. Against the per-litre capacities that is 25 × 24 = 600 films, 25 × 80 = 2000 resin-coated 8 × 10 prints and 25 × 40 = 1000 fibre prints — which is exactly what the availability paragraph claims.

The replenishment figures check too, using the sheet’s own equivalence of 40 fibre sheets to 2 m², that is 20 sheets per square metre. 25 000 mL at 45 mL per film is 555 films, against a claimed 550. At 500 mL/m² for fibre it is 50 m², which is 1000 sheets exactly. At the manual resin-coated rate of 250 mL/m² it is 100 m², or 2000 sheets; at the top of the machine range, 450 mL/m², it is 55.6 m², or about 1100 — and the sheet claims 1000 to 2000, which is precisely that span.

Every one of those is arithmetic on ILFORD’s own published figures, not an estimate of the course’s. It is worth doing because it establishes that the tables and the marketing paragraph on this sheet were derived from one another, which is not something you may assume about a manufacturer’s literature.

What is not disclosed

The concentration of the fixing agent. The sheet names ammonium thiosulphate and stops. The safety data sheet gives a band, 35-50%, which exists to classify a mixture and not to describe one: section 11 of that sheet records that every toxicological endpoint on it was reached by calculation method from the bands, so the bands are chosen to make a conservative classification. Nowhere in ILFORD’s literature is there a molarity, a grams-per-litre, or a figure for either working dilution.

What else is in the bottle. The composition table names three substances. It does not name water. It names nothing else. A safety data sheet lists what drives a classification, not what is in the container, so this is not a contradiction — but it does mean that a reader who subtracts the bands from a hundred has produced a number that no document supports and that describes nothing. This page will not do that arithmetic and no reader should. The correct statement is the plain one: three substances are named, everything else is not, and the boundary is where the naming stops.

The buffer. The working solution holds pH 5.0 to 5.5 at 1+4 and at 1+9 — the same window at two concentrations differing by a factor of two. Something in the bottle is holding it there. The sheet names acetic acid at <2% and bisulphite at 1-5%, and says nothing about which of them, in what ratio, with what conjugate base, is doing the holding.

The concentrate’s own physical properties. Section 9 of the safety data sheet records pH, colour, odour, density, relative density and solubility all as not known, and gives appearance as Liquid. That is the reverse of the arrangement on the ILFOSTOP page, where the concentrate’s pH and specific gravity are published and the working solution’s are not.

Whether the two documents describe the same product. The technical sheet’s page footer is dated July 2010. The safety data sheet was issued on 18 July 2024 and carries three product codes. Fourteen years separate them, HARMAN publishes no statement that the formulation is unchanged across that span, and a reader has no way to establish it. This is not an allegation that the product changed; it is the observation that the question cannot be answered from the published record, which is a different and more useful thing to know.

The identity of anything not classified. Ammonium thiosulphate is listed with Not classified against it and no pictogram. A component that carries no hazard has no classification reason to be on a hazard sheet at all, so its presence there is a courtesy of HARMAN’s rather than an obligation — and the courtesy stops at the name and the band.

Disclosed components

Three, in the order the safety data sheet prints them, with the concentration against each being the band the sheet states and not a quantity.

Ammonium thiosulphate, CAS 7783-18-8, EC 231-982-0, 35-50%. This is the fixing agent, and it is the one component whose presence ILFORD states outside the hazard paperwork: the technical sheet’s own sentence is that the fixing agent is ammonium thiosulphate and that the product contains no sodium thiosulphate. The composition table lists it as Not classified, with no hazard statement and no pictogram, which is why a bottle of rapid fixer carries no orange diamond. It is its encyclopaedia page that has the chemistry; three properties matter here. It supplies the same thiosulphate ion hypo supplies, so the fixing reaction is not a different reaction. It is far more soluble than the sodium salt, so a stronger bath is available before the solid runs out of solvent, which is what makes a liquid concentrate a sensible way to sell it at all. And it carries an ammonium ion, which is the half of the molecule that makes the bath rapid and also the half that will give off ammonia if the solution is ever made alkaline.

Sodium bisulphite, CAS 7631-90-5, EC 231-548-0, 1-5%. The sheet does not name it that way. It prints “sodium hydrogensulphite … % sodium bisulphite … %”, and the ellipses are not a redaction: that is the CLP Annex VI entry name for a solution of unstated strength, carried across verbatim into the composition table, and the same name appears again in section 15.1 under the UK REACH Annex XVII restrictions line. The course’s encyclopaedia calls the substance sodium bisulfite and the sheet’s own hazard line for it is Acute Tox. 4, H302, pictogram GHS07. Its function in an acid fixing bath is not in doubt and is not an inference peculiar to this product: bisulphite supplies both the sulphite and the acid a fixing bath needs — the property F-24’s page traces to Kodak’s 1928 handbook, and the reason that formula is built from it. The sulphite half keeps the thiosulphate from being thrown out as sulphur when it meets acid; the acid half is what makes the bath an acid bath. What HARMAN does not say is how much of each job this 1-5% is doing here.

Acetic acid, CAS 64-19-7, EC 200-580-7, <2%. Printed as “acetic acid … %”, the same Annex VI naming convention, and it is the component that carries the harshest hazard line on the sheet — Flam. Liq. 3 with H226, and Skin Corr. 1A with H314, pictograms GHS02 and GHS05 — while the mixture itself is classified as not dangerous at all. That gap is the whole subject of the GHS and CLP page: a corrosive substance below its classification threshold in a mixture does not make the mixture corrosive. Its encyclopaedia page has the properties; the International Chemical Safety Card records that a solution in water is a weak acid and that the substance attacks many metals. It is the same acid ILFORD tells you to add, at 50 per cent, to a fixer bath whose pH has drifted upward — which is a rare case of a maker naming, in its own technical sheet, a substance it also lists on its hazard sheet. What the acetic acid is doing there, HARMAN does not say, and neither will this page beyond the obvious. It is an acid in an acid fixer. Whether it sets the pH, buffers it with the bisulphite, or is a residue of the manufacture is not stated, and a page that picked one would be reconstructing a formulation by inference.

And what a fixer bottle usually contains that is not in this table. Commercial rapid fixers are often sold with a sequestrant or an anti-sludging agent, and hardening fixers with alum and a boric acid buffer. Nothing of the kind appears in this table, and the absence proves nothing either way — the table lists what drives a classification, and an unclassified additive would not appear whether it were there or not. The one thing that is established about this product’s formulation beyond the three names is negative and comes from the technical sheet: it contains no sodium thiosulphate, and it is not to be used with a hardener.

Behaviour

It is fast, and the speed is the product. Two to five minutes for film at 1+4, and thirty seconds for a resin-coated print at the same dilution. The difference between that and a sodium bath belongs to the ammonium ion, and the previous lesson in Part XI is where the comparison is made and is candid that no open source this course holds explains the mechanism.

It is tolerant of temperature and intolerant of nothing much else. 18 to 40 °C is a range within which the maker makes no adjustment to the published times at all. Compare that with a developer, where two degrees is a correction. A fixer is dissolving a salt rather than running a redox reaction against a fog threshold, and the latitude is the chemistry showing through the instruction.

It is buffered, and the sheet demonstrates it without saying so. pH 5.0 to 5.5 at 1+4, and pH 5.0 to 5.5 at 1+9. The specific gravity falls by more than half between those two dilutions, so the solution really is twice as dilute, and the pH does not move. A solution whose pH is insensitive to a two-fold dilution is a buffered solution — that is the course’s inference from ILFORD’s published figures, marked as an inference under Rule 7, not a statement of HARMAN’s. What it is buffered with is not published.

It exhausts three ways at once. Silver and halide accumulate; carried-over solutions dilute it; and carried-over developer raises the pH. Only the first is what most people picture, and only the third can be corrected in the tray — a few drops of 50 per cent acetic acid, added gradually with thorough stirring, and never past the published pH limits.

Its concentration can drift in both directions in a machine. If the specific gravity is too low the bath is too dilute and is restored by stirring in fresh concentrate; in a high-temperature processor evaporation concentrates it and the fix is to top up with water. In a roller transport machine, where stirring is not possible, the sheet says the moving parts and the recirculation do enough.

Its capacity per litre does not change with dilution — for paper. 80 resin-coated sheets per litre at 1+4, and 80 per litre at 1+9. Arithmetically that means a bottle of concentrate diluted 1+9 fixes twice as many sheets as the same bottle at 1+4, at double the time each; and ILFORD’s own availability paragraph is computed at 1+4 and never mentions it. Why the capacity is dilution-independent, the sheet does not say. The obvious reading is that the limit is the silver loading in grams per litre rather than the thiosulphate available, which is consistent with the whole silver-limit section — but that reading is the course’s and is marked as such.

It goes on working past the point where it should be trusted. This is the property that makes the clearing-time test necessary rather than optional. A bath at twice its fresh clearing time still clears film; it simply leaves behind complexes that will not wash out. Nothing visible in the tray distinguishes the two states.

Image characteristics

A fixer contributes nothing to the image and decides how much of it survives. There is no signature to describe in the way a developer’s is described. What there is instead is a list of failures the bath can leave behind, each with its own appearance and its own delay.

Under-fixing shows immediately and mildly, and permanently and badly. Undissolved halide leaves a milky or opalescent cast that is visible against a light box on film, and on paper as a grey veil in what should be white. Left in place it darkens. Under-fixing and residual silver and yellow staining are the entries.

Over-fixing has no published limit here, and the course does not invent one. ILFORD’s times are average minimum times, and the only upper bound the sheet offers is the machine note that a longer time “should not cause any process problems provided it is not excessive”. No number is attached to “excessive” anywhere in the document, so none appears on this page.

Residual thiosulphate is the failure you cannot see for years. It is a wash problem rather than a fix problem, but the fixer sets its size: a print that came out of a bath at 0.5 g/L of silver has a different washing problem from one that came out at 4. Residual thiosulphate is the entry and the residual hypo and silver SOP is the procedure.

The unhardened emulsion is a characteristic in itself. Nothing in this bath cross-links the gelatin, so the layer that comes out of it swells more, dries more freely, takes a toner or a selenium bath faster and more evenly, and is more easily damaged wet. That is a trade ILFORD makes deliberately — the HYPAM sheet’s reasoning is that modern camera films are sufficiently hardened at manufacture — and it is why frilling and reticulation are worth reading before a warm-weather session.

On a print-out image it is actively destructive, which is not a defect of the product but a fact about acid fixers, and the reason the alternative-process side of this course reaches for a plain hypo bath instead.

The mechanism

The fixing reaction is the ordinary one, and the counter-ion does not enter it.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Thiosulfate takes the silver out of the halide's solubility equilibrium

How fixer works derives it properly. Nothing about that argument changes when the cation is ammonium rather than sodium, which is why the sheet’s own description of the product can be so short.

Why it is rapid is a gap this course states rather than fills. The acceleration belongs to the ammonium ion, and Wall’s 1924 formulary is where that is established rather than assumed: adding 2.5 to 5 per cent of ammonium chloride to a 20 per cent hypo bath increases the rapidity of fixing, 10 per cent ammonia water does the same, and neither has any effect on a 40 per cent hypo bath — so it is the cation and not the anion doing the work, and only where thiosulphate is not already in large excess. Wall gives that as an effect and calls the period explanation a supposition. The ammonium thiosulphate page sets out what is evidenced and what is not. Whether the ion changes the rate at which the silver–thiosulphate complex forms, changes which complex forms, or acts on diffusion into the gelatin, the course could not verify from any open source it holds, and does not guess.

Why the bath is acid at all is about the bath before it, not this one. Alkaline developer walks in on every sheet and every film. An acid bath neutralises it on arrival, which is what keeps the fixer’s own chemistry inside its window and is why a stop bath pays for itself in fixer life rather than in print quality. ILFORD recommends ILFOSTOP between the two and prints the whole ILFOSTOP table inside this fixer’s own sheet.

Why the acid is weak, and why the pH window has a floor as well as a ceiling. Thiosulphate in strong acid decomposes and throws out sulphur — the bath goes milky and then sludges. So the acid in a fixing bath is chosen to be weak and to be accompanied by sulphite, which is exactly what a bisulphite delivers in one salt, and Part XI’s account of sulfurisation carries the 1928 equation and the evidence for it. ILFORD’s instruction when correcting a drifted bath carries the same logic in one clause: add acetic acid gradually with stirring, and “do not lower the pH of the fixer bath too far, the limits are given above”. A ceiling, because a bath whose acid is spent stains prints brown — F-24’s page says so of any acid fixer — and because an ammonium bath pushed alkaline gives off ammonia; a floor, because sulphur comes out.

⚗What the disclosed components let this page say, and where it stops

A bath disclosing ammonium thiosulphate, a bisulphite and acetic acid is, in the ordinary vocabulary of photographic chemistry, a non-hardening acid rapid fixer — and that is not an inference at all, because ILFORD’s technical sheet uses those exact words in its first paragraph.

The inference begins one step later, and this page marks it as one under Rule 7. The pH window that does not move between 1+4 and 1+9 is the behaviour of a buffered solution; a bisulphite and a weak acid are the ingredients a formulator would use to buffer a fixing bath at pH 5; the two facts are consistent. They are consistent, not conclusive. HARMAN states no buffer, no ratio and no conjugate base, the bands on the sheet were chosen to classify a hazard, and there may be a fourth substance in the bottle that carries no hazard and therefore no line in the table. Naming a buffer pair here would be reconstructing a formulation out of a hazard document, which is precisely what this page kind exists to refuse.

Why silver limits are so much tighter for paper than for film. Both are about what is left in the material after washing, and paper has a fibrous base that holds solution where film has an inert support. ILFORD’s own numbers say it: 8 to 10 g/L in a film bath, below 2 g/L for commercially permanent fibre prints, below 0.5 g/L for maximum stability — and 4 to 6 g/L for resin-coated paper, whose base “is protected on both sides by an impervious polythene coating”. The polythene is the whole of the difference, which is a neat demonstration that the limit is about the base and not about the emulsion.

Why electrolytic recovery can poison the bath it is cleaning. Drive too much current through a solution whose silver has already fallen low and the cell starts reducing something else; the fixer’s active ingredients break down, silver sulphide plates onto the cathode, and in the worst case hydrogen sulphide comes off. ILFORD names all three consequences. It is the clearest published statement in this corpus that a fixer is a chemical system rather than a consumable, and that taking the silver out is an operation on it rather than a service to it.

The nearest open formula

Kodak’s F-24, and the comparison is the reason this page exists.

F-24 is 240 g of sodium thiosulphate (pentahydrated), 10 g of sodium sulphite and 25 g of sodium bisulphite, made to a litre — a non-hardening acid fixing bath which Kodak’s J-1 says may be used for films, plates or papers when no hardening is desired. It is built from the same three functional parts as the product on this page: a thiosulphate, a sulphite, and an acid that arrives inside the sulphite. It differs in the one term that matters most, the cation, and that difference is the whole of the speed.

	Kodak F-24	ILFORD RAPID FIXER
Composition	240 g sodium thiosulphate, 10 g sulphite, 25 g bisulphite per litre, published	Ammonium thiosulphate at 35-50% of a concentrate, bisulphite 1-5%, acetic acid <2%; amounts not published
Fixing agent	Sodium thiosulphate, weighed by you	Ammonium thiosulphate, named by the maker, concentration not published
Acid and preservative	Sulphite and bisulphite, both weighed	Bisulphite named at a band; acetic acid named at a band
Hardener	None, by design	None, and prohibited: “must not be used with fix hardeners”
How it is made	Dissolved in warm water in the printed order, made to volume	1+4, or 1+9 for manual paper work, stirred thoroughly into water
Published fixing time	None — J-1 gives F-24 no time at all	Five: film 2–5 min; RC ½ or 1 min; FB 1 or 2 min
Published pH	None	5.0–5.5 at both dilutions
Published density	None	SG 1.070–1.080 at 1+4, 1.030–1.040 at 1+9
Published capacity	26 sheets of 8 × 10 per litre, and only if a stop bath is used	24 films, or 80 RC or 40 FB sheets of 8 × 10 per litre
Replenishment	None published	Four rates, plus a machine range and a silver-recovery reduction
Keeping	2 months stock in a stoppered bottle; 1 week in a tray or tank	6 months, 2 months, 1 month or 7 days by container; concentrate 2 years
Speed	Sodium bath; no published time to compare	2–5 minutes on film
What you can change	The thiosulphate concentration, the acid load, and therefore everything	The dilution, the temperature and the time

The last row is the argument, and the “published fixing time” row is its sharpest form. Kodak prints a complete composition for F-24 and no fixing time; ILFORD prints five fixing times and no composition. Neither document is dishonest, and neither is sufficient. The reader who mixes F-24 gets a bath whose contents are entirely known and whose behaviour must be measured; the reader who buys RAPID FIXER gets a bath whose behaviour is documented in more detail than almost anything else in this library and whose contents are a band on a hazard sheet.

The measurement that closes the gap is the same one in both directions, and ILFORD publishes it on this very sheet: the clearing time, and the rule that fixing time is twice it. Part XI’s clearing-time and capacity experiment runs it against both baths, and the clearing-time SOP is the procedure. Do that, and F-24 acquires a fixing time for your film and RAPID FIXER acquires a capacity for your work — which is exactly the half each document is missing.

Safety

Level A, and for once the reasoning is almost entirely the maker’s.

HARMAN classifies the mixture as not dangerous for supply or use. Section 2.2 gives no hazard pictogram, no signal word and no hazard statement — three lines reading None — and section 2.3 records other hazards as none known. That is the shortest classification of any product in this formulary, shorter even than ILFOSTOP, which at least carries H319. On the classification rubric Level A takes substances whose classification is at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled, and names silver-bearing fixer collected for recovery as a Level A waste stream in as many words. An unclassified mixture whose most severe component sits below the threshold that would classify it meets the first, and this product is the second.

And yet the sheet still prints five precautionary statements, which is worth pausing on: P102, keep out of reach of children; P280, wear protective gloves, protective clothing, eye protection or face protection; P302+P352, if on skin wash with plenty of water; P305+P351+P338, if in eyes rinse cautiously with water for several minutes, removing contact lenses if present and easy to do; and P501, dispose of contents in accordance with local, state or national legislation. A mixture that is not classified is not required to carry any of those. HARMAN prints them anyway, and its own technical sheets say the same thing in plainer words: photographic chemicals are not hazardous when used correctly, and it is recommended that gloves, eye protection and an apron or overall are worn when handling and mixing all chemicals.

Two occupational exposure limits appear on a sheet for an unclassified product. Section 8.1.1 carries acetic acid at a long-term limit of 10 ppm (25 mg/m³) and a short-term limit of 20 ppm (50 mg/m³), and sodium hydrogen sulphite at a long-term limit of 5 mg/m³, both taken from the United Kingdom’s EH40/2005. Those are limits for the substances, not measurements of this product, and no limit is assigned to the mixture. They are on the page because two of the three named components have them — which is the useful lesson: an unclassified mixture is not a mixture of unregulated substances.

Personal protection, as the sheet specifies it. Eye protection with side protection to EN ISO 16321-1. Adequate ventilation. Skin protection recorded as not normally required, and section 6 qualifying that with “wear suitable gloves if prolonged skin contact is likely” — a distinction between a splash and a session that the glove selection page takes seriously even where the sheet does not. Respiratory protection: normally none necessary.

First aid, in the sheet’s own words. Inhalation: if breathing is difficult, move to fresh air and rest in a position comfortable for breathing. Skin: wash with water. Eyes: flush with water for at least fifteen minutes — notably longer than the “several minutes” its stop-bath stablemate asks for. Ingestion: wash out the mouth with water. Symptoms and effects: none anticipated, treat symptomatically. The first-aid page has the course’s fuller procedure and the eyewash SOP has the technique.

In a fire, the sheet says no special hazard is anticipated but that heating may cause decomposition, and directs extinguishing media as appropriate to the surrounding fire.

⚠The two gases, neither of which is on this product's safety data sheet

Nothing in section 2 of the safety data sheet mentions a gas, and nothing needs to, because neither gas is a property of the fixer sitting in its bottle. Both are properties of what a darkroom does with it.

Sulphur dioxide, from acid meeting a thiosulphate or a sulphite. That reaction is available in every direction here: this bath is acid and contains both a thiosulphate and a bisulphite, so pouring a strong acid into it, or pouring it into a waste bottle that already holds an acid, or letting a tray over-concentrate and then acidifying it, are all the same mistake. The incompatibilities page has the reaction and the limits, which are among the tightest gas limits in this course.

Ammonia, from an ammonium salt meeting an alkali. Splash a carbonate developer or a hydroxide into a rapid fixer and the smell is the warning. The ammonium thiosulphate page carries the rule: an ammonium bath pushed alkaline gives off ammonia.

And one the sheet does name, in its technical rather than its hazard document: hydrogen sulphide, from vigorous electrolysis during silver recovery. If you run a recovery cell, read ILFORD’s own paragraph on it rather than this summary.

The control for all three is procedural rather than chemical: keep the baths in the order they belong in, never pour one waste stream into another, and do not let a tray stand and concentrate.

Storage

The concentrate: full, tightly sealed, upright, cool. ILFORD’s figures are two years for a full, unopened bottle stored at 5 to 20 °C, and six months from opening — and the second clock starts at the first pour, not when the bottle is half empty. The safety data sheet adds only ambient, stable under normal conditions, and out of reach of children.

The working solution, by container, and the containers are the interesting part. Six months in a full tightly capped bottle. Two months in a tank or dish with a floating lid. One month in a half-full tightly capped bottle. Seven days in an open dish or tray. Two of those four figures describe the same solution differing only in how much air is above it, and the ratio between them is six to one.

A replenished tank has no keeping figure worth the name, which is why the sheet gives a rule instead: a properly replenished bath in regular use has a very long life, and should be replaced after twelve months in the processing tank regardless.

Buy for the darkroom you have. A 500 mL bottle makes 2.5 litres at 1+4, which the capacity table says is 60 films or 200 resin-coated 8 × 10 prints. A 5 litre bottle makes 600 films’ worth. If your year is thirty films, the six-month opened-concentrate figure will beat you to the bottom of the larger bottle by a wide margin, and the cheaper purchase is the smaller one.

Label the bottle with the date it was diluted and the work it has taken. Neither number is on the bottle and both decide whether the bath is still good. The labelling SOP and the batch record SOP are the procedures, and an unlabelled or undated container is the entry for what to do when they were skipped.

Incompatibilities

Fix hardeners, by the maker’s own prohibition. “ILFORD RAPID FIXER must not be used with fix hardeners.” This is the flattest instruction on the sheet and it has no stated reason. The sibling product shows what the sentence is doing: HYPAM, otherwise the same class of bath, explicitly may take a hardener into its working strength solution — and even there, never into the concentrate. Take the prohibition at face value and do not treat the HYPAM paragraph as permission for this bottle.

Alkali, in any form, because of the ammonium. Carbonate and hydroxide developers, ammonia, carelessly dosed neutralisation before disposal. The product of that meeting is ammonia gas, and it is also the end of the bath.

Strong acid, because of the thiosulphate and the sulphite. Sulphur dioxide, and a bath that goes milky and then sludges. The intended acid contact — a small carried-over volume of stop bath into a large fixer — is a different thing from mixing the two in a bottle, and the incompatibilities page governs.

Developer, in both directions. Fixer carried back into a developer on a pair of tongs does not dilute it, it stops it. Developer carried forward into the fixer is the third of ILFORD’s three exhaustion mechanisms. Dedicated tongs, dedicated measures and the wash-everything rule are the control; the darkroom opening and closing SOP is where the course puts it.

Metals, because of the acid. The International Chemical Safety Card records that acetic acid attacks many metals; the practical rule for any acid fixer is plastic or glass for trays, funnels, measures and storage.

Its own silver, past a point. A bath at 8 g/L is not a bath at 2 g/L with more silver in it; it is a bath whose remaining capacity for fibre paper is zero. That is an incompatibility between the bath and the work, and it is the one this sheet spends the most words on.

✕What sections 7.1 and 10.5 of the sheet say, and why they are not the last word

Section 7.1, precautions for safe handling, reads “Not known.” Section 10.5, incompatible materials, reads “Not known.” Section 7.2 gives incompatible materials as “None known.”

Read literally, that says an ammonium thiosulphate concentrate has no handling precautions and no chemical incompatibilities, which is false in a room whose previous tray is an alkaline developer and whose waste bottle may hold an acid.

A safety data sheet answers a narrow question — what makes this product itself unstable or dangerous in its own container — and by that standard the answers are defensible. It is not a compatibility chart for a process. HARMAN’s own technical sheet is the operative instruction here: it tells you to wear gloves and eye protection, to keep the fixer away from hardeners, to correct pH with acid only gradually and within limits, and to watch for hydrogen sulphide at a recovery cell. The course’s incompatibilities page governs the rest.

Waste

This is a silver-bearing stream, and that single fact outranks everything the safety data sheet says about disposal. Section 13 of that sheet reads “Dispose at suitable refuse site” and section 13.2 adds “No special precautions are required for this product” — and both are statements about the product as supplied, an unused concentrate in a sealed bottle. A used fixer bath is a different material: it contains up to 8 to 10 g of dissolved silver per litre by ILFORD’s own film figure, and silver is the reason a photographic effluent is regulated at all.

Bottle it separately, and never into the stop-bath bottle. ILFORD’s instruction to domestic users is that chemicals should be bottled separately and appropriately labelled and taken to the chemical cupboard at a Household Waste and Recycling Centre, with a note that a few authorities collect on request; its instruction to business users is stronger — different waste chemicals should not be mixed, and should be kept in separate, appropriately labelled containers. The silver-bearing waste SOP is the course’s procedure and the general chemical waste SOP covers the rest.

Recovery is a real option and ILFORD documents it properly. Electrolytic recovery, off-line or in-line; treated fixer that can be recycled or reused; 50 to 100 ppm of silver achievable in a well-set-up system; secondary and tertiary ion exchange and metal exchange on the fix and wash overflows reaching about 3 ppm, which the sheet says allows processing waste discharges to meet the most demanding effluent controls. It also gives the failure mode — over-current, sulphiding, hydrogen sulphide — and tells you to consult the equipment supplier rather than a fixer sheet for the settings.

What the sheet says about the environment. Low toxicity to aquatic invertebrates, to fish and to algae; other adverse effects, none known; not classified as a marine pollutant; and, from section 6.2, do not release large quantities into surface water or into drains. That is a mild ecological profile for the mixture as supplied, and it is not a licence for a silver-loaded bath.

The disposal caveat governs and local regulation decides. A silver discharge consent is a jurisdictional matter, and no manufacturer’s sheet — including this one — can settle it for your drain.

Troubleshooting

Film that comes out milky or opalescent. Under-fixing. Run the clearing test before doing anything else: if the clearing time in your bath is more than twice what it was fresh, the bath is finished and no amount of extra time will rescue it. Under-fixing and exhausted fixer are the entries, and Part XI’s break/fix page walks the diagnosis.

Prints that yellow or stain months later. Residual silver, residual thiosulphate, or both — which is to say a bath that went past its silver limit, a wash that was short, or the two compounding. ILFORD’s sulphide test on a washed print is the discriminator it publishes; the residual hypo and silver SOP is the procedure and Kodak’s HT-2 is the open-formula equivalent of the test solution.

A bath that has gone cloudy or smells of sulphur. The pH has been driven too far down, usually by a strong acid getting in or by a tray that stood and concentrated. The sheet’s own warning is not to lower the pH past the published limits when correcting it. Discard it; sulphur in a bath does not redissolve.

A whiff of ammonia over the tray. Alkali has got in — a carbonate developer splashed across, or a neutralisation done into the wrong bottle. Ventilate, and treat the bath as finished.

The count says the bath is fine and the clearing time says it is not. Believe the clearing time. ILFORD’s capacities are for ILFORD’s own materials at ILFORD’s own carry-over; yours are what you measure. A fibre printer’s bath dies at half the sheet count of a resin-coated printer’s, and both figures on the sheet are correct.

Specific gravity drifting up in a warm processor. Evaporation. Top up with water, not with concentrate — the opposite correction to a bath reading low.

A fixing time or a dilution carried over from a fixer you used to buy. The commonest way to under-fix. A dilution, a time and a capacity belong together and to one bottle: Foma’s FOMAFIX, also a liquid rapid fixer concentrate, is diluted 1+5 for manual processing and 1+4 only for automatic, and its published capacity is 17 films of 135-36 per litre against ILFORD’s 24. Part XI’s survey makes the point at length.

A bath mixed at the wrong strength. Sixty millilitres into 240 is 1+4; sixty into 300 is not, and neither is 120 into 600 read carelessly off the jug. A misread dilution ratio is the entry, and the fixer mixing SOP is the procedure that prevents it.

Small clear or purple circles on a film. Air bells at the moment of pouring, not a fixer fault — purple spots from fixer air bubbles is the entry, and the fix is the tap on the bench that ILFORD’s own beginner sheet asks for after each inversion.

A print-out image that vanished in the fix. Not a fault either: this is an acid fixer doing what acid fixers do to finely divided silver. The entry is here, and the answer is a different bath.

Experiments

Measure the working pH and specific gravity against ILFORD’s windows. Mix a litre at 1+4, calibrate a meter that day with the calibration SOP, and read pH and SG within the hour. You are checking two published numbers, 5.0–5.5 and 1.070–1.080, and — more usefully — you are creating the reference measurement ILFORD tells you to create, against which every later bath of yours is judged.

Test the buffering claim this page marks as an inference. Measure the pH at 1+4, then dilute a sample to 1+9 and measure again. ILFORD says both are 5.0 to 5.5. If your two readings agree while the specific gravity halves, you have reproduced the observation the inference rests on. Then add measured increments of used print developer to a known volume and plot pH against added volume: the shape of that curve is the acid reserve, in the only units that matter, and it is a real measurement of a product whose composition you do not know.

Find your own clearing time and your own fixing time. ILFORD’s method, on this sheet, in normal light: a drop on scrap film, thirty to sixty seconds to a clear spot, then immerse and time the rest. Double it. Do it for each film you use, because “general purpose film, 2 to 5 minutes” is a range that contains your answer and does not give it. The clearing-time SOP is the procedure.

Run the capacity down against the published 24 films per litre. Fix films through one litre of 1+4, re-measuring the clearing time every few films, and stop when it passes twice the fresh figure. Part XI’s experiment sets out the protocol. ILFORD says 24; the interesting question is where your curve turns and what your carry-over did to it.

Test the claim that 1+9 gives the same capacity per litre as 1+4. Two litres of working solution, one at each dilution, matched prints, each fixed for its own published time, clearing time or a residual-silver test as the endpoint. It is one of the few places in this course where a manufacturer publishes a figure that invites a direct experiment, and it decides whether the cheaper dilution is genuinely cheaper.

This against F-24, mixed from scratch. Same film, same developer, same stop, same wash; measure the clearing time in each. You will have measured, on your own bench, the difference the ammonium ion makes — and given F-24 the published fixing time Kodak never printed for it. Record both in the formula version record; a bought product gets a line in that record too, with its dilution, its bottle date and how much work the bath had already taken.

Two-bath against one-bath, on fibre paper, judged by the sulphide test. Fix one set of prints in a single bath run to ILFORD’s fibre capacity and another set through two baths for half the time each, wash both identically, and read the test spots against a reference print. The two-bath rotation SOP is the procedure. This is the experiment that turns the archival advice from a rule into a result.

Weigh the carry-over that exhausts the bath. Drain ten sheets of resin-coated and ten of fibre for a fixed count, weigh them wet against dry, and compare the ratio with ILFORD’s two-to-one capacity ratio between the papers. If your ratio is different, you have found the reason your capacity does not match the sheet’s — and, incidentally, measured the third of ILFORD’s three exhaustion mechanisms.