ILFORD ILFOSTOP

This is a product page, not a formula page. Every stop bath elsewhere in this library arrives as a quantity with a source beside it — seventeen millilitres of glacial acetic acid, nineteen point two grams of citric acid — and an account of why that quantity and not another. Nobody outside Harman knows the quantity in this bottle. What follows is what its maker publishes, which for a stop bath is a surprising amount and is spread across five of HARMAN’s own sheets; what its maker’s safety data sheet discloses, which is two substances and no more; what the two documents together still leave unknown, including the one ingredient the product is actually sold on; and the open citric 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
Citric acid (anhydrous)named on the sheet as Citric acidCAS 77-92-9The acid, and the one component whose presence ILFORD states outside the hazard sheet — its own description of the product is "a low odour citric acid stop bath"	10-30%	H319, H335
2-phenoxyethanolCAS 122-99-6	1-5%	H302, H319

Not disclosed. HARMAN publishes no composition for ILFOSTOP. The safety data sheet names two components, citric acid and 2-phenoxyethanol, and gives each the concentration band it needs to classify a hazard, which is not an assay: section 11 records that every classification on the sheet was reached by calculation from those bands. It names nothing else at all — not the water, and not the pH-sensitive indicator dye that ILFORD's own technical sheet says the product contains. No acid concentration, no molarity, no working-solution pH, no identity for the dye, no pH at which the dye turns, no function for the 2-phenoxyethanol and no formulation date appears anywhere in ILFORD's literature. A reader who wants to know how many moles of acid are in the tray, or to change that number and watch the capacity move with it, has to mix an open formula rather than buy this one.

Nearest open formula. Citric Acid Stop Bath — Both are citric acid stop baths, and neither one's concentration is published — ILFORD's because the formulation is a trade secret, the course's because the course never had a manufacturer's figure to publish and would not invent one. What differs is which half you can see. ILFOSTOP arrives as a concentrate at pH 2.1 with one published dilution, a published time, three published capacities, a five-year keeping life and a dye that reports its own exhaustion, and its acid content appears on no document HARMAN prints. The course's bath is 19.2 g of anhydrous citric acid made up to a litre — a molarity you chose, can check and can change — with no published capacity, no published working pH and no indicator whatever. One is a measured performance from an unmeasurable composition; the other is a known composition with unmeasured performance, and Part X's capacity experiment is the bench where the two are put side by side.

Purpose

To end development immediately, and to protect the fixer that follows. ILFORD’s own sentence puts the two jobs in that order: “After development we recommend that films and papers are rinsed in an acid stop bath to stop development immediately and neutralise the developer to help maintain the activity of the fixer bath.” The second clause is the one that pays for the tray. A print or a film carries a measurable volume of alkaline developer into the next bath, and an acid fixer that keeps being fed alkali does not last as long as it should — ILFORD says so directly, that ILFOSTOP “helps to maintain the activity and prolong the life of the fixer solution by reducing carry over of excess developer (alkaline) into the fixer bath (acidic)”.

Three words in the description carry the product. Low odour: it is a citric bath rather than an acetic one, and in a small dark room that is not a marketing point but the difference between a session you can breathe through and one you cannot. Citric acid stop bath: five words that are the only statement of composition HARMAN makes anywhere, and they name a substance without naming an amount. Indicator dye: it tells you when it is finished, which is the one thing an ordinary stop bath will never do, because a spent stop bath looks exactly like a fresh one.

That last property has a longer history than the product. Kodak’s own handbooks sold a separate testing solution that turned from yellow to purple when a bath’s acidity fell too low to stop development — the SB-5 page has the story — and warned that the appearance of a stop bath does not change until well beyond its useful life. ILFOSTOP’s contribution is to put that test inside the bath.

Recommended uses

Dish or tray processing of paper, and deep tank processing of film, at 1+19 and about 20 °C. Those are the two applications ILFORD names in the phrase “specifically recommended for”, and they are the two where the immersion is long enough that a trace of dye left in the emulsion will wash back out again.

Film in a spiral tank, for ten seconds with two inversions. ILFORD’s beginner film sheet is the only document in the corpus that gives an agitation instruction for a stop bath: pour it in at 20 °C, turn the tank upside down twice, pour it out after ten seconds. The volume must completely cover the spiral. The same sheet says the time “is not critical. It must be at least 10 seconds”, which is the plainest statement anywhere in ILFORD’s literature that ten seconds is a floor and not a target.

Prints in a dish under a safelight, where the dye earns its place. ILFORD’s own qualification is that the colour change “can be useful when dish/tray processing paper” and “can be ignored for replenished machine processors”. A tray is a large, evenly lit, repeatedly observed sample and the eye is good at a hue shift it has been told to expect; a machine’s tank is none of those things.

Across more than one session. Both beginner sheets say it explicitly — the stop bath and the fixer can be made up and used again, and after processing you can store the solution to use it again. That is a genuine difference from a print developer, which is mixed for the evening and poured away, and it is why the seven-working-day figure exists at all.

Not for machine processing — and here HARMAN’s own literature disagrees with itself, which is set out in full below. The reason given on the sundries sheet is specific and worth taking seriously whatever machine you are using: the short fix and wash times of a machine may leave a residual dye stain on films and prints, and the longer times of dish, tray and tank work minimise that risk.

When another formula is preferable

• When you want to understand a stop bath rather than buy one, mix the citric acid stop bath. It is the same acid at a concentration you chose and can change, and it is the comparison this page is built around.
• When the bath is for film in a tank and nothing else, the indicator buys you nothing. You cannot see a stainless steel tank’s contents while the film is in it, and a bath whose only advantage over a hand-mixed one is a colour you never look at is a bath you are paying a premium for. ILFORD’s own answer to that is ILFOSTOP PRO, the same product without the dye and with half again the capacity; the course has no page for it, because HARMAN publishes no composition for that one either.
• When you want the formula with a maker’s name and a maker’s capacity on it, SB-1 is seventeen millilitres of glacial acetic acid in a litre of water, published by Kodak Limited with a rinse time and a useful life.
• After a caustic developer, SB-1a is Kodak’s answer — three times the acid, for a bath that will be fed far more alkali per sheet — and no commercial stop bath is sold in two strengths for that reason.
• When the emulsion must not be allowed to swell, SB-5 carries forty-five grams of sodium sulfate to the litre and stops the film without swelling it, which matters in warm weather and matters nowhere else.
• When the fix and the wash will both be short, the dye is a liability rather than an asset, and the maker says so.
• When no stop bath is possible at all, ILFORD sanctions a water rinse and prices it: the risk of processing-related marks and stains goes up, the water must be completely changed at very frequent intervals, and fewer fixing problems will be seen if the fixer’s activity is monitored and adequate replenishment rates are used. Part X sets out both sides.

What the maker publishes

For a bath that costs a few pence a litre this is a great deal, and the quantity of it is exactly what makes a page about a secret formula useful rather than merely careful. It is also spread across five separate ILFORD sheets, which is why this page reads all five.

One dilution, and only one. 1+19, for film and for paper, in a dish, a tank or a deep tank. There is no second dilution for economy, no second dilution for control and no different strength for fibre paper. Whatever else is uncertain about this product, the strength you use it at is not.

A temperature range rather than a temperature. 18 to 24 °C on the technical sheet, which is wider than ILFORD allows any of its developers, and the beginner film sheet widens it further to about 20 °C, preferably no warmer than 25 and no cooler than 15. A stop bath is a neutralisation and not a rate-controlled reaction; the tolerance is the chemistry showing through the instruction.

Ten seconds, stated as a minimum. “The process time given is the minimum required, a longer time in the stop bath may be used and should not cause any process problems provided it is not excessive.” The qualification that follows is the interesting half: care must be taken, because the indicator dye may give a slight stain to some products, and if staining occurs thorough washing will remove it.

The concentrate’s pH and specific gravity, which is the rarest kind of figure on this page: pH 2.1, specific gravity 1.101 to 1.111 at 20 °C. ILFORD also prints the caveat, and the caveat is better advice than the number — the figures were obtained under carefully controlled laboratory conditions, may differ slightly from measurements made by users in their own working areas, and the right use of them is to make your own control measurement on your own solution and compare later ones against it.

Three capacities, per litre of working solution, unreplenished: 15 films of 135-36, 60 sheets of 8 × 10 inch resin-coated paper, and 30 sheets of 8 × 10 inch fibre paper. The HYPAM fixer sheet adds a fourth that appears nowhere else in the corpus — 0.75 m² (8 ft²) of other specialist, X-ray and graphic art materials.

What a bottle makes. ILFOSTOP is sold in 500 mL bottles of concentrate; one makes 10 litres of working solution, enough for 150 films of 135-36 or 120 format, or 600 8 × 10 resin-coated prints, or 300 8 × 10 fibre prints. All three follow exactly from the per-litre figures times ten, which is not true of every availability paragraph ILFORD prints.

Four keeping figures. Five years for the concentrate in a full airtight bottle. Twelve months in a half-full tightly capped one — the clock starts when the air space appears, not when the seal breaks. Seven working days at working strength. And the safety data sheet’s own laconic entry, storage temperature ambient, storage life stable under normal conditions.

The mixing method, in the order ILFORD does it. Work out how much the session needs, making sure it is enough to fill the dish about half full, cover the spirals completely, or fill the deep tank. Measure the concentrate with the smallest cylinder that will hold it — ILFORD’s own reasoning is that 100 mL is more accurately measured in a 100 mL cylinder than in a 1000 mL one. Add the concentrate to the mixing vessel, and use a large measuring jug so the total is checked as you go. Measure the dilution water with hot and cold to arrive at 20 °C. Rinse the concentrate cylinder out into the mixing vessel with some of that water, then add the rest and stir. At 1+19 a film of concentrate left behind in a cylinder is a larger fraction of the dose than it is at 1+9, which is the general lesson the mixing SOP draws from this paragraph.

Two worked dilutions, on the beginner sheets. 15 mL of ILFOSTOP plus 285 mL of water for a one-reel tank; 30 mL plus 570 mL for a tray. Both are 1+19 and both are given as arithmetic rather than as a ratio, which is a kindness the technical sheet does not extend.

The indicator, described and not identified. “ILFOSTOP contains an indicator dye that is pH sensitive and changes colour from yellow to purple as the stop bath becomes exhausted.” That sentence is the whole of what ILFORD says about it in three separate documents.

The sibling. ILFOSTOP PRO is the same dilution, the same temperature range and the same ten seconds, without the dye, and with capacities of 22 films, 90 resin-coated sheets and 45 fibre sheets per litre against ILFOSTOP’s 15, 60 and 30. That is one and a half times the capacity for the same dilution and the same contact time.

!Three ILFORD sheets, three answers about machine processing

The sundries sheet (December 2017) says ILFOSTOP is not recommended for machine processing applications, because the short fix and wash times often used there may leave a residual dye stain.

The RAPID FIXER sheet (2010) says ILFOSTOP PRO is recommended for all machine processing applications, which is the same policy stated from the other side — use the one without the dye.

The HYPAM sheet (2017) prints the same table and says “ILFOSTOP is also recommended for all machine processing applications”, then adds in the next paragraph that the indicator “can be ignored for replenished machine processors”.

Two of the three agree in substance and the third contradicts them. The course reads the HYPAM line as a dropped PRO — a typographical loss, in a sentence that sits directly above a table whose second column is ILFOSTOP PRO — and that reading is an inference under Rule 7, not a HARMAN statement. What is not an inference is the underlying reason, which all three sheets agree on: the risk is a dye stain, and it is a function of how long the material spends in the fix and the wash afterwards, not of the stop bath’s chemistry.

What is not disclosed

The acid concentration, which is the whole of the formula for a bath with one working ingredient. A stop bath is not a superadditive system with a dozen interacting terms; it is an acid in water, and the single number that describes it is how much acid. That number is on no ILFORD document.

The working solution’s pH. The sheet gives 2.1 for the concentrate and stops. Diluting a weak acid twenty-fold does not raise its pH by the logarithm of twenty, because a weak acid re-ionises as it is diluted, so the working figure cannot be derived from the concentrate figure without knowing the acid’s concentration and its pK_a values — and the concentration is precisely what is withheld. That reasoning is the course’s; ILFORD offers none. The course’s own citric bath, whose molarity is known, has a computed starting pH near 1.96 in Part X’s capacity experiment, and nothing licenses transferring it to this one.

The dye. Not its identity, not its concentration, not the pH at which it turns, and not the direction of the change in pH units. It appears on the technical sheet as a property and on the safety data sheet not at all — a substance present in the mixture that the composition table does not name, because at its concentration it does not have to be. Part X records that bromocresol purple’s published transition matches the described colour change and refuses to call that an identification; nothing here revises that.

What the 2-phenoxyethanol is for. The sheet names it, bands it and classifies it. It offers no function, and neither does any other ILFORD document.

Everything measurable in section 9. Appearance is “Liquid”. Colour: Not known. Odour: Not known. pH: Not known. Density, solubility, viscosity, decomposition temperature: all Not known. The company that publishes a concentrate pH of 2.1 and a specific gravity to four significant figures in its technical sheet declines to publish either in its hazard sheet, and the company that sells the product on being low odour and on changing colour records both odour and colour as unknown.

!Why the two bands cannot be added up

There are only two numbers in the composition table, and the temptation to treat them as an assay is correspondingly stronger. Do the arithmetic once and it goes away.

Citric acid is banded at 10–30 % and 2-phenoxyethanol at 1–5 %. Taken at their lower limits the two named substances account for 11 per cent of the bottle; taken at their upper limits, 35 per cent. So somewhere between 65 and 89 per cent of what is in the bottle is not named by any substance on the sheet — most of it water, on any sensible reading, and some of it the dye that the technical sheet says is there. Both totals are the course’s arithmetic on ILFORD’s own printed figures, and neither is a composition.

Worse for the reconstructionist: the band is a factor of three wide. A working solution at 1+19 built from a 10 per cent concentrate holds a third of the acid of one built from a 30 per cent concentrate, and those two baths would have visibly different capacities and measurably different pH values. The band is not a range of manufacturing tolerance. It is a band chosen so that the mixture’s classification can be calculated conservatively, and section 11 confirms the calculation is exactly what happened.

A safety data sheet read as a hazard document is evidence. Reassembled into a recipe it is a fabrication with citations attached, and this page does not attempt it.

What can be constrained, and what cannot. Part X’s capacity experiment measures the volume of developer a bath will absorb before it stops working, and from a published films-per-litre figure and your own measured carryover you can bound ILFOSTOP’s proton reserve. You still cannot get its concentration, because you do not know how many of citric acid’s three protons its working pH range actually delivers. Knowing exactly how far a coincidence of numbers licenses you to go is the difference between a result and an anecdote.

Disclosed components

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

Citric acid, CAS 77-92-9, EC 201-069-1, 10-30 %. This is the acid, and it is the one component whose presence the maker states outside the hazard sheet: the technical sheet’s description is “a low odour citric acid stop bath”, so naming citric acid as the working substance is ILFORD’s statement and not a reading of a hazard table. It is also the entry that carries the product’s classification — Eye Irrit. 2, H319, and STOT SE 3, H335, with pictogram GHS07 — and the mixture’s own classification is H319 alone, so the respiratory-irritation statement does not survive dilution into the mixture at this band. Three properties of the substance matter here and its encyclopaedia page has the rest. It is triprotic, which is why a citric bath holds a large neutralising reserve for its mass. It is solid and non-volatile — the International Chemical Safety Card records evaporation at 20 °C as negligible and 59 g dissolving in 100 mL of water — which is the whole of the low-odour claim. And its solution attacks metals, which is why the tray, the funnel and the storage bottle are plastic or glass. The aggregated GHS picture is milder than the sheet’s: across 4,373 ECHA notifications PubChem records H319 in 84.7 per cent and H335 in 23 per cent, with 359 reports finding it meets no GHS criterion at all.

2-phenoxyethanol, CAS 122-99-6, EC 204-589-7, 1-5 %. The course has no encyclopaedia page for it, because outside this bottle it is not a photographic chemical. It is a glycol ether: an oily colourless liquid with a characteristic odour, relative density about 1.11, dissolving in water to about 2.6 g per 100 mL at 20 °C. Its own aggregated classification is harsher than the sheet’s line suggests — signal word Danger, with H302, H318 and H335 across 4,299 ECHA notifications, Acute Tox. 4 in 99.8 per cent of them — and the reason ILFOSTOP is nevertheless labelled Warning with H319 alone is that the substance is present below the concentration at which those statements carry through into a mixture. That relationship, between a component’s classification and a product’s, is the whole subject of the GHS and CLP page.

What it is doing there, HARMAN does not say, and neither will this page. The public record gives the substance two industrial identities that would each be at home in this product — PubChem lists it under the EPA’s Safer Chemical functional use classes as a preservative and as a solvent, and HSDB records its use as a solvent for inks and dyes. A citrate solution that must keep for five years in a bottle and seven days in an open tray has a preservation problem; a bath that carries a dye has a dye-solubility problem. Both readings are plausible and neither is an identification: the maker states no function, the two roles are not mutually exclusive, and a page that picked one would be reconstructing a formulation by inference. It is named, it is banded, and that is the end of what is known.

And one component that is in the bottle and not in the table: the dye. ILFORD says the product contains a pH-sensitive indicator; the composition table names citric acid and 2-phenoxyethanol and nothing else. There is no contradiction — a safety data sheet lists the substances that drive the classification, not the contents — but it is worth stating plainly, because it is the clearest possible demonstration of what these tables are and are not. The most distinctive ingredient in the product does not appear on the document that lists its ingredients.

Behaviour

Ten seconds is a floor, and the plateau above it is wide. ILFORD calls the published time the minimum required and says a longer time causes no process problems provided it is not excessive. The only cost of over-immersion that the maker names is cosmetic and reversible: a slight dye stain on some products, removed by thorough washing. Compare that with a developer, where the time is a point on a slope, and the difference tells you what kind of reaction is happening in the tray.

It exhausts by counting and reports by colour. The counting figures are 15 films of 135-36, 60 resin-coated 8 × 10 prints or 30 fibre 8 × 10 prints to the litre. The reporting is the dye going yellow to purple. Notice that the fibre figure is exactly half the resin-coated one, which is the signature of a capacity set by carryover rather than by anything about the paper: fibre base holds about twice the solution that a resin-coated sheet does, and Part X’s experiment turns that observation into a measurement.

Seven working days is the other limit, and it is independent of the count. A tray that has taken ten prints in a week is finished on the calendar even though it is a sixth of the way through its capacity. ILFORD gives no reason. The concentrate’s five-year keeping and the working solution’s seven days are three orders of magnitude apart, which is a large enough gap to be about the dilution itself rather than about the acid.

The indicator is a warning device and a poor gauge. A dye transition is spread over more than a pH unit, it begins only after the bath has climbed well past its starting point, and the reading is made by eye under a safelight. Against that, it is the only monitoring most darkrooms will ever do. Part X makes the argument in full; the practical form of it is that a bath that has turned is certainly finished, and a bath that has not turned is not certainly fresh.

Low odour is a real property, not a claim about strength. The bath is exactly as acid as it needs to be; what it does not do is put acid vapour into the room, because the acid is a solid dissolved in water rather than a volatile liquid. In a windowless darkroom over a three-hour printing session that is the difference the product is sold on.

It is reused, and it is the only bath in an ILFORD beginner sequence that is. The developer is mixed for the session and poured away; the stop bath and the fixer are stored and used again.

Image characteristics

A stop bath has none, and saying so plainly is worth a paragraph. It dissolves no silver halide, removes nothing from the emulsion, changes no image tone and makes no print more permanent. Every tonal property of the finished print was decided in the trays either side of it. What this bath protects is the reproducibility of those decisions: a development that runs on for an unmeasured extra twenty seconds in a water rinse is a print whose development time you do not know.

It has, uniquely among the stop baths in this library, one documented way of leaving a mark. The indicator dye can deposit a slight stain on some products where the immersion is excessive or the fix and wash that follow are short. ILFORD’s remedy is thorough washing, and its prevention is the reason the product is not offered for machine processing. This is a processing artefact and not an image property, and the place to chase it is the troubleshooting index rather than a characteristic curve.

The mechanism

The stopping. A developing agent works as its anion, and the fraction of it that is ionised is set by the pH of the bath against the agent’s pK_a. Drop the pH by seven or eight units and the population of the active species collapses; development does not slow, it switches off. Part X works the arithmetic through and this page does not repeat it. The rate limit is diffusion — the acid has to get through the gelatin — which is why the answer is ten seconds and not ten milliseconds, and why the maker asks for agitation on the way in.

The reserve, and why a triprotic acid is a good choice for it. The design rule is the one SB-1’s page quotes from Kodak’s 1928 handbook, and it has not been improved on: the acidity of a solution depends on how much of the hydrogen is dissociated, but the quantity of alkali an acid can neutralise depends on the total hydrogen present. A stop bath is a neutralising job, so what it wants is a large quantity of a weakly dissociated acid. Citric acid carries three ionisable protons per molecule against acetic acid’s one, so a citric bath holds three times the reserve per mole. How many moles ILFOSTOP holds is exactly the number HARMAN does not publish, so the reserve can be bounded from the published capacity and cannot be calculated from the composition.

The fixer protection. An acid fixer works within a pH window. Alkaline developer carried in on film or paper walks it upward, and past the top of that window the fixer starts to fail in ways that show up on the print rather than in the tray — Part XI has the fixer side of it. Ten seconds of acid between the two baths removes most of the carried-over alkali before it arrives. ILFORD’s alternative, where a stop bath cannot be used at all, is to monitor the fixer’s activity and raise the replenishment rate, and its remedy for a fixer that has already gone alkaline is a few drops of 50 per cent acetic acid with thorough stirring — added gradually, and not past the published pH limits.

The dye, as far as the evidence goes and no further. A pH-sensitive dye that runs from yellow to purple as a bath exhausts is behaving the way a sulfonphthalein indicator behaves, with a transition somewhere above the working pH of a fresh bath and below the pH at which stopping fails. That is a statement about the class of compound the described behaviour belongs to, and it is an inference of the course’s, marked as one under Rule 7. HARMAN names no dye. Part X records that bromocresol purple’s published range matches the description and declines to call it an identification, and this page holds the same line.

A caution about what “pH 2.1” licenses. It is the concentrate’s figure, and a stop bath is not used as a concentrate. It tells you the concentrate is strongly enough acid to be worth respecting in the bottle. It does not tell you the working pH, it does not tell you the concentration, and any calculation that runs from it to a molarity has assumed the answer somewhere in the middle.

The nearest open formula

The course’s citric acid stop bath, and the comparison is the reason this page exists.

That bath is 19.2 g of anhydrous citric acid made up to a litre, and it is not a reconstruction of this product. It was derived from Kodak’s SB-1 by matching titratable protons: SB-1 carries about 0.300 mol of a monoprotic acid, citric acid has three ionisable protons, so 0.100 mol of it — 19.2 g at 192.12 g/mol — carries the same reserve. The arithmetic is published on that page and can be checked or rejected. Nothing in it came from ILFORD.

	The course’s citric bath	ILFORD ILFOSTOP
Composition	19.2 g of citric acid per litre, published with its derivation	Citric acid at 10-30 % of a concentrate, and 2-phenoxyethanol at 1-5 %; the amounts are not published
Acid	Citric, and you chose the molarity	Citric, named by the maker; molarity not published
Preservative	None	2-phenoxyethanol named, function not stated
Indicator	None, and the course will not invent one	Present, described as yellow to purple, not identified
How it is made	Weighed and dissolved to volume	1+19 from a concentrate
Published pH	None; a computed starting figure near 1.96 in Part X	2.1 for the concentrate; nothing for the working solution
Published time	None; ten seconds is a starting point taken from published figures for other baths	10 seconds at 20 °C, given as a minimum
Published capacity	None	15 films, 60 RC or 30 FB sheets of 8 × 10 per litre
Keeping	Not published	5 years sealed, 12 months half full, 7 working days in use
What you can change	The molarity, and therefore the reserve and the pH	The temperature and how long you leave things in it

The last row is the argument. Buying the concentrate gets you a documented performance — a capacity, a keeping life, a working temperature range and an exhaustion signal — from a bath whose one important number is unknowable. Mixing the open bath gets you that number, and nothing else: no capacity, no keeping figure, no indicator, and a working pH the course computed rather than measured. Part X’s capacity experiment is where a reader turns one into the other, by measuring on their own bench what neither document gives them.

Safety

Level A, and the reasoning belongs on the page rather than in a database.

HARMAN classifies the mixture Eye Irrit. 2 and nothing else: pictogram GHS07, signal word Warning, one hazard statement, H319, causes serious eye irritation. Section 2.3, other hazards, reads none known. That is the shortest classification of any product in this formulary, and it is what places the page at Level A on the classification rubric: the criterion is substances that are at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled, and an eye irritant at the top of the sheet with a harmful-if-swallowed component below the label threshold sits inside it.

The concentrate is the part to respect. It is where the acid is at ten to thirty per cent and where the whole classification lives. Decant it over the sink. At 1+19 the working bath is a dilute citric solution; the concentrate is not.

Personal protection, as the sheet specifies it. Eye protection with side protection to EN ISO 16321-1. Impervious gloves to EN 374, with the breakthrough time taken from the glove maker rather than assumed — the glove selection page is where that goes. Protective clothing. Hands and exposed skin washed thoroughly after handling.

Ventilation and eyewash. The sheet asks for adequate ventilation and for a washing facility or water for eye and skin cleaning to be present. It also records that normally no personal respiratory protection is necessary, and that no occupational exposure limit is assigned to anything in the product — two statements you will not find on an acetic stop bath’s paperwork.

First aid, in the sheet’s own words. For inhalation, skin contact and ingestion: treat symptomatically. For the eyes, which is the only route the product is classified for: rinse cautiously with water for several minutes, remove contact lenses if present and easy to do, continue rinsing, and if irritation persists get medical advice. The first-aid page has the course’s fuller procedure and the eyewash SOP has the technique.

In a fire, the sheet states the product may decompose giving off toxic and irritant vapours, and directs extinguishing media as appropriate to the surrounding fire.

⚠The hazard that is not in this product's classification

Nothing on ILFOSTOP’s safety data sheet mentions sulfur dioxide, and nothing needs to: the gas is not a property of the stop bath, it is a property of acid meeting sulfite or thiosulfate. That meeting happens in a darkroom whenever a stop bath is carried into a fixer, poured into a waste bottle that already holds spent fixer, or splashed into a developer holding sodium sulfite.

The incompatibilities page has the reaction and the limits, which are among the tightest gas limits in this course. The control 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 over-concentrate by standing.

Note also what a low-odour bath costs you here. An acetic bath announces itself. A citric one does not, so the smell that would have told you a tray was over-strength or that a waste bottle had been mixed is absent. A property that makes the session pleasanter also removes a warning.

Storage

The concentrate, capped, upright, at ambient temperature and out of reach of children, which is the safety data sheet’s own storage instruction and the only one it gives. ILFORD’s technical figures are five years in a full airtight bottle and twelve months once the bottle is half full and tightly capped — the clock is started by the air space, not by the seal, so a 500 mL bottle bought and half used is on a twelve-month life from that point.

Buy for the darkroom you have. A 500 mL bottle makes ten litres of working solution. If a year’s printing is thirty prints, the twelve-month figure will run out long before the bottle does.

The working solution is stored, and this is the exception. Seven working days in a capped bottle or a covered tray, reused across sessions, which the beginner sheets state and the technical sheet quantifies. Both limits apply at once: the count and the calendar, whichever arrives first.

Label everything, with the product, the dilution and the date mixed, using the labelling SOP. Three trays of clear liquid under a safelight are indistinguishable, and this one is the only one that will eventually tell you its own age by turning purple — which is not a substitute for a label, because a fresh bath and a mislabelled fresh bath look the same. An unlabelled or undated container is the failure that costs a session.

Keep the metal out of it. The safety card records that a citric acid solution attacks metals, so the storage bottle, the funnel and the tray are plastic or glass. This applies to the working solution as much as to the concentrate.

Incompatibilities

Developer, in both directions, and ILFORD says so in one line of its own. “Do not let developer become contaminated with a stop bath solution.” A few drops of acid carried back on a pair of tongs does not merely dilute a print developer, it moves it out of the pH region where it works at all. The same sheet asks for all utensils, measuring and mixing vessels to be thoroughly washed after use, and for dedicated equipment wherever possible.

Fixer and sulfite, because of the gas. Acid meeting sodium thiosulfate or sodium sulfite releases sulfur dioxide, and the darkroom arrangement that does it fastest is a waste container holding both. The incompatibilities page governs. In the process sequence the transfer is intended and controlled — a small carried-over volume into a large acid fixer — and that is a different thing from mixing the two in a bottle.

Metals, per the International Chemical Safety Card: a citric acid solution in water is a medium strong acid and attacks them. No metal trays, no metal funnels, no metal storage.

Alkali of any kind, including a bicarbonate neutralisation done carelessly. If a spent acid bath is to be neutralised before disposal, sodium bicarbonate is the reagent and slow addition is the technique, because the mixture foams. That procedure belongs to the disposal page and to local regulation, not to a manufacturer’s sheet.

✕What sections 7.2 and 10.5 of the sheet say, and why they are not the last word

Section 7.2 gives incompatible materials as “None known” and section 10.5 gives them as “Not known”. Read literally, that says a citric acid concentrate has no chemical incompatibilities, which is false in a room where the next tray is a thiosulfate and the previous one is an alkali.

A safety data sheet answers a narrow question — what makes this product itself unstable or dangerous, in its own container — and it answers it well. It is not a compatibility chart for a process. The course’s incompatibilities page governs here, and ILFORD’s own technical sheet, which does tell you to keep developer and stop bath apart and to wash every vessel, is the operative instruction.

Waste

Bottled separately, and never into the fixer bottle. Spent stop bath is an acidic stream carrying oxidised developer components and a dye. Spent fixer is a silver-bearing stream that is worth money at a refiner and that must not meet an acid. ILFORD’s own instruction to domestic users is that chemicals should be bottled separately and appropriately labelled, and its instruction to business users puts the same rule more strongly: different waste chemicals should not be mixed, and should be kept in separate, appropriately labelled containers.

What the safety data sheet says to do with it: send it to a licensed recycler, reclaimer or incinerator, or dispose at a suitable refuse site, in accordance with local, state or national legislation. Section 6 adds that the material and its container must be disposed of in a safe way, and that a spillage is adsorbed onto sand, earth or another suitable adsorbent and transferred to a container for disposal.

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. That is a milder ecological profile than any developer in this library, and it is not a licence — the pH is still the issue, and a sewer authority’s window is set on pH before it is set on toxicity.

What ILFORD tells United Kingdom domestic users: bottle each waste chemical separately, label it, and leave it in the chemical cupboard at a Household Waste and Recycling Centre; a few authorities will collect on request.

The dye is the one thing that makes this stream visible, and a purple bottle on a shelf is a useful reminder that a spent stop bath is a waste stream rather than water.

The disposal caveat governs and local regulation decides. The general chemical waste SOP gives the procedure this course follows.

Troubleshooting

A faint stain on prints or films that was not there before. The indicator dye, left behind because the immersion was long or the fix and wash were short. ILFORD’s remedy is thorough washing, and its prevention is to keep the stop bath to its ten seconds and the wash to its full time. If the stain appears on toned work rather than on plain prints, start at mottle revealed after toning and uneven toning.

Development visibly continuing after the material goes into the bath. The bath is exhausted and has gone alkaline. On this product it should have warned you first, so ask a second question: has it turned purple and been used anyway, or has it failed without turning? The first is a discipline problem, the second is worth investigating, because a dye that has not moved on a bath that has stopped working is a dye whose transition sits below the pH where stopping fails.

A count that disagrees with the colour. The published figures are ILFORD’s for ILFORD’s own materials; yours are what you measure. A fibre printer’s bath dies at half the sheet count of a resin-coated printer’s, and both are correct.

The fixer going alkaline, or prints staining later. Check the stop bath before you blame the fixer: an exhausted stop bath is upstream of every fixing problem. Exhausted fixer, under-fixing and residual thiosulfate all have their own entries, and Part XI’s break/fix page walks the diagnosis.

A bath mixed at the wrong strength. Fifteen millilitres into 285 is 1+19; fifteen into 300 is not, and neither is thirty into 600 read carelessly off the tray. A misread dilution ratio is the entry, and the stop bath preparation SOP is the procedure that prevents it.

A tray that has been standing. Seven working days is the limit regardless of how few sheets went through it, and a tray left uncovered also concentrates by evaporation, which pushes the pH the wrong way and makes the dye’s reading meaningless.

Streaks on a print that stop at the tongs’ reach. Not this bath. Agitation, and usually a sheet left floating rather than pushed under — mottled development and squeegee lines on prints are the entries.

Experiments

Measure the working pH ILFORD does not publish. Mix a litre at 1+19, calibrate a meter that day using the calibration SOP, and record the figure. It is the single number the maker withholds that you can obtain in five minutes, and every later measurement of your own baths is compared against your own first one.

Titrate the working solution and find its proton reserve. Standard alkali into a measured volume of working bath, plotting pH against added volume, gives you the total titratable protons per litre. That is a real, publishable measurement of the product. It is not its citric acid concentration, because you do not know how many of the three protons the working range delivers, and knowing which of those two things you have measured is the whole point of the exercise.

Run the capacity down against ILFORD’s figure. Add developer to a litre of working bath in measured increments, reading pH as you go, exactly as Part X’s capacity experiment sets out, and convert the exhaustion volume into sheets using your own measured carryover. ILFORD says 60 resin-coated sheets; the interesting question is where your curve turns and why it differs.

Watch the dye against the curve. Do the same titration and record the added volume at which the colour first shifts and the volume at which it is unambiguously purple. Put both marks on the pH curve. You will have measured the transition range of a dye nobody has named, in the only units that matter — how much developer the bath had absorbed when it warned you — and you will find out whether the warning arrives with capacity to spare.

This against the course’s citric bath. Two trays, one negative, matched prints, ten seconds in each, and the same fixer and wash after both. If the prints are indistinguishable, the product’s advantage is entirely in the capacity, the keeping and the indicator, which is a defensible thing to buy. Record it 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.

Test the low-odour claim honestly. SB-1 in one tray and this in another, both mixed to strength, a third person to place them, and the door shut. It is a subjective test and it is worth doing anyway, because the reason to prefer a citric bath in a small dark room is a subjective one and it should be measured on the person it affects.

Check the fibre-to-RC ratio on your own paper. ILFORD’s fibre capacity is exactly half its resin-coated one. Weigh ten sheets of each dry, process them, drain them for a fixed count and weigh them wet. If your carryover ratio is not two to one, you have found the reason your capacity does not match the sheet’s.