Stop Bath Formulations and the Fixer Interaction

Every stop bath does the same job with the same number of moles. What differs is which acid supplies them, what else that acid does while it is in the tray, and what it leaves on the film when it goes into the fixer. Five answers are in common use, one of them being no stop bath at all, and each is defensible on its own terms once you can say what those terms are.

Acetic acid, and the three strengths it is sold at

Kodak’s 1928 primer states the design rule in one sentence and it is still the reason acetic acid is the traditional choice: 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 — so, since a large quantity of a weak acid is what the job needs, the best acid for the purpose is acetic acid. That is the distinction Part III makes as strong against weak, applied to a bath whose whole function is neutralising capacity.

It is sold in three strengths and the differences matter more than the label suggests. Glacial, at about 99.5 per cent, freezes at 16.7 °C — which is where the name comes from, and why a bottle can set solid in an unheated darkroom in winter. 80 per cent and 28 per cent are the classic dilutions; the 1928 primer says 28 per cent is prepared by diluting three parts of glacial acid with eight parts of water, and warns that it must not be confused with a differently made “commercial 28 per cent” acid. Modern photographic stop concentrates are sold at strengths the maker states on the bottle and in its safety data sheet, which is the number to work from.

🧮Working the dilution, and where the density has to appear

Take the target as 1 L of 0.30 mol/L acetic acid, which is Kodak’s SB-1 to within two per cent. 0.30 mol × 60.05 g/mol = 18.0 g of acetic acid in the litre. Everything below is a way of getting 18.0 g into a graduate.

From glacial, by volume. PubChem’s experimental density is 1.049 g/mL at 20 °C, and the acid is about 99.5 per cent, so a millilitre carries 1.049 × 0.995 = 1.044 g. 18.0 ÷ 1.044 = 17.2 mL, which is why Kodak wrote 17. The course does not use this route: see the hazard note below.

From a stated per cent, by mass. A product declared as 28 per cent w/w needs 18.0 ÷ 0.28 = 64.3 g of the solution, weighed. An 80 per cent product needs 22.5 g. This route needs no density at all, which is the point: when you do not have a reliable density for a solution, weigh it. Part II’s balance is more accurate than most darkroom graduates anyway.

Checking Kodak’s own recipe, which is where the correction lives. Three parts glacial to eight parts water: if the parts are volumes, that is 3 × 1.049 × 0.995 = 3.13 g of acid in 3.15 + 8.00 = 11.15 g of solution, a mass fraction of 28.1 per cent. The recipe is measured by volume and the result is quoted by mass, and the only thing that reconciles them is the density. A “per cent” with no basis attached is the commonest silent error in darkroom arithmetic, and Part III has a section on it.

⚠Why this course buys acetic acid already diluted

Glacial acetic acid’s aggregated GHS classification, from 5,076 ECHA notifications on PubChem, is H314, causes severe skin burns and eye damage, in more than 99.9 per cent of reports, and H226, flammable liquid and vapour, in 99.7 per cent. Its flash point is 39 °C and its explosive range is 6.0 to 17 per cent by volume in air. The International Chemical Safety Card adds the sentence that matters in a small room: a harmful contamination of the air can be reached rather quickly on evaporation at 20 °C.

The exposure limits are correspondingly tight. HSE’s EH40 gives acetic acid 10 ppm as an eight-hour average and 20 ppm over fifteen minutes; the safety card’s EU figure is the same and its TLV entry is 10 ppm with a 15 ppm short-term value. A university environmental-health guidance sheet for photographic chemicals puts the practical conclusion first among its precautions for stop baths: purchase dilute solutions of acetic acid rather than concentrated ones.

So the course’s position is explicit. Handling glacial acetic acid is Level B work; the pages in this part are Level A and assume a pre-diluted product whose strength is stated on the label and in its own safety data sheet. A working bath at about 1.8 per cent w/v is a different proposition from the concentrate, and the control that makes it so is having bought it already dilute.

The vapour is also the reason the odour is not merely an aesthetic complaint. Acetic acid smells of vinegar at concentrations far below the exposure limit, so a darkroom that smells strongly of stop bath is telling you something about its ventilation, not about its stop bath.

Citric acid, and what it buys

ILFORD’s sundries sheet describes ILFOSTOP in five words that contain the whole commercial argument: a low odour citric acid stop bath. Citric acid is a solid, it does not evaporate — the safety card records evaporation at 20 °C as negligible — and its aggregated classification is H319 in 84.7 per cent of 4,373 reports and H335 in 23 per cent, with 359 of those reports finding that it meets no GHS criterion at all. Against glacial acetic acid’s H314 and H226 that is not a close comparison.

Three properties come with it and none of them is a marketing point.

It is triprotic. Citric acid’s three pK_a values are 2.87, 4.35 and 5.68 in the IUPAC dataset at 20 °C, against acetic acid’s single 4.76. One mole of citric acid therefore supplies up to three moles of neutralising capacity, and it does so across a broad, stepped range rather than a single plateau. That is the reason a citric bath can hold much less material by weight and still take the same work, and it is exactly what the experiment in this part sets out to test.

C₆H₈O₇ ⇌ C₆H₇O₇⁻ + H⁺

Citric acid's first ionisation, pKa 2.87; two more follow at 4.35 and 5.68

It is a chelator. The citric acid page records citrate’s other identity as a calcium chelator, which is why it turns up in clearing baths and water treatments as well as in stop baths. In a stop bath it is of no photographic consequence most of the time and occasionally useful: hard water’s calcium is held in solution rather than deposited as a scum. It also means a citric bath standing in a metal tray or funnel is attacking it — the safety card lists metals among the materials it attacks — so plastic or glass, always.

It grows things. Citrate is a nutrient, and a dilute citrate solution left standing is a better medium for mould than a dilute acetate one. This is the one comparison on the page the course could not source: no manufacturer sheet, safety data sheet or standard reference read for this course states it, and it is repeated widely enough in darkroom practice to be worth naming and worth distrusting in equal measure. What is published is the working life of the commercial product — ILFORD gives ILFOSTOP working strength seven working days, and the concentrate five years in a full airtight bottle or twelve months in a half-full one — and that seven-day figure is the number to work to, whatever the mechanism behind it turns out to be.

!The sulfur dioxide claim, examined

It is often said that a citric stop bath promotes sulfur dioxide release or pushes a fixer towards sulfurisation. Here is what the sources actually support.

Sourced: acid meeting sulfite or thiosulfate releases sulfur dioxide. The incompatibilities page sets out the reaction and EH40’s limit of 0.5 ppm over eight hours and 1 ppm over fifteen minutes — among the tightest gas limits in this course. A university guidance sheet advises a water rinse between developer and stop specifically to reduce sulfur dioxide formation, and says that a fixer contaminated with acid from the stop bath releases the gas more rapidly.

Not sourced: any statement, in any document read for this course, attributing that to citric acid as a substance. What the chemistry says is that the gas comes out of sulfite when the pH it meets is low enough, and that the pH is set by the acid’s strength and concentration together. Citric acid’s first pK_a is 1.9 units below acetic acid’s, so a citric bath at the same molarity does sit lower — but the two products are not sold at the same molarity, and a working acetic bath mixed twice as strong as it should be will sit lower than a correctly mixed citric one.

So the honest form of the claim is about pH, not about citric acid, and the control is the same either way: mix to the stated strength, do not let a bath over-concentrate by evaporation, and keep acid away from spent fixer. Kodak’s toning manual makes the over-concentration point independently, naming a tray left more than three days or a tank left more than a month.

The indicator, and what a colour can report

An indicator stop bath is an ordinary stop bath carrying a pH-sensitive dye. ILFORD states that ILFOSTOP’s dye changes from yellow to purple as the bath becomes exhausted. Kodak’s environmental guidance states that KODAK Indicator Stop Bath contains acetic acid and has a very low pH of 1.0 as supplied. Neither document names the dye.

Bromocresol purple is the dye whose published behaviour matches the described change: PubChem gives it as a pH indicator across 5.2 to 6.8, yellow at 5.2 and purple at 6.8, with a pK_a of 6.3. The course records the match and stops there. It is not an identification of either product’s contents, and Part III already made the same refusal about ILFOSTOP; nothing here revises it.

What a dye can and cannot tell you follows from the numbers rather than from the brand. A transition spread over one and a half pH units is a soft-edged instrument, the change begins only after the bath has climbed past pH 5 — which the previous lesson showed is most of the way through the reserve — and the reading is made by eye under a safelight. Against that: a tray is a large, evenly lit sample, the eye is very good at noticing a hue shift it has been told to expect, and the alternative in most darkrooms is no monitoring at all. It is a good warning device and a poor gauge, and Kodak’s toning manual recommends it in exactly those terms — as a bath that signals when to change.

One documented cost. ILFORD says ILFOSTOP is not recommended for machine processing, because the short fix and wash times often used there may leave a residual dye stain on films and prints; the longer times of dish, tray and tank work minimise the risk, and where staining does occur, thorough washing removes it. Bromocresol purple itself, if that is what a bath contains, carries H315, H319 and H335 in over 92 per cent of its 101 ECHA reports, which is one more reason not to leave a concentrate open.

No stop bath at all

Two versions of this exist and they are not the same.

The water rinse is the one ILFORD sanctions. Where a stop bath cannot be included, a water bath or water rinse can be substituted — and the sheet then prices it: using a water bath increases the risk of seeing processing-related marks and stains, 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. The fixer sheet says the same thing from the other side about machine processing. Read together, they are not saying the stop bath is optional; they are saying the load can be moved onto the fixer if you are prepared to manage the fixer instead.

There is one honest advantage to a water rinse, and it belongs on the safety side rather than the quality side: a guidance sheet for photographic chemicals recommends a water rinse step between developer and stop bath to reduce sulfur dioxide formation. Less sulfite arriving in the acid means less gas. A rinse before the stop is a different thing from a rinse instead of it, and it costs capacity nothing.

The alkaline-fixer workflow is the second version, and here the course has to be careful. The argument as usually stated is that an alkaline fixer must not receive acid carryover, so a thorough water rinse replaces the stop. The chemistry behind that is sound and follows from what is already established: an acid arriving in an alkaline thiosulfate bath spends the alkalinity that keeps it alkaline, and thiosulfate at low pH decomposes to sulfur dioxide and colloidal sulfur, which is sulfurisation. But no manufacturer or standards document read for this course states the practice, and the course will not present reasoning as if it were a published instruction. Treat it as chemistry that points one way, and follow the instructions supplied with whatever fixer you actually own.

The hardening stop baths, and why they are not reproduced

Kodak Limited’s 1949 handbook carries two: SB-3, a hardening bath of 30 g of potassium chrome alum per litre, agitated for a few seconds immediately after immersion and giving maximum hardening in three to five minutes in a fresh bath; and SB-4, a tropical version adding 140 g of crystalline (or 60 g of anhydrous) sodium sulfate, agitated for 30 to 45 seconds, left three minutes, and replaced after the equivalent of twenty 10 × 8 inch films per gallon or scum marks will result.

They are worth reading for two reasons that have nothing to do with using them.

The first is that they report their own exhaustion by colour, without a dye. The 1949 handbook records that the freshly made bath is violet-blue and keeps indefinitely unused, while a partially used bath deteriorates over a few days to yellow-green; the 1928 primer says the same and adds that when the colour has changed the bath ceases to harden and should be replaced. That is the indicator-stop-bath idea arrived at eighty years earlier by a completely different route — a change in the chromium complex rather than an added dye.

The second is that the 1928 primer records the interference that ends the technique: chrome alum loses its hardening power in the presence of sulfite, and a chromium scum of chromium hydroxide forms on the film if it is not agitated on immersion, produced by the chrome alum meeting the alkaline developer carried over on the film.

☠Historically significant, not reproduced here

This course does not give a hardening stop bath as a procedure. Three reasons, in order of weight.

The chromium policy governs. The course’s ruling permits chromium(III), as chrome alum, at Level B and only as an in-emulsion hardener where a lesson genuinely requires one. A tray of 30 g/L standing open through a printing session is not that, and it puts chromium into a waste stream that has to be collected rather than diluted.

The need has largely gone. ILFORD’s HYPAM sheet states that a fix hardener is no longer generally recommended because modern camera films are sufficiently hardened at manufacture. A bath designed for the soft emulsions of 1928 in a tropical darkroom is solving a problem most readers do not have.

A correction, on the record. This course’s own research plan expected to find SB-1a described as a chrome alum hardening stop bath. It is not. In Kodak Limited’s 1949 handbook SB-1A is an acid stop bath for Reflex plates, 50 c.c. of glacial acetic acid per litre — three times SB-1’s strength and no alum in it. The chrome alum baths are SB-3 and SB-4. And no SB-5 formula appears anywhere on this page, because the mirror of Kodak’s J-1 formulary that this course holds is a scan with no extractable text layer: it cannot be read, so nothing is quoted from it.

What the stop bath does to the fixer

This is the interaction that decides whether the bath earns its tray, and the 1928 primer measured it in a way nobody has improved on for clarity. Its account of an acid hardening fixer has four findings.

There is an optimum, and it is not the maximum. With a given quantity of alum, hardening increases as acetic acid is added up to a maximum, beyond which it decreases until the bath does not harden at all. But a certain minimum quantity of acid is needed to give the bath a long useful life before aluminium sulfite precipitates — and that quantity is usually greater than the one giving maximum hardening. The formula is a compromise between two curves that peak in different places.

So a fixer’s hardening rises and then falls in use. The primer draws the consequence: with the addition of developer, the hardening ability of most fixing baths at first increases to a maximum, beyond which it falls off rapidly. Carried-over alkali is moving the bath along the acid axis.

Too little acid causes two named faults. A white scum of aluminium sulfite on films or prints comes from insufficient rinsing after development, too low an acid concentration in the fixing bath, or insufficient agitation on first immersion. And dichroic fog — yellowish green by reflected light, reddish pink by transmitted — appears if the fixing bath does not contain acid, or is old and carries an excess of dissolved silver; it never occurs, the primer says, in a fresh acid fixing bath or if the film is rinsed before fixing.

Too much acid causes a different one. Thiosulfate at low pH decomposes to sulfur dioxide and colloidal sulfur. ILFORD publishes RAPID FIXER at pH 5.0 to 5.5 and, in the same breath as telling you how to lower a drifted pH with 50 per cent acetic acid, says not to lower it too far, the limits being the published ones. A stop bath keeps a fixer inside that window from the alkaline side; mixing stop bath into fixer drives it out through the acid side.

⚗One reaction, two directions

Na₂S₂O₃ + 2 CH₃COOH → 2 CH₃COONa + H₂O + SO₂ + S

What an acid does to a thiosulfate bath if it goes too far — the milkiness is the sulfur

The same acid that protects the fixer by neutralising alkali on the way in will destroy it if it arrives undiluted. The incompatibilities page calls this the single most likely accident in a home darkroom, because the two liquids are adjacent in the tray sequence and adjacent again in the waste corner. Kodak’s toning manual makes the waste-corner version explicit for sulfide toners, whose combination with a stop or fixing bath generates hydrogen sulfide.

Choosing one

The five options, and the one property that decides each

1. 1Acetic, pre-diluted — decided by vapour: 10 ppm over eight hours, and a smell well below that
2. 2Citric, weighed solid — decided by three protons per molecule and negligible evaporation at 20 °C
3. 3Indicator — decided by whether anything else would monitor the bath
4. 4Water rinse — decided by willingness to manage the fixer instead
5. 5Hardening — decided by the chromium, which stays out of the tray

Every row does the same chemistry. What separates them is odour, hazard, monitoring and waste — which is why the choice is a darkroom decision rather than a chemical one.

For film, in a spiral tank, the case for citric is strongest: ten seconds with the lid on, no vapour in a small room, and a bath you mix from a jar of solid rather than storing a corrosive liquid. The indicator is nearly useless here, because you cannot see the colour of a solution inside a tank without pouring it out.

For prints, in an open tray under a safelight, the indicator earns its place — the tray is a large evenly lit sample and the session is long enough for a bath to be spent within it. Either acid will carry the dye.

On cost, the honest comparison is materials rather than prices, which move. A litre of working bath needs about 18 g of acetic acid or about 19 g of anhydrous citric acid. Both are negligible against a sheet of paper. The real difference is freight and storage: citric acid is bought dry, so you are not paying to ship and store water, and a jar of solid does not have a shelf life measured in months. The planner carries what numbers the course has.

And on capacity, note what ILFORD’s own table shows when the two products are set side by side at the same 1+19 dilution: ILFOSTOP gives 15 films, 60 RC sheets and 30 fibre sheets per litre, and ILFOSTOP PRO gives 22, 90 and 45. Same dilution, same ten seconds, one and a half times the capacity. Nothing but more acid in the concentrate can do that, and it is the cleanest published demonstration that capacity is a quantity of acid rather than a quality of the bath.

Recap

Acetic acid is the traditional choice because the job needs a large quantity of a weak acid, and it is sold glacial, at 80 and at 28 per cent, with the 28 per cent made by volume and quoted by mass — a conversion that needs the density and is the commonest silent error in mixing. The concentrate is corrosive and flammable with a tight vapour limit, so this course buys it dilute. Citric acid is a solid with three ionisable protons, no appreciable vapour, a much milder classification and a chelating habit; the claim that it promotes sulfur dioxide is really a claim about pH, and the claim that it grows mould is one the course could not source. An indicator reports exhaustion late but reports it, which beats not knowing; no manufacturer names its dye, and bromocresol purple’s published 5.2 to 6.8 transition is a match rather than an identification. A water rinse is sanctioned by the manufacturer alongside frequent changes and a managed fixer, and a rinse before the stop cuts sulfur dioxide at no cost in capacity. Hardening stop baths are historical study here, on the chromium ruling. And the whole point of the acid, from the fixer’s side, is to hold it inside a published pH window that too little acid and too much acid both leave, by different faults.

Next: measuring it. Two baths, a meter, and a litre of developer added twenty-five millilitres at a time until the reserve is gone.

Check your understanding

Question 1. A bottle is labelled "acetic acid 28% w/w". You want 1 L of a working bath at 0.30 mol/L. What is the most reliable way to measure the acid, and how much?

• Measure 64.3 mL in a graduate, because 28 per cent solutions have a density close to 1
• Weigh 64.3 g of the solution, because 0.30 mol is 18.0 g of acetic acid and 18.0 ÷ 0.28 = 64.3 g — and no density is needed if you weigh
• Measure 300 mL, because 0.30 mol/L means 30 per cent
• Weigh 18.0 g of the solution, since that is the mass of acid required

Show the answer and why

Answer: Weigh 64.3 g of the solution, because 0.30 mol is 18.0 g of acetic acid and 18.0 ÷ 0.28 = 64.3 g — and no density is needed if you weigh

A per cent by mass converts to a mass directly and to a volume only through a density you would have to look up and may not have. 0.30 mol × 60.05 g/mol = 18.0 g of acid; dividing by the mass fraction gives 64.3 g of solution. Option 4 forgets that 72 per cent of the bottle is water. Option 1 is the trap the whole callout is about: it happens to be close for a dilute solution and is wrong in principle, and the same habit applied to an 80 per cent product, whose density is well below 1.049, would be wrong in practice too.

Question 2. ILFORD publishes ILFOSTOP and ILFOSTOP PRO at the same 1+19 dilution, the same 10 seconds and the same temperature range, but at 15 and 22 films per litre. What does the difference measure?

• A stronger indicator dye in the PRO version
• More acid in the PRO concentrate, since dilution, time and temperature are all held constant and capacity is a quantity of acid
• A lower working pH, which stops development faster
• A difference in the films the two products were tested on

Show the answer and why

Answer: More acid in the PRO concentrate, since dilution, time and temperature are all held constant and capacity is a quantity of acid

This is as close to a controlled experiment as a product table gets: everything the user does is identical and only the concentrate differs, so the extra capacity has to be in the bottle. It cannot be pH, because pH is a ratio and capacity is a total — a bath twice as concentrated is only about 0.15 units lower in pH but has twice the moles. That distinction, between the intensity of the acidity and the quantity of it, is the one Kodak made in 1928 when it chose a weak acid on purpose, and it is what the experiment in this part measures directly.

Question 3. A printer switches from acetic to citric stop and notices a faint scum on prints that were previously clean, in a darkroom with hard water. Which property of citric acid is the most likely explanation, and is it a fault?

• Its three pKa values, which make the bath too acidic — a fault to correct by diluting further
• Its chelating action, which holds calcium in solution rather than depositing it — so if anything it should reduce a hard-water scum, and the cause should be sought elsewhere
• Its tendency to grow mould, which is well documented by the manufacturers
• Its lack of odour, which means the bath cannot be judged by smell

Show the answer and why

Answer: Its chelating action, which holds calcium in solution rather than depositing it — so if anything it should reduce a hard-water scum, and the cause should be sought elsewhere

Citrate is a calcium chelator, so the direction of the effect is the opposite of the one proposed: it should hold hard-water calcium in solution. The exercise is in refusing a plausible story that runs the wrong way. Look instead at the things the sources do name — an over-concentrated bath, insufficient agitation in the first few seconds (Kodak's toning manual attributes a base mottle to exactly that), or a fixer whose acid reserve has drifted, which the 1928 primer ties to a white aluminium sulfite scum. And note that the mould claim in option 3 is precisely the one this course could not source.

Question 4. Kodak's 1928 primer says that adding acetic acid to an alum fixing bath raises its hardening to a maximum and then reduces it, but that the acid needed for a long useful life is more than the acid that gives maximum hardening. What does that tell you about the published formula?

• That the formula is wrong and should be corrected
• That the formula is a deliberate compromise between two properties that peak at different acid concentrations, so changing the acid trades one against the other rather than improving the bath
• That more acid is always better, since useful life matters more than hardening
• That the hardening is unrelated to the acid

Show the answer and why

Answer: That the formula is a deliberate compromise between two properties that peak at different acid concentrations, so changing the acid trades one against the other rather than improving the bath

Two curves against the same axis, peaking in different places, and the published quantity sits between them nearer the life peak. That is why a historical formula is reproduced rather than adjusted: the number encodes a decision about which property to favour, and someone who "improves" the acid content is silently re-making that decision. It also explains a behaviour readers notice and misread — that a fixer's hardening gets better before it gets worse, because carried-over alkali is walking the bath back down the acid axis through the hardening maximum.

Question 5. Which of these is a claim this course states it could NOT source, and therefore does not assert?

• That bromocresol purple changes from yellow to purple between pH 5.2 and 6.8
• That ILFOSTOP is a citric acid stop bath containing a pH-sensitive indicator dye
• That a dilute citrate stop bath is more prone to growing mould in storage than an acetate one
• That ILFORD RAPID FIXER is published at pH 5.0 to 5.5

Show the answer and why

Answer: That a dilute citrate stop bath is more prone to growing mould in storage than an acetate one

The other three are quoted from named documents: PubChem for the dye's transition range and pKa, ILFORD's sundries sheet for ILFOSTOP's description, and ILFORD's fixer sheet for the pH. The mould claim is widely repeated in practice and appears in no manufacturer sheet, safety data sheet or reference work read for this course, so it is named as unsourced and the published seven-working-day life of the working solution is used instead. Knowing which sentence on a page is carrying a citation and which is carrying a habit is most of what reading photographic literature critically consists of.

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