Kodak SB-1a

Fifty millilitres of glacial acetic acid in a litre of water, where SB-1 uses seventeen. Three times the acid, and Kodak’s published capacity is 40 sheets against SB-1’s 90. Those two facts, read together, are the most useful thing in this entry: a stop bath’s life is not a property of the stop bath.

Ingredient	Quantity	Form the source specifies
Water	1000 mL	the acid is added to it
Acetic acid	50 mL	glacial

Purpose

To stop development on a plate that arrives carrying an unusually large load of alkali. Kodak Limited’s header gives it as an “acid stop bath for Reflex plates”; its keeping-properties table gives the capacity as 40 sheets per 160 fluid ounces and annotates it used after D-8. Those are two different statements about what the bath is for, both from the same handbook, and both are recorded here.

Recommended uses

Plates, at working strength. The handbook publishes no immersion time for this formula, so the course supplies none; SB-1’s published five seconds for prints, and its 1928 companion’s one to two minutes for a large batch, are the nearest published figures and are for a different bath and a different material.

The honest recommendation is narrower than the formula looks. A three-times-strength acetic bath is worth mixing when the developer before it is unusually alkaline — a caustic, maximum-contrast developer, which is what D-8 is — and is otherwise three times the vapour, three times the material and three times the waste for a job SB-1 already does.

When another formula is preferable

• For ordinary developers on paper, plates or film, SB-1 is the formula Kodak publishes and the one whose capacity, rinse time and effect on the fixer are all documented.
• For a bath with no vapour, the citric equivalent the course derives from SB-1. At three times the strength, an acetic bath’s vapour becomes the dominant consideration in a small darkroom, which is exactly when the case for a solid acid is strongest.
• For a hardening bath, which this is not — see the correction below.

Mixing

Acid into water, always. Measure the litre of water, then add the 50 mL of acid, and stir.

At this concentration the arithmetic for a pre-diluted product matters more than it does for SB-1, because there is three times as much of it to get wrong:

50 mL × 1.049 g/mL × 0.995 = 52.2 g

Acetic acid delivered

52.2 g ÷ 0.28 = 186 g of the solution, weighed

From a 28 per cent w/w product

52.2 g ÷ 0.80 = 65 g of the solution, weighed

From an 80 per cent w/w product

Weigh the solution rather than measuring it: a per cent by mass converts to a mass directly and to a volume only through a density you would have to look up. Note also that 186 g of a 28 per cent product is nearly a fifth of the finished litre, so at that strength the added water is no longer negligible and the bath should be made up to volume rather than by addition.

⚠Three times the acid is three times the vapour

Everything the SB-1 page says about glacial acetic acid applies here with the concentration trebled. The concentrate carries H314, causes severe skin burns and eye damage, in more than 99.9 per cent of 5,076 ECHA reports and H226, flammable liquid and vapour, in 99.7 per cent; its flash point is 39 °C; and the International Chemical Safety Card records that a harmful contamination of the air can be reached rather quickly on evaporation at 20 °C. HSE’s EH40 sets 10 ppm over eight hours and 20 ppm over fifteen minutes.

A working bath at about 5 per cent w/v is not the concentrate, but it is three times the vapour pressure of the ordinary stop bath standing in the next tray. Cover the dish between plates, work with the extraction running, and keep the bath in a tank rather than an open tray where the process allows. The course’s practice of buying acetic acid already diluted, set out in Part X, applies here too and matters more.

Behaviour

It stops development on contact, like any acid stop bath, and Part X explains why that takes seconds rather than minutes.

Its capacity is less than half SB-1’s despite carrying three times the acid, and that is the whole lesson of the page. Working it out is a small piece of arithmetic on two published numbers:

0.290 mol/L × 4.546 L = 1.32 mol, for 90 sheets = 14.7 mmol per sheet

SB-1, acid in a full bath

0.869 mol/L × 4.546 L = 3.95 mol, for 40 sheets = 98.8 mmol per sheet

SB-1a, acid in a full bath

About six and a half times as much neutralising capacity is spent on each sheet. That is the course’s own arithmetic on Kodak’s two published capacities, and it is arithmetic rather than chemistry: it says how much alkali arrived, not what the alkali was.

⚗What arrives from D-8, and why the strength follows from it

Kodak’s table names the developer, so the composition can be looked up in the same handbook.

D-8 is a single-solution hydroquinone caustic developer for maximum contrast on process materials: per litre of stock, 180 g of crystalline (or 90 g of anhydrous) sodium sulfite, 45 g of hydroquinone, 37.5 g of sodium hydroxide and 30 g of potassium bromide, used at two parts of stock to one of water.

That working bath therefore holds about 25 g/L of sodium hydroxide, which is 0.625 mol/L of a strong base, alongside about 60 g/L of sulfite, another 0.48 mol/L of proton acceptor. Set against a paper developer like D-72 at 1:2, carrying 0.21 mol/L of carbonate and 0.12 mol/L of sulfite, the difference is of the right order to account for the capacity ratio above.

The causal step is the course’s reading and is marked as one. Kodak states the two capacities and names the developer; it does not state that the strength was chosen because of the caustic. What the course can say without inference is that a plate leaving a hydroxide developer brings far more alkali into the tray than a print leaving a carbonate one, that Kodak trebled the acid, and that its own published capacity still fell by more than half.

It keeps like SB-1: three days in a dish, a month in a tank and indefinitely in a stoppered bottle whether full or half full. Acid is not destroyed by air, which is why a stop bath is the one solution in this formulary a half-empty bottle does not spoil.

Image characteristics

None, and the reason is worth restating here rather than assumed from the SB-1 page: a stop bath dissolves no silver halide and removes nothing from the emulsion. On a maximum-contrast process plate, where the whole point is a hard, clean separation between exposed and unexposed areas, an unreliable stop is nonetheless visible in the result — because a plate that goes on developing for an unmeasured extra ten seconds in a caustic developer’s carryover is a plate whose density has moved. This bath’s contribution to the image is that it makes the development time mean something.

The mechanism

50 mL × 1.049 g/mL × 0.995 = 52.2 g

Acetic acid delivered

52.2 g ÷ 60.05 g/mol = 0.869 mol, in about 1.05 L, so 0.828 mol/L

As moles

A weak acid at three times the concentration, not a stronger acid. Kodak did not reach for hydrochloric or sulfuric acid, and the reason is the one its 1928 primer states for the whole class: the quantity of alkali an acid can neutralise depends on the total hydrogen present, not on how much of it is dissociated. Trebling a weak acid trebles the reservoir. Substituting a strong acid at the same pH would have given a bath with a fraction of the capacity and a far more aggressive one at that — and a plate arriving from a caustic developer needs reservoir above all.

Tripling the concentration moves the pH very little. A ten-fold change in the concentration of a weak acid moves its pH by about half a unit, so a three-fold change moves it by roughly a quarter. That is the distinction between the intensity of an acid and the quantity of it, which Part X’s capacity experiment is built to demonstrate: the bath that starts lower is not the bath that lasts longer.

And what the extra acid does to the carried-over sulfite. D-8’s working bath holds about 0.48 mol/L of sulfite, and acid meeting sulfite releases sulfur dioxide, whose workplace limit is 0.5 ppm over eight hours — among the tightest in this course. A stronger stop bath meeting a high-sulfite developer is the worst combination on that axis in the whole formulary. The controls are in Part X and on the incompatibilities page, and a water rinse before the stop is the one that costs nothing.

Function of every ingredient

Acetic acid, 50 mL of the glacial acid. The whole formula, and the only variable in it. It is a weak acid, chosen for reservoir rather than for strength, and at 0.83 mol/L it holds three times SB-1’s reserve of neutralising capacity for the same volume of bath. It is also volatile, and at this concentration that is the dominant practical property: three times the acid in an open dish is three times the vapour in the room, against an eight-hour exposure limit of 10 ppm. More acid buys more capacity, drives more sulfur dioxide out of the sulfite arriving with each plate, and worsens the vapour; less returns the bath towards SB-1 and, after a caustic developer, exhausts it quickly. Its own encyclopaedia entry is here.

Water, 1000 mL. The source’s own first line, and the thing the acid is added to rather than a make-up volume: the finished bath is about 1050 mL. At this strength that five per cent is worth knowing about, and a bath made up to a litre rather than from a litre is five per cent stronger than Kodak’s.

Interactions

With the developer, which is the whole of its capacity. Kodak’s own annotation ties the published 40 sheets to D-8, and the arithmetic above shows how much of the difference from SB-1 that accounts for. Change the developer and the capacity changes; it is not a property of this tray.

With sulfite, and this is the hazard interaction. D-8’s working bath is about 0.48 mol/L in sulfite, and a strong-ish acid bath receiving it releases sulfur dioxide. Ventilation, a covered dish and — best of all — a water rinse before the stop, which costs the bath nothing in capacity.

With the fixer, in the usual way. Carrying acid forward protects the fixer’s pH; pouring stop bath into fixer decomposes the thiosulfate to sulfur dioxide and colloidal sulfur. See incompatibilities.

With metals. Acetic acid attacks them, and at this concentration faster. Plastic or glass throughout, and never a metal funnel.

Variants

SB-1 is the ordinary-strength member of the pair, at 17 mL per litre, and is the one to use unless the developer before it is unusual. The two together are a published, controlled pair on a single variable, which is rare enough to be worth exploiting — see Experiments.

The citric equivalent is the course’s own variant of SB-1, matched on titratable protons. A citric bath at three times that strength would be the natural equivalent of this page, and the course does not publish one, because tripling a formula the course itself derived would be building an inference on an inference. The arithmetic is on that page for anyone who wants to do it deliberately and record it as their own.

!A correction the course records rather than hides

This course’s own research plan expected SB-1a to be 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 at 50 c.c. of glacial acetic acid per litre, with no alum of any kind in it. The chrome alum baths are SB-3, a hardening bath of 30 g of potassium chrome alum per litre, and SB-4, its tropical version.

Neither of those is published as a formula in this course. Under the course’s chromium ruling, chromium(III) is permitted at Level B only as an in-emulsion hardener where a lesson genuinely requires one, and a tray of 30 g/L standing open through a session is not that. Part X sets out the full reasoning, including the two other reasons — that ILFORD states a fix hardener is no longer generally recommended for modern films, and that the 1928 primer records chrome alum losing its hardening power in the presence of sulfite.

And there is no SB-5 in this formulary. Kodak’s standard formulary J-1 would be the source for it; the mirror this course holds is a 56-page scan with no extractable text layer, so nothing can be read from it and nothing is quoted from it. A formula the course cannot read is a formula the course does not publish.

Safety

Level B, on glacial acetic acid — H314 in more than 99.9 per cent of 5,076 ECHA reports, H226 in 99.7 per cent, a flash point of 39 °C, and an eight-hour exposure limit of 10 ppm with 20 ppm over fifteen minutes.

The working bath at this strength deserves more respect than SB-1’s. About 5 per cent w/v is still a dilute solution, but it is three times the vapour source, and the tray it usually stands beside holds a caustic developer. Splash goggles rather than glasses, nitrile gloves, an apron, and the extraction running. Cover the dish between plates.

And the sulfur dioxide route is the one to plan for, because the developer this bath is documented against is a high-sulfite one. A water rinse between the developer and the stop reduces it and costs nothing. The classification rubric sets what Level B assumes, the gloves page covers the choice, and the first aid page covers a splash.

Storage

Indefinitely in a stoppered bottle, full or half full; three days in a dish and a month in a tank. Glass or plastic, never metal. Label with the formula, the strength and the date per the labelling SOP, and label it clearly enough that it is never mistaken for SB-1 — the two are indistinguishable by eye and differ by a factor of three.

Store the concentrate as the flammable liquid it is: cool, away from ignition sources, oxidisers and bases, in a labelled container with its safety data sheet to hand. Glacial acid freezes at 16.7 °C, so a bottle can set solid in an unheated darkroom; warm it gently and whole.

Incompatibilities

Spent fixer, above everything. Acid meeting thiosulfate decomposes it to sulfur dioxide and colloidal sulfur. Never pour stop bath into a fixer bottle and never share a waste container between the two. See chemical incompatibilities.

Sulfide toners, which generate hydrogen sulfide on meeting an acid bath.

Developer, in the backwards direction. One pair of tongs per tray, never interchanged. A splash of this bath in a caustic developer is a violent neutralisation as well as a spoiled solution.

Metals, which acetic acid attacks.

Carbonates and bicarbonates, which fizz. Neutralising this bath with sodium bicarbonate before disposal works and foams vigorously at this strength; add it slowly, in a well-ventilated area, with gloves, goggles and an apron.

Waste

A dilute solution of acetate, sulfite and — after D-8 — sodium hydroxide already neutralised to sodium acetate, at a pH near neutral once the bath is spent. It carries no silver.

Keep it out of the fixer stream, both for the reaction above and because spent fixer is a recoverable silver-bearing solution that acid contamination spoils. Its own labelled container, per the general chemical waste SOP, and the disposal caveat governs. Local regulation decides.

Troubleshooting

Development that visibly continues in the stop bath. The bath is exhausted, or the plate was not agitated on immersion. After a caustic developer this happens sooner than experience with paper would suggest — the published capacity is 40 sheets per 4.55 litres.

A sharp smell and stinging eyes at the tray. Acetic acid vapour, sulfur dioxide, or both. Stop, ventilate, cover the dish, and consider a water rinse before the stop.

A bath that smells stronger over a session. Evaporation concentrating an already strong bath. This is the formula in the library where a tray left uncovered matters most.

Plates showing uneven density from edge to centre. Not usually the stop bath, but agitate on immersion before looking elsewhere: a plate lying still in any bath has a boundary layer on it.

A bath that never seems to exhaust. Check that it is being used after the developer it was measured against. Behind an ordinary carbonate developer this bath has roughly three times SB-1’s reserve and will simply last a long time — which is a reason to use SB-1 instead, not a reason to be pleased.

Experiments

The published controlled pair. SB-1 and SB-1a differ in exactly one quantity, and Kodak published a capacity for each. Exhaust both against the same developer, using Part X’s capacity method, and the ratio of the two capacities should come out at the ratio of their acid contents, three to one. If it does, you have confirmed that capacity is a quantity of acid; and you have also shown that Kodak’s own 90-and-40 pair is telling you about two different developers rather than about two different baths.

Measure the carryover from a caustic developer. Weigh a dry plate, wet it in a hydroxide developer, drain it for your usual count, weigh it again, and multiply the gain by the developer’s hydroxide and sulfite concentrations. Compare the result with the 98.8 mmol per sheet this page computes from Kodak’s table. This is the arithmetic that turns a published capacity into an understanding.

Test the quarter-unit prediction. Measure the pH of SB-1 and SB-1a with the same meter. The weak acid relationship predicts about a quarter of a unit between them for a three-fold concentration change. A much larger difference means something is wrong with a bath or with the meter’s calibration at low pH, which the capacity experiment also warns about.

Put a water rinse in front of it. Run one session with the plates going straight from developer to stop, and one with a five-second water rinse between. Count sheets to exhaustion in each. The rinse costs nothing and should extend the bath’s life measurably, and it is the same intervention a university guidance sheet recommends for reducing sulfur dioxide.