Kodak SB-1

One ingredient, and the whole of the argument for it is in a sentence Kodak wrote in 1928: 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 weak acid — which is why the answer has been acetic acid for a century. Part X works that through and this page does not repeat it.

Ingredient	Quantity	Form the source specifies
Water	1000 mL	the acid is added to it
Acetic acid	17 mL	glacial

Purpose

To end development immediately, and to keep the fixer that follows from going alkaline. Kodak Limited’s header covers all three materials — “acid stop bath for papers, plates and films” — and the 1928 primer argues the first job explicitly: an acid rinse is strongly recommended instead of a water rinse because it arrests development immediately, whereas in a water rinse development continues if the rinsing is unduly prolonged.

Recommended uses

Prints, five seconds, which is Kodak Limited’s entire instruction for use. The 1928 primer is more detailed and worth having: 5 to 10 seconds when handling only a few prints, but 1 to 2 minutes for large batches, with the prints moved and separated so that the solution reaches all of every print. And, reassuringly, no harm is done if prints are left in the bath for ten or twenty minutes.

For film in a spiral tank the same bath works and the same ten-second order of magnitude applies; the course’s practical comparison of the options for film against paper is in Part X.

When another formula is preferable

• For a plate leaving a caustic developer, SB-1a is Kodak’s own three-times-stronger version, and the reason it needs to be three times stronger is the best short lesson in stop-bath capacity in this formulary.
• For a bath with no vapour, the citric equivalent the course derives from this one. Citric acid is a solid, it does not evaporate, and its classification is far lighter than glacial acetic acid’s.
• For a bath that tells you when it is spent, an indicator stop bath. The course cannot publish one as a formula: no manufacturer names its dye or its transition pH, and Part X sets out what can and cannot be said about that.
• Where no stop bath is possible at all, a water rinse changed very frequently, with the fixer monitored instead — again, Part X, which prices the substitution using the manufacturer’s own words.

Mixing

Acid into water, always, and never the reverse. Measure the litre of water first, then add the 17 mL of acid to it. Stir.

⚠This course buys its acetic acid already diluted, and so should you

The formula prints glacial acetic acid, and glacial acetic acid is the reason this page is Level B rather than Level A. Its aggregated GHS classification, from 5,076 ECHA notifications, 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. 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. HSE’s EH40 gives it 10 ppm as an eight-hour average and 20 ppm over fifteen minutes.

Part X’s ruling is that the course’s own practice is to buy a pre-diluted product of stated strength, which makes the tray work Level A. The arithmetic for doing that is below. The formula is printed as Kodak printed it because Rule 6 does not permit a historical formula to be silently restated; the safer route is given as a route, and labelled as the course’s.

🧮Getting 17 millilitres' worth of acid out of a bottle that is not glacial

17 mL of glacial acid, at PubChem’s experimental density of 1.049 g/mL and about 99.5 per cent purity, is 17.7 g of acetic acid. Everything below is a way of getting 17.7 g into a litre.

17.7 g ÷ 0.28 = 63.2 g of the solution, weighed

From a 28 per cent w/w product

17.7 g ÷ 0.80 = 22.1 g of the solution, weighed

From an 80 per cent w/w product

Weigh it 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 and may not have. Part II’s balance is more accurate than most darkroom graduates anyway, and the habit of measuring a stated per cent by volume is the commonest silent error in darkroom arithmetic.

The 1928 primer’s footnote is where this trap lives in the historical literature: 28 per cent acid is made by diluting three parts of glacial acid with eight parts of water. Those are volumes, and the 28 per cent is a mass fraction; only the density reconciles them, and Part X does the reconciliation.

Behaviour

It works in seconds because it has almost nothing to do. A print or a film carries out of the developer a thin film of alkaline solution and a swollen gelatin layer holding more of it. The acid has only to reach into that layer and drop the pH below the region where the developing agent reduces silver. Part X measures how far that is.

Its capacity is a quantity of acid against a quantity of alkali, and both sources say so. Kodak Limited’s table gives 90 sheets of 8 × 10 inches per 160 fluid ounces, and — unusually in that table — gives the same figure for a narrow dish and a deep tank, which is a clue that this bath is not limited by standing in air the way a developer is. The 1928 primer gives about seventy-five 8 × 10 prints per gallon and then explains what the figure depends on: the quantity of alkali carried over, which depends on the carbonate content of the developer, the volume the print retains, and the draining time. Those are three variables in your darkroom, not in Kodak’s, which is why the capacity experiment exists.

It keeps almost indefinitely, and that is genuinely unusual. Three days in a dish, a month in a tank and indefinite in a stoppered bottle whether full or half full. Every developer in this formulary has its keeping figures cut by a half-empty bottle, because a developer is destroyed by air. Acetic acid is not.

⚗The measured effect on the fixer, from 1928

The second job of a stop bath is usually asserted. Kodak measured it.

The primer gives the capacity of its acid hardening fixer F-1 as one hundred 8 × 10 prints per gallon, provided a thorough water rinse precedes fixation. It then gives the capacity of the same fixer when SB-1 is used instead: one hundred and twenty-five 8 × 10 prints per gallon.

A twenty-five per cent gain in fixer life, from ten seconds and about a penny’s worth of acid. The mechanism is spelled out in the same passage: without a rinse, developer is carried over, the alkali in it rapidly neutralises the acid in the fixing bath, a white sludge forms, the bath becomes alkaline — and prints fixed in an alkaline bath are likely to become stained brown.

The primer also gives the test, and it needs no meter: dip a strip of blue litmus paper in the rinse bath. If it turns red the bath is acid; if it stays blue the bath is alkaline and is finished. Kodak’s conclusion is that sludging of the fixing bath will never occur if the rinse bath is always kept acid.

Image characteristics

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

The mechanism

17 mL × 1.049 g/mL × 0.995 = 17.7 g

Acetic acid delivered

17.7 g ÷ 60.05 g/mol = 0.295 mol, in about 1.017 L, so 0.290 mol/L

As moles

A weak acid, deliberately. Acetic acid’s pK_a is 4.76, so in a 0.29 mol/L solution only a small fraction of the acid is dissociated at any instant and the pH sits near 2.6. That is the point. A strong acid at the same pH would hold a hundredth of the neutralising capacity, because its protons are all already in solution and there is no reservoir behind them. As carried-over alkali consumes free protons, the undissociated acetic acid dissociates further to replace them, and the pH barely moves until the reservoir is nearly gone. That is the shape of the curve the capacity experiment measures.

What arrives from the developer is mostly sulfite, not carbonate. A litre of D-72 at 1:2 carries 15 g/L of sodium sulfite and 22.5 g/L of sodium carbonate; a litre of D-163 at 1:3 carries 18.75 and 16.3. Both of those basic species take up protons, and in the fine-grain film developers the sulfite dominates overwhelmingly. The consequence is set out in Part X: sulfite meeting acid releases sulfur dioxide, whose workplace limit is one of the tightest in this course, and the control is to mix to the stated strength, not to let a bath over-concentrate, and to keep acid away from spent fixer.

And the pH the print sees is not the pH of the tray. The acid has to diffuse into a swollen gelatin layer that is full of alkaline developer. That is why the instruction for a large batch is one to two minutes rather than five seconds, and why the prints must be moved and separated: a print lying flat against another is a print whose emulsion the bath has not reached.

Function of every ingredient

Acetic acid, 17 mL of the glacial acid. The whole formula. It is a weak acid, and that is a requirement rather than a compromise: the job is to neutralise a quantity of alkali, and neutralising capacity is a count of total available protons rather than a pH. It is also volatile, which is the source of every hazard on this page and the reason the course prefers the citric equivalent for tank work. More acid gives a bath with more capacity and a lower pH, and drives more sulfur dioxide out of the sulfite that arrives with each print; less gives a bath that exhausts early and, once it has gone alkaline, one that does nothing at all while looking exactly the same. Its own encyclopaedia entry is here, and it carries the hazard data this page’s Safety section draws on.

Water, 1000 mL. Not an ingredient in the ordinary sense, but the source’s own first line, and worth one remark: Kodak writes water 1000 c.c. and then adds the acid to it, so the bath is about 1017 mL and no final volume is published. Nothing in this formulary depends on the two per cent, but a reader comparing this page’s molarity with a calculation of their own should know which volume the course divided by.

Interactions

With the developer, by design. The bath’s whole capacity is spent on sodium carbonate and sodium sulfite carried over on wet prints. A developer with more carbonate exhausts a stop bath faster, which is why the 1928 primer’s capacity figure names the developer type.

With the fixer, measurably. Twenty-five per cent more prints per gallon of F-1, as above. In the other direction, stop bath poured into a fixer will drive sodium thiosulfate below the pH at which it is stable and decompose it to sulfur dioxide and colloidal sulfur. Part X’s account of the fixer window is the place for that, and the incompatibilities page calls it the single most likely accident in a home darkroom.

With itself, over time. Very little. The keeping figures are indefinite in a bottle, which is why a made-up bottle of stop bath is a reasonable thing to keep on a shelf in a way that a made-up developer is not.

Variants

The 1928 printing of the same formula, at a different strength. Kodak’s US primer gives SB-1 as 48.0 c.c. of 28 per cent acetic acid to 1.0 litre of water. That is about 13.4 g of acetic acid per litre against the 1949 handbook’s 17.7 g — the 1949 bath is about a third stronger. Both are Kodak, twenty-one years apart, under the same designation.

The course prints the 1949 figure as the formula, for one reason and it is not that the 1949 is better: Part X already works to about 0.3 mol/L and derives 17 mL from it, and the capacity experiment is built on that arithmetic. Printing a different SB-1 here would make the lessons and the formulary disagree. If you mix the 1928 version, expect roughly three quarters of the capacity, and note that its 75-prints-per-gallon figure and the 1949 handbook’s 90-sheets-per-160-fluid-ounces are not directly comparable anyway — different volumes, different materials, different criteria.

The avoirdupois column of the 1949 handbook reads 1 oz 120 minims per 80 fluid ounces, which works out at about 15.6 c.c. per litre rather than 17. The handbook itself warns in capitals that its two columns “are not exact equivalents” and that one or the other must be used exclusively. The course prints the metric column throughout the formulary and records the discrepancy rather than resolving it.

SB-1a is Kodak’s stronger stop bath for plates leaving a caustic developer, at 50 c.c. of glacial acid per litre. It is a different formula rather than a variant of this one — Kodak numbered it separately and published it for a different material — and it has its own entry.

The citric equivalent is the course’s own variant of this formula, matched on titratable protons, and is identified as such on its own page.

Indicator versions. Not published here. No manufacturer in the course’s corpus names the dye in its indicator stop bath or the pH at which it changes, so there is nothing to print; Part X explains what is known and where the limit is.

Safety

Level B, on the concentrate rather than the bath. Acetic acid in its glacial form 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, its explosive range is 6.0 to 17 per cent by volume in air, 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.

The working bath is a different proposition from the concentrate, and the control that makes it so is buying the acid already dilute — which is the course’s stated practice and the reason the Part X lessons are Level A. A university guidance sheet for photographic chemicals puts the same advice first among its stop-bath precautions: purchase dilute solutions of acetic acid rather than concentrated ones.

The vapour is not merely an odour problem. Acetic acid smells strongly of vinegar well below its exposure limit, so a darkroom that reeks of stop bath is reporting on its ventilation rather than on its chemistry — but a covered tray, an open door and a working extract are the answer, not tolerance.

And the one interaction that must never happen: acid into spent fixer. See Incompatibilities.

Storage

Indefinitely, in a stoppered bottle, full or half full — the only solution in this formulary with that entry in Kodak’s table. Three days once it is standing in a dish and a month in a covered tank.

Glass or plastic, never metal: acetic acid attacks metals, and a bath standing in a metal funnel or tray is dissolving it. Label with the formula, the strength and the date per the labelling SOP.

Store the concentrate as the flammable liquid it is: cool, away from ignition sources and away from oxidisers and bases, in a labelled container, with its own safety data sheet to hand. Glacial acid also freezes at 16.7 °C, so a bottle can set solid in an unheated darkroom in winter — warm it gently and whole, and do not chip at it.

Incompatibilities

Spent fixer, above everything. Acid meeting thiosulfate decomposes it to sulfur dioxide and colloidal sulfur; acid meeting sulfite releases sulfur dioxide directly. Sulfur dioxide’s workplace limit is 0.5 ppm over eight hours, among the tightest in this course. Never pour stop bath into a fixer bottle, and never share a waste container between the two. The incompatibilities page has the reactions and the reasoning.

Sulfide toners, which generate hydrogen sulfide on meeting an acid bath. Kodak’s toning manual says plainly that sulfide-type toners are not to be discarded with stop baths or fixing baths.

Developer, in the other direction. The whole point of the tray is to carry acid forward, and none of it must travel back: one pair of tongs per tray, never interchanged. A splash of this bath in a litre of paper developer drops the pH out of the region where hydroquinone works.

Metals, which acetic acid attacks. Plastic or glass throughout.

Carbonates, which fizz. Neutralising spent stop bath with sodium bicarbonate works and foams; Kodak’s own environmental guidance says to add it slowly, in a well-ventilated area, with gloves, goggles and an apron.

Waste

A dilute solution of acetate, sulfite and carbonate at a pH near neutral once it is spent — which is the point of a bath whose exhaustion is defined as having gone alkaline. It carries no silver.

Keep it out of the fixer stream, both because of 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, and this course cannot tell you what it says where you are.

Troubleshooting

Prints staining brown after fixing. The 1928 primer’s own diagnosis: developer carried over, the fixer gone alkaline, a white sludge in it. The stop bath is either exhausted or was skipped. Test it with blue litmus — red is acid and working, blue is spent.

A bath that smells much stronger than usual. Over-concentrated, most often by evaporation from a tray left standing. Kodak’s toning manual names three days in a tray and a month in a tank as the points past which evaporation begins to matter, and an over-concentrated stop bath is both a vapour problem and, on the toning manual’s account, a cause of mottle in the base of a print.

Development that seems to continue after the print leaves the developer. The bath is spent, or the print went in and out too fast to be wetted through, or prints are stuck together. Move them and separate them, which is the primer’s instruction.

Blisters or a fine mottle on a print. Look at the acid strength and at the agitation in the first few seconds rather than at the developer; the toning manual attributes base mottle to an over-concentrated bath or to insufficient agitation on immersion.

A tray that has gone cloudy or smells of rotten eggs. Something sulfur-bearing has been mixed into it. Stop, ventilate, and read the incompatibilities page.

Experiments

Measure the capacity in your own darkroom. Part X’s capacity experiment is exactly this formula against a measured litre of developer, with a pH meter and a plotted curve, and it converts Kodak’s 90 sheets into a number about your paper, your drain time and your developer.

Test the litmus claim against the meter. Run the bath to exhaustion, testing with blue litmus and with a meter at every step. Where does the paper stop turning red, and what pH is that? You are calibrating a hundred-year-old field test, and the answer tells you how much warning it gives.

Weigh a print’s carryover. Weigh a dry sheet, wet it in developer, drain it for the count you actually use, and weigh it again. The gain in grams is the carryover in millilitres. Multiply by the carbonate and sulfite concentrations of your developer and you have the moles of alkali per print — which, divided into the 0.29 mol in a litre, is your capacity, computed rather than quoted.

The 1928 against the 1949. Mix both strengths and exhaust them side by side with the same developer. The ratio of the two capacities should be the ratio of their acid contents, about 4 to 3. If it is not, the difference is telling you something about how exhaustion is being judged, which is the more interesting result.