Kodak SB-5

Every other stop bath in this formulary is an acid and some water. This one is an acid, some water and forty-five grams of a salt that does nothing to the developer at all — and the salt outweighs the acid it is mixed with by nearly five to one. Kodak’s own subtitle names a mechanism rather than a job: Nonswelling Acid Rinse for Photofinishing. That mechanism is worth the page, because the problem it solves is one that the acid in an ordinary stop bath makes slightly worse.

Ingredient	Quantity	Form the source specifies
Acetic acid (glacial)	32 mL of a 28% solution	Kodak's 28 per cent acetic acid, which its own footnote makes by diluting three parts of glacial acid with eight parts of water
Sodium sulfate (anhydrous)	45 g or 105 g (crystalline sodium sulfate, the decahydrate — Kodak's own alternative, printed as 3 and a half ounces per 32 ounces of solution)	desiccated, which is Kodak's word for the anhydrous salt
Water	to make 1000 mL	Kodak prints water twice, and the two lines are different statements. "Water 16 ounces / 500 c.c." heads the list and is the volume the acid and the salt are dissolved in; "Water to make 32 ounces / 1.0 liter" closes it and is the make-up volume that sets the strength. This field holds the make-up volume, because that is the one every concentration on this page is computed against; the 500 mL is in the Mixing section. Kodak specifies no temperature for either.

Purpose

Two jobs in one tray, and the second one is the reason the formula exists.

The first job is the ordinary one, and Kodak’s 1946 handbook and its 1977 J-1 state it in nearly the same words: the primary purpose of an acid rinse between development and fixation is to check development instantly by neutralising the developer carried over, which also protects the acidity of the fixing bath from exhaustion.

The second job is to do that without swelling the gelatin. Kodak’s 1928 primer sets out the difficulty plainly: a gelatin layer placed in water swells, and a small quantity of either an acid or an alkali will produce a considerable increase in the swelling — so the developer and the fixing bath both tend to swell the emulsion, especially when they are warm. An acid stop bath sits between two swelling baths and is itself acid. In a hand darkroom at 18 °C that costs nothing. In a photofinishing plant running hundreds of rolls a day through machines in a warm room, a soft, swollen emulsion is scratches, frilling, drying marks and torn film.

SB-5’s answer is not to harden the gelatin — it contains no alum, and Kodak’s 1977 printing calls it in so many words a non-hardening stop bath — but to hold the swelling down while the film is in the bath, using a concentrated neutral salt. It is the cheap, reversible, non-chromium half of a problem whose expensive half is SB-4, the chrome alum tropical hardener that Part X sets out and declines to publish.

Recommended uses

Films and plates, between the developer and the fixer. Kodak never offers it for paper, and three things in the documents say so: the subtitle is for photofinishing, the instruction for use speaks of films, and the capacity is quoted in rolls — while in the same 1946 table it is SB-1 that is marked papers only.

The time is the one thing the two Kodak printings disagree about, and they disagree by a factor of six.

• 1946 Reference Handbook: agitate the films when first immersed, and leave them about three minutes before transfer to the fixing bath. No temperature is given.
• 1977 J-1: treat the films or plates for about 30 seconds with agitation, at 18.5 to 21 °C, between developing and fixing — and the bath is for use up to 26.5 °C (80 °F).

Both are Kodak’s. The course prints both and does not choose, because there is no evidence on which to choose: the emulsions of 1946 and of 1977 are not the same materials, and a three-minute soak that was prudent for a 1946 roll film is a long time to leave a modern hardened film in an acid.

✦If you are going to mix this, mix it for the reason it exists

A thirty-second rinse in SB-5 and a thirty-second rinse in SB-1 stop development equally well; SB-1 does it with more acid and less trouble. The case for SB-5 is a warm darkroom, a soft or hand-coated emulsion, or a long run of film through the same tray — anywhere the swelling of the gelatin is the thing you are managing rather than an afterthought. If your darkroom sits at 20 °C and your film is a modern hardened one, this formula is a lesson rather than a recommendation.

When another formula is preferable

• For prints, always SB-1. Kodak’s own table restricts SB-1 to papers and SB-5’s capacity to rolls, and paper carries several times as much developer out of the tray as film does. Nothing in this formula is designed for a print.
• After a highly alkaline developer, SB-1a, which Kodak’s 1977 printing recommends by name for line materials. SB-5 carries two-thirds of SB-1’s acid and about a quarter of SB-1a’s; it is not built to absorb a caustic carry-over.
• For a bath with no acetic vapour at all, the course’s citric equivalent, which is a weighed solid. It carries no sulfate and makes no claim about swelling.
• For genuine tropical processing, Kodak’s answer was SB-4, a chrome alum hardening bath, and its route is set out in the handbook’s high-temperature instructions. This course does not publish a chromium tray: the chromium ruling and Part X’s comparison give the reasoning in full.
• For most modern film in a temperate darkroom, an ordinary acid rinse. Modern camera films are hardened at manufacture to a degree that 1946 emulsions were not, and Part X sets out the manufacturer’s own statement that a fix hardener is no longer generally recommended. The same argument weakens the case for an anti-swelling stop bath.

Mixing

Five hundred millilitres of water first, then the acid, then the salt, then water to a litre. That is Kodak’s printed order and there is no reason to depart from it.

1. Measure 500 mL of water into the vessel. Kodak specifies no temperature; cold or tepid tap water is what the formula assumes.
2. Add 32 mL of 28 per cent acetic acid to the water, never the other way round. See the safety note below on where that acid comes from.
3. Dissolve 45 g of desiccated (anhydrous) sodium sulfate, or 105 g of the crystalline salt if that is what you have. Stir until the solution is clear; it is a long way below saturation and will go in without heat.
4. Make up to 1000 mL with cold water.

🧮What 32 millilitres of 28 per cent acid actually is

Kodak’s footnote is the only definition of the strength it gives: 28 per cent acetic acid is made by diluting three parts of glacial acetic acid with eight parts of water. Those are volumes, so three parts in eleven of what you measure out is glacial acid.

32.0 mL × 3/11 = 8.73 mL of glacial acetic acid

Glacial acid in the bath

8.73 mL × 1.049 g/mL × 0.995 = 9.11 g

As a mass, at PubChem's density and about 99.5 per cent purity

9.11 g ÷ 60.05 g/mol = 0.152 mol/L

As moles, in the made-up litre

So SB-5 delivers about 0.15 mol of acetic acid per litre — two-thirds of the 0.23 mol/L that Kodak’s American SB-1 delivers from 48 mL of the same acid, and about half of the 0.30 mol/L in Kodak Limited’s London SB-1, which is written as 17 mL of the glacial acid.

The 28 per cent and the three-to-eight do not reconcile without a density, and Part X works that through: a per cent by mass converts to a mass directly and to a volume only through a density you would have to look up. The reliable move, if your bottle is labelled by mass, is to weigh the acid: 9.11 g of acetic acid from a 28 per cent w/w product is 9.11 ÷ 0.28 = 32.5 g of solution, weighed.

One more warning from the 1928 primer, still worth having: 28 per cent acid made by diluting the glacial acid is not the same product as the “commercial 28 per cent acetic acid” redistilled from the destructive distillation of wood, and vinegar is not a substitute for either.

⚠The acid is the whole of the hazard, and it need not be handled neat

The ingredient Kodak names is a concentrate, not a working dilution, and that is why this page is Level B rather than Level A. If you make the 28 per cent acid yourself you are handling glacial acetic acid, whose aggregated GHS classification across 5,076 ECHA reports is H314, causes severe skin burns and eye damage, in over 99.9 per cent of them 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 is reached rather quickly on evaporation at 20 °C. HSE’s EH40 sets 10 ppm over eight hours and 20 ppm over fifteen minutes.

Acid into water, slowly, goggles and gloves on, with the extractor running. Buying acetic acid already dilute is this course’s stated practice and it is exactly what this formula’s own ingredient line asks for. The finished tray, at under one per cent of glacial acid by volume, is Level A work.

⚗Two water lines, one field

Kodak prints Water 500 c.c. at the head of the list and Water to make 1.0 liter at the foot, and the 1928 primer states the convention behind the second: “cold water to make” at the end of a formula insures dilution to a definite volume, and therefore a known concentration each time it is mixed. The formula object on this page records the make-up volume, because that is what every concentration below is computed against. The 500 mL is a working instruction, not a strength.

Behaviour

It keeps essentially for ever and is spent by what walks into it. Kodak’s 1946 table gives SB-5 indefinite keeping in a stoppered bottle — and, unusually, heads the two bottle columns 65 °F and 75 °F instead of full and half full, which is Kodak quietly acknowledging that a bath with nothing oxidisable in it does not care how much air is above it. In use it is three days in a tray and a month in a gallon tank, the same as every other Kodak stop bath.

The capacity is stated twice by Kodak and the two statements do not agree.

Printing	Sheets of 8 × 10 in	Rolls	Carry-over stated
1946 Reference Handbook	100 per US gallon (tray and tank alike)	about 25 per quart	about 24 fl oz (720 c.c.) of developer
1977 publication J-1	13 per litre [50 per gallon]	about 13 per litre	none given

Half, in a generation, with no explanation printed. What the 1977 table does show is that Kodak rated SB-5a — the same bath with double the acetic acid — at 26 [100], which is the 1946 figure to the number. Whether Kodak halved the formula’s acid and kept the name, or kept the formula and revised the rating down, cannot be settled from the documents this course holds, and this page does not guess. What it will not do is print an average of the two.

🧮The number almost no source gives: how much developer a film carries

Kodak’s 1946 entry states both halves of a capacity for once — the count and the volume that produces it. Twenty-five rolls per quart, and by then about 720 mL of developer has gone into the bath.

720 mL ÷ 25 rolls = about 29 mL of developer carried in per roll

Per roll

720 mL of developer in a 946 mL quart — the tray is about two-fifths developer by volume

What the bath has become when it is spent

Twenty-nine millilitres is a great deal for a strip of film to hold, several times what a bare film surface retains, so this figure is almost certainly counting everything a photofinishing rack carries over — the hangers, the clips and the solution bridging between them — and not the emulsion alone. That reading is the course’s, not Kodak’s; the two published numbers are Kodak’s. Either way the lesson holds and is the whole of stop-bath capacity: the bath dies of what the film brings with it, and the way to make it live longer is to drain properly, which Part X’s capacity experiment turns into a measurement about your own darkroom.

Do not compare its capacity with SB-1’s. SB-1 is rated on paper prints and SB-5 on film, and a fibre-base print carries several times the solution a film does. The two numbers are answers to different questions.

Image characteristics

No image characteristics, and one physical one. Like every stop bath, SB-5 dissolves no silver halide, removes nothing from the emulsion, changes no image tone and makes no print more permanent. Everything about the negative was decided in the two trays either side of it.

What it does change is the state of the gelatin while the film is wet — and that is a real photographic property even though it is not a tonal one. A less swollen emulsion is a tougher one: it scratches less, frills less at the edges, holds its dimensions better, and is less likely to show the drying marks and reticulated grain patterns that a warm-to-cold shock produces. Those are the defects this bath was sold against, and they belong to the troubleshooting atlas — reticulation, frilling — rather than to a tone scale.

The one thing it explicitly does not do is harden. Kodak’s 1977 printing says so in its first sentence. Whatever restraint the sulfate imposes ends when the film leaves the tray, which is why SB-5 is followed by an acid hardening fixer such as F-5 and not by a plain hypo bath.

The mechanism

The acid half

32.0 mL × 3/11 × 1.049 g/mL × 0.995 = 9.11 g

Acetic acid delivered

9.11 g ÷ 60.05 g/mol = 0.152 mol/L

As a concentration in the made-up litre

A weak acid, deliberately, for the reason Kodak gave in 1928 and Part X works through: 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. At a pK_a of 4.76 (IUPAC), a 0.152 mol/L solution of acetic acid alone would sit near pH 2.8 by the standard weak-acid approximation, ½(pK_a − log C) — about 0.14 of a pH unit above the 2.65 the same arithmetic gives for SB-1, and far enough below the region where Metol and hydroquinone reduce silver that development stops on contact.

The salt half

45.0 g ÷ 142.04 g/mol = 0.317 mol/L

Sodium sulfate delivered

½ × [(0.634 × 1²) + (0.317 × 2²)] = 0.95 mol/L

Ionic strength, I = ½ Σ c z²

Nearly one molar in ionic strength, from a salt that takes no part in any photographic reaction. That is the formula’s whole second idea, and it works on gelatin rather than on silver.

A gelatin layer swells because water is drawn into it: the polymer network holds fixed ionic groups and counter-ions, the water outside is comparatively pure, and the difference in water activity drives water in until the network’s elasticity balances it. Two things follow from putting a concentrated neutral salt in the outside solution. First, the water activity outside falls, so less water is drawn in. Second, the high ionic strength screens the charges on the gelatin chains, so they repel one another less and the network does not expand as far. Acid makes both worse — it protonates carboxylate groups and shifts the gelatin away from its isoelectric point — which is exactly why an acid rinse swells gelatin and why an acid rinse that does not is worth a formula number.

⁇How much of this is Kodak's account and how much is the course's

Kodak’s, in print: that the bath is a nonswelling acid rinse; that acid and alkali both increase the swelling of gelatin and warmth increases it further; that the secret of tropical processing lies in preventing abnormal swelling; and that adding sodium sulfate to a developer allows it to be used at higher temperatures. The dose table on Processing page 16 is Kodak quantifying the effect.

The course’s: the account above of why a concentrated neutral salt restrains swelling — water activity and charge screening. That is ordinary colloid chemistry and it is not stated in any Kodak document this course has read. The sodium sulfate entry makes the same distinction about the same salt in Kodak’s tropical developer D-13, and for the same reason.

Why forty-five grams

Kodak’s own high-temperature table, on Processing page 16 of the 1946 handbook and page 19 of the 1977 J-1, prescribes sodium sulfate for the developer against temperature:

Developer	Temperature	Sodium sulfate, desiccated
D-11, D-19, D-61a, D-76	24 to 26.5 °C (75–80 °F)	50 g/L
the same	26.5 to 29.5 °C (80–85 °F)	75 g/L
the same	29.5 to 32 °C (85–90 °F)	100 g/L
DK-50, DK-60a, D-72 (1:1)	24 to 26.5 °C	100 g/L

SB-5 carries 45 g/L, and the 1977 printing caps its use at 26.5 °C — the top of that first row. A film leaving a sulfate-loaded developer at about 50 g/L and entering a rinse at 45 g/L crosses almost no concentration step at all, so there is no osmotic jolt at the moment of transfer. Kodak nowhere states that pairing; the numbers are Kodak’s and the observation is the course’s. It also squares with Kodak’s first precaution for high-temperature work, that developer, stop bath, fixing bath and wash water must all be within about 2.5 °C of one another: this is the same instruction applied to concentration instead of to temperature.

One honest limit on that reading. Kodak’s own high-temperature route does not send the film into SB-5. It sends it into SB-4, a chrome alum hardening rinse. SB-5 is offered for photofinishing, at ordinary to warm room temperature, where a chromium bath on a production line is a different kind of problem.

Function of every ingredient

Acetic acid, 32 mL of the 28 per cent solution — about 9.1 g of acid, or 0.15 mol, per litre. This is the working half of the bath. It is a weak acid and that is a requirement rather than a compromise: stopping development needs a pH low enough to shut the developing agents down, but lasting needs a large reservoir of undissociated acid that can go on releasing protons as carried-over carbonate and sulfite consume them. A strong acid at the same pH would hold a hundredth of the reserve. More acid buys capacity and a lower pH — which is precisely what Kodak did in SB-5a, doubling this line for photofinishing and doubling the rated capacity with it — but it also drives more sulfur dioxide out of the sulfite arriving with each film, and, because acid swells gelatin, it works against the salt. Less acid gives a bath that goes alkaline early and then does nothing at all while looking exactly the same. It is also the only volatile thing in the formula and therefore the source of every hazard on this page.

Sodium sulfate, 45 g of the desiccated salt, or 105 g of the crystals. The formula’s second idea, and by weight the bulk of it: five times as much salt as acid. It is photographically inert — it reduces nothing, dissolves nothing, restrains nothing, and is in fact the end product that sodium sulfite becomes when a preservative has done its job. Its entire function is physical: at 0.317 mol/L it brings the bath to an ionic strength near 0.95 mol/L, lowering the activity of the water outside the emulsion and screening the charges inside it, so that the gelatin takes up less water while it is being acidified. More sulfate restrains the swelling further, and Kodak’s own developer table shows the dose being raised with temperature in 25 g/L steps — but past some point it makes the bath a stronger solution than the ones on either side of it, and a large concentration step between trays is the osmotic shock the bath exists to avoid. Less sulfate leaves an ordinary, rather weak acetic acid stop bath: SB-1 at two-thirds strength, and no reason to number it separately.

✕The one substitution that ruins this formula, and it is a spelling

Sodium sulfate is CAS 7757-82-6. Sodium sulfite is CAS 7757-83-7. One letter, one digit, two white crystalline solids. Forty-five grams of sulfite in this bath would not make a nonswelling rinse: it would make a litre of acidified sulfite, which liberates sulfur dioxide directly — a gas whose workplace exposure limit is among the tightest in this course. Check the CAS number on the jar, not the name.

Water, 500 mL to dissolve in and enough to make 1000 mL. Not an ingredient in the ordinary sense, but Kodak prints it twice and the two lines mean different things — the first is a working volume, the second is the strength of the bath. Every concentration on this page is computed against the litre.

Interactions

With the developer, twice over. The obvious interaction is the one every stop bath has: the acid is spent on sodium carbonate and sodium sulfite carried in on wet film, and a developer with more alkali exhausts the bath faster. The interaction peculiar to SB-5 is the salt one: if the developer is itself sulfate-loaded for warm-weather work, the rinse and the developer are at nearly the same ionic strength and the film crosses no osmotic step. If the developer is a plain one at 20 °C, SB-5 is the more concentrated of the two solutions and the step runs the other way. Both readings are the course’s, drawn from Kodak’s numbers rather than from a Kodak statement; see the note under The mechanism.

With the fixer, by omission. SB-5 hardens nothing. It carries no alum and Kodak calls it non-hardening in as many words, so all the hardening in the sequence has to come from the fixer. Kodak’s instruction for SB-5 names no fixer — it says only before transfer to the fixing bath — but the fixer the same handbook names wherever hardening matters is F-5, with its potassium alum and boric acid, and that is the bath this course would put after it. The sulfate carried forward into the fixer is inert there too.

With sulfite, in the direction that matters for your lungs. Acid meeting sulfite releases sulfur dioxide, and every film brings sulfite in from the developer. This is a general property of acid stop baths rather than a peculiarity of this one, and the control is the same: mix to strength, do not let a tray concentrate by evaporation, and ventilate.

With gelatin, which is the point. See The mechanism.

Variants

Kodak’s own: SB-5a. Printed immediately after SB-5 on page 38 of the 1977 J-1 — “For photofinishing, use double above quantities of KODAK 28% Acetic Acid” — and rated in the same publication’s table at 26 [100] sheets per litre [gallon], twice SB-5’s. It is not published here as a formula of its own, for a precise reason: the copy of J-1 this course holds does not carry SB-5’s quantities in that printing, so “double the above” has no printed above in that document, and doubling the 1946 figure to 64 mL would be the course’s arithmetic dressed as Kodak’s. The relationship is recorded; the number is not invented.

The crystalline sulfate is Kodak’s alternative form, not a variant. 3½ ounces per 32 ounces of solution (105 grams per liter) of the crystals in place of 45 g of the desiccated salt. The arithmetic is worth a glance, because Kodak’s factor moved:

105 ÷ 45 = 140 ÷ 60 = 2.33

Kodak's 1946 factor, on SB-5 and SB-4 alike

(142.04 + 10 × 18.015) ÷ 142.04 = 322.19 ÷ 142.04 = 2.27

The exact hydrate factor, Na₂SO₄·10H₂O ÷ Na₂SO₄

2.25, as printed

Kodak's 1977 factor, on SB-4

So the crystalline route as printed in 1946 delivers about three per cent more sulfate than the desiccated one, and by 1977 Kodak had rounded the other way. Three per cent is far below anything this bath can notice, and the drift is recorded only because a reader doing the conversion themselves should know which number they are reproducing.

The two printings of SB-5 itself are a disagreement, not a variant. Three minutes against thirty seconds, 100 sheets per gallon against 50, 25 rolls per quart against 13. Same designation, same maker, thirty-one years apart. This formulary has a field for two sources that agree and none for two that do not, so the disagreement lives in the capacity and development arrays, in this section, and in the schema’s own written record of what it cannot yet say — which is to say, in prose.

SB-4 is the hardening cousin, not a variant of this. Kodak’s tropical hardening bath is 30 g of chrome alum with 140 g of crystalline (or 60 g of anhydrous) sodium sulfate per litre: the same anti-swelling salt at a higher dose, plus a chromium(III) hardener that actually crosslinks the gelatin. It has its own number, its own material and its own hazard profile, and the course’s chromium ruling keeps it out of a tray.

Safety

Level B, and the level belongs to the bottle rather than to the tray. The working bath is under one per cent glacial-acid-equivalent by volume, plus a salt with no classification; that is Level A work. What lifts the page is the ingredient line itself, which names a 28 per cent concentrate, and Kodak’s only stated way of obtaining it, which is to dilute the glacial acid three parts in eleven.

Acetic acid is the hazard. In its glacial form its aggregated ECHA classification is Danger, with H314 (severe skin burns and eye damage) in more than 99.9 per cent of 5,076 company reports and H226 (flammable liquid and vapour) in 99.7 per cent. Flash point 39 °C. The International Chemical Safety Card notes that a harmful air concentration can be reached rather quickly on evaporation at 20 °C. EH40 sets 10 ppm over eight hours and 20 ppm over fifteen minutes. Goggles, gloves, apron, extraction, acid into water; and if you can buy the 28 per cent solution ready made, buy it.

Sodium sulfate is not classified, on a broad but shallow evidence base: 4,127 of 4,277 company reports say it meets no GHS hazard criterion, and Chemical Safety Card 0952 leaves its classification box empty. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit for it. The one handling instruction that matters is the dust: the safety card records that a nuisance-causing airborne concentration is reached quickly, so weigh 45 g without raising a cloud, and do it away from the tray.

The real risk on this page is not toxicological. It is picking up the wrong white jar. See the callout under Function of every ingredient.

Storage

Indefinitely in a stoppered bottle, at 65 °F and at 75 °F alike, which is Kodak’s own way of saying that this bath has nothing in it that air can spoil. Three days standing in a tray, a month in a covered tank. Both Kodak printings give the same figures.

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

A deposit in a cold bottle is the sulfate, not spoilage. Sodium sulfate has an unusual solubility curve: HSDB puts its maximum at 33 °C, with the salt less soluble both above and below that temperature, so a bottle that has stood in a cold darkroom holds less in solution than the same bottle at room temperature. The course’s sources give no figure for cold water, so this page does not say at what temperature a 45 g/L bath would begin to deposit — only what to do if one has. Kodak’s own storage instruction covers exactly the case: a precipitate that separates from a solution stored at a low temperature should not be discarded, but should be redissolved by warming before the solution is used. Warm the whole bottle gently, shake, and check that the liquid is clear before it goes in a tray.

Store the concentrated acid as the flammable liquid it is: cool, labelled, away from ignition sources, oxidisers and bases, with its safety data sheet to hand.

Incompatibilities

Spent fixer, above everything else. Acid meeting sodium thiosulfate drives it below the pH at which it is stable and decomposes it:

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

What an acid does to a thiosulfate bath: the milkiness is colloidal sulfur, the smell is sulfur dioxide

Sulfur dioxide’s workplace limit is 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 calls this the single most likely accident in a home darkroom.

Sulfide toners, which give off hydrogen sulfide on meeting any acid bath. Kodak’s toning literature is explicit that sulfide toners are not to be discarded with stop baths or fixing baths.

The developer, in the reverse direction. The whole purpose of the tray is to carry acid forward and none of it backward. 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, including the funnel.

Carbonates, which fizz. Neutralising spent stop bath with bicarbonate works and foams; add it slowly, in a ventilated place, gloved.

Waste

A dilute solution of sodium acetate and sodium sulfate, plus whatever carbonate and sulfite the film carried in, at a pH near neutral once the bath is spent — because “spent” for a stop bath means “neutralised”. It carries no silver and no heavy metal. The sulfate in it is chemically the same species that leaves every darkroom in exhausted developer anyway, as the sodium sulfate entry explains.

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. Local regulation decides what happens next and this course cannot tell you what it says where you are; the disposal caveat governs.

Troubleshooting

Film soft, frilling at the edges, or scratching easily in the fixer. The condition SB-5 exists to prevent, so check the bath first: has it been diluted by carry-over from a wet tank, and is the sulfate actually in it? Then check Kodak’s own precaution, which is a temperature one: developer, stop bath, fixing bath and wash water within about 2.5 °C of one another. A nonswelling rinse cannot save an emulsion that meets a wash ten degrees colder than the developer.

Crystals or a haze in the storage bottle. Sodium sulfate coming out of solution in a cold room, not decomposition. Warm the whole bottle and redissolve, per Storage above.

Development visibly continuing after the film enters the bath. The bath is exhausted and has gone alkaline. It looks identical when spent — Kodak’s own warning is that the appearance of a stop bath does not change until well beyond its useful life, which is why the company sold a testing solution that turns from yellow to purple when the acidity falls too low to stop development. Blue litmus is the field version: red means acid and working, blue means finished.

A white sludge in the fixing bath and brown stains appearing on the negatives later. The classic symptom of an exhausted or skipped stop bath: developer alkali reaches the fixer, neutralises its acid, and aluminium sulfite precipitates. Replace the stop bath, then the fixer.

A strong vinegar smell that was not there yesterday. The tray has concentrated by evaporation. Three days is Kodak’s limit for a stop bath standing in a tray, and the reason is as much this as exhaustion.

Thirty seconds or three minutes? Both are Kodak’s, in different decades. If you must pick one for a modern hardened film, the shorter time is the more conservative choice — less time in an acid, and the same stopping action — and if you are working with a soft or self-coated emulsion in a warm room, the longer soak is what the bath was designed around. Record which you used; the lab notebook exists for exactly this kind of undecidable.

Experiments

Weigh the swelling, which is the only claim on this page that a home darkroom can test directly. Take four strips of the same film, undeveloped and fixed out so the emulsion is clear. Blot each dry and weigh it. Soak one in water, one in SB-1, one in SB-5 and one in SB-5 with the sulfate left out — all at the same temperature, all for three minutes. Blot each identically and weigh again. The gain in milligrams is the water taken up. If the salt does what its name claims, the SB-5 strip gains least and the sulfate-free control gains most of the three acid baths. This is the whole formula in one balance reading.

Measure the salt effect on the acid. Make 0.152 mol/L acetic acid — 9.11 g of the acid in a litre — and measure its pH. Then dissolve 45 g of sodium sulfate in it and measure again. The pH should fall, because a high ionic strength stabilises the ions of a dissociating weak acid and shifts its apparent pK_a down. How far it falls is a number this course does not publish, because it depends on your meter’s calibration in a concentrated salt solution — which is itself the lesson.

Test the two capacities against each other. Run Part X’s capacity experiment on SB-5 with a measured D-76 as the alkali, and plot pH against millilitres of developer added. Where does the curve break — at the 1946 figure, at the 1977 figure, or somewhere between? You will not settle what Kodak did, but you will find out which figure describes your darkroom.

Measure your own carry-over and check Kodak’s 720 millilitres. Weigh a dry film strip, run it through the developer, drain it for the count you actually use, and weigh it again. The gain in grams is very nearly the carry-over in millilitres. Multiply by the number of rolls in Kodak’s figure and see how close you get to 24 fluid ounces — and if you are an order of magnitude short, as you probably will be, you have just demonstrated that Kodak was measuring a machine and you are measuring a film.

Compare against the sulfate-free control, photographically. Process two identical films at 26 °C, one through SB-5 and one through SB-1 at the same acid molarity, keeping every other bath the same. Dry them and examine the surfaces under raking light for reticulation, drying marks and softening. This is the experiment Kodak’s photofinishing customers were running every day, and it is the only one that tests the formula against the reason it was written.