Citric Acid Stop Bath

This is a variant, and it is the course’s own. No manufacturer publishes the composition of a citric acid stop bath, and this page does not reconstruct one. What it publishes is a substitution the course made deliberately, from a formula it can cite and a set of dissociation constants it can cite, with the arithmetic shown so that you can check it or reject it.

Ingredient	Quantity	Form
Citric acid	19.2 g	anhydrous — or 21.0 g of the monohydrate
Water to make	1000 mL

!What this page is, under Rule 6

Rule 6 says a formula is published only with traceable provenance, that historical formulations are never silently changed, and that a safer or more practical variant is identified as a variant, with what it is based on, why it differs and what the consequence is. This page is the fourth of those.

What is cited: Kodak’s SB-1, at 17 mL of glacial acetic acid per litre of water, from Kodak Limited’s 1949 handbook. The pK_a values of both acids, from the IUPAC digitised dissociation-constant dataset. Citric acid’s molar mass and solubility, from the International Chemical Safety Card.

What is arithmetic: the 19.2 g. It is 0.100 mol of a triprotic acid, chosen because SB-1 holds about 0.30 mol of a monoprotic one, and it is worked out in full below.

What is not here, and will not be: any statement about what is in a commercial citric stop bath. ILFORD describes ILFOSTOP as a low-odour citric acid stop bath containing a pH-sensitive indicator dye and publishes its dilution, its time, its capacities, its concentrate pH and its keeping figures. It does not publish the acid concentration, the dye, or the pH at which the dye changes, and Part X is explicit that the course will not supply any of them. A plausible reconstruction of a proprietary product would be a Rule 6 violation whatever it was labelled, and this bath is not one: it is derived from Kodak, not from ILFORD.

Purpose

To do SB-1’s job — end development immediately, and protect the fixer from carried-over alkali — without keeping a corrosive, flammable, volatile liquid in the darkroom. The substitution buys three things and costs two, and both lists are below.

Recommended uses

Papers, plates and films, at working strength, wherever SB-1 would be used.

No immersion time is published for this bath, because no one has published one. Kodak’s five seconds is for SB-1; ILFORD’s ten seconds at 20 °C is for its own product at its own dilution. Neither transfers, and the course does not pretend otherwise. What can be said is that both published figures are of the order of ten seconds for a bath in this molarity range, that this bath’s starting pH is lower than SB-1’s rather than higher, and that no source suggests a slower action. Start at ten seconds with agitation, and if you want a defensible number, measure it: develop matched prints, stop them at 2, 5, 10 and 30 seconds, and find where the density stops changing.

It is the better choice for film in a tank, where there is no advantage to an indicator you cannot see and no reason to have acetic vapour in a small room. For prints in an open tray under a safelight, the argument is more balanced, and Part X sets out both sides.

When another formula is preferable

• When you want to follow a published formula exactly, SB-1 is the one with a maker’s name on it, a maker’s rinse time, a maker’s capacity and a maker’s measured effect on the fixer. Everything on this page beyond the derivation is a prediction.
• After a caustic developer, SB-1a is Kodak’s answer, and the course has not derived a citric equivalent of it, because tripling a formula the course itself constructed would be an inference built on an inference.
• When you want a bath that reports its own exhaustion, a commercial indicator stop bath does that and this one does not. The course cannot publish an indicator formula, and it will not invent a dye for you to add.
• When hard water is not an issue and vapour is not an issue, the acetic original is cheaper to understand, because everything about it is documented.

Mixing

1. Put about 800 mL of water in the vessel.
2. Weigh 19.2 g of anhydrous citric acid, or 21.0 g of the monohydrate — read the jar, because the two are different articles and the difference is nine per cent by mass.
3. Dissolve. It goes readily: the safety card gives 59 g per 100 mL at 20 °C, so this bath is at about a thirtieth of saturation and needs no warming.
4. Make up to 1.000 L.

Follow the mixing SOP for the transfer and the labelling SOP for the bottle.

🧮Where 19.2 grams comes from, in four lines

17 mL × 1.049 g/mL × 0.995 ÷ 60.05 g/mol = 0.295 mol, in about 1.017 L

SB-1's acid content

0.300 mol of acetic acid per litre, which has one ionisable proton, so 0.300 mol of protons

Rounded, as Part X works to

0.300 mol of protons ÷ 3 protons per molecule = 0.100 mol of citric acid

Matching with a triprotic acid

0.100 mol × 192.12 g/mol = 19.2 g anhydrous, or × 210.14 g/mol = 21.0 g of the monohydrate

As a mass

Matched on protons, not on grams and not on pH. That is the whole design and it is also the reason the substitution is testable rather than merely plausible. Capacity is a count of protons available to neutralise arriving alkali; pH is a ratio, not a total. The two baths are built to hold the same reserve and to start about 0.7 pH units apart — near 2.63 for the acetic and near 1.96 for the citric, on the computed curves in Part X’s capacity experiment. If pH predicted capacity, the citric bath would win. It should not, and that mismatch is what makes the experiment a test.

Behaviour

Predicted, and labelled as prediction. The course has computed the titration curves for both baths against a litre of D-76 stock and has not yet measured them. Part X publishes the computed curves with the assumptions stated: the citric bath starts about 0.7 units lower, the two curves cross at about 245 mL of added developer near pH 5.0, and from there on they stay within a tenth of a unit of each other. The prediction is that the two baths reach exhaustion at essentially the same volume of carried-over developer — which is to say, at the same capacity.

No capacity is published for this bath and none is claimed. Kodak’s 90 sheets of 8 × 10 inches per 160 fluid ounces belongs to SB-1. If the proton-matching is sound, this bath should reach the same figure; if it comes in early, the matching is wrong or something other than protons limits the bath. That is the experiment, and until it is run this page has a blank where a capacity would be.

No shelf life is published for this bath either. The nearest published figure for anything citric is ILFORD’s seven working days for the working strength of its own product — a different bath at an undisclosed concentration, quoted here as the only published number of its kind and not as this bath’s figure. What the course can say from its own sources is that a stop bath is not destroyed by air the way a developer is: Kodak gives SB-1 indefinite keeping in a stoppered bottle, full or half full. Whatever limits a citric bath’s life, it is not oxidation of the acid.

And one claim the course could not source at all. It is widely said that a dilute citrate solution is a better medium for mould than a dilute acetate one. No manufacturer sheet, safety data sheet or reference work read for this course states it, and Part X names it as unsourced. It is repeated often enough to be worth knowing about and distrusting in equal measure. The practical response is to keep the bottle clean, dated and stoppered, and to discard a bath that looks or smells wrong rather than to reason about a mechanism nobody has published.

Image characteristics

None, and the same paragraph applies as to any stop bath: it dissolves no silver halide, removes nothing from the emulsion, changes no image tone and makes no print more permanent. There is one material difference from a commercial indicator bath worth stating, and it is in this bath’s favour: ILFORD records that its own indicator product is not recommended for machine processing because the short fix and wash times used there may leave a residual dye stain on films and prints. A bath with no dye in it cannot stain.

The mechanism

19.2 g/L ÷ 192.12 g/mol = 0.100 mol/L

Citric acid

0.100 mol/L × 3 = 0.300 mol/L

Titratable protons

2.87, 4.35 and 5.68 — against acetic acid’s single 4.76 at 25 °C

pKa values, IUPAC, 20 °C

Three protons, released in three overlapping steps. That is the substantive chemical difference and it has two consequences. The first is arithmetic: a triprotic acid delivers the same reserve from a third of the moles, which is why 19.2 g does the work of about 18 g of acetic acid.

The second is the shape of the exhaustion curve. A monoprotic weak acid has one buffer plateau, centred on its pK_a; a triprotic acid has three, at 2.87, 4.35 and 5.68, and because those constants are close together the plateaus merge into a long, gently rising region rather than a single flat one. Over the pH range a stop bath actually traverses — from about 2 to about 6 — the citric system averages a little over two protons released per molecule, which is the number that matters rather than the three on the label. Part X does that calculation.

Why the lower starting pH is not an advantage. The first pK_a is 1.9 units below acetic acid’s, so at the same total proton reserve the citric bath begins about 0.7 units more acidic. That looks stronger and is not: the extra acidity is intensity, not quantity, and the alkali arriving on each print consumes protons regardless of how many of them were free at the start. Kodak stated the general principle 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.

Where the lower starting pH does matter, and it is a hazard rather than a benefit. Sulfite arrives in the tray on every print, and the fraction of sulfur(IV) present as dissolved sulfur dioxide rises sharply as pH falls. On Part X’s computation, immediately after the first addition of developer the acetic bath sits near pH 3.6 with about 1.6 per cent of its sulfur(IV) as sulfur dioxide, and the citric bath near pH 2.4 with about 19 per cent — twelve times as much. The difference is transient, because the curves converge, and it is real. The controls are the same either way: mix to strength, do not let a bath over-concentrate, ventilate, and put a water rinse between the developer and the stop if you can.

A property acetic acid does not have. Citrate is a calcium chelator, which the citric acid page records. In a stop bath it is of no photographic consequence most of the time and occasionally useful, because hard water’s calcium is held in solution rather than deposited. 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.

Function of every ingredient

Citric acid, 19.2 g anhydrous. The whole formula, and it is doing three separable things.

It is the proton reservoir, at 0.300 mol/L of titratable protons — matched by construction to SB-1’s acetic acid. That reserve is what neutralises the sodium carbonate and sodium sulfite arriving on each wet print, and it is what the bath’s capacity consists of.

It is a solid at room temperature with negligible evaporation, which is the reason the substitution exists. The safety card records evaporation at 20 °C as negligible, against acetic acid’s note that a harmful contamination of the air can be reached rather quickly at the same temperature. A jar of white powder on a shelf is a different proposition from a bottle of corrosive flammable liquid, and it does not freeze solid in an unheated darkroom in winter as glacial acetic acid does at 16.7 °C.

And it is a chelator, as above — incidental in this application, and the reason for keeping it away from metal.

More citric acid gives more capacity, a lower pH and more sulfur dioxide from the sulfite that arrives with each print; less gives a bath that exhausts early and then, having gone alkaline, does nothing at all while looking exactly the same. Read the jar for the hydrate: the anhydrous acid and the monohydrate differ by nine per cent, and 21.0 g of the monohydrate is the same 0.100 mol as 19.2 g of the anhydrous.

Water to 1000 mL. Made up to volume, not added to. This differs from SB-1, where Kodak’s instruction is to add the acid to a litre of water and the finished bath is about 1017 mL. The course makes this one up to the mark because the whole point of it is a stated molarity, and a two per cent error in the volume would be a two per cent error in the thing being matched.

Interactions

With the developer, which is the whole of its capacity. Every mole of carbonate and sulfite that arrives spends the reserve. A developer with more alkali exhausts the bath faster, exactly as for the acetic original, and SB-1a is the published demonstration of how far that goes.

With sulfite, at low pH. Discussed above: the transient sulfur dioxide fraction is higher in this bath than in the acetic one immediately after each addition. Ventilate, and prefer a covered tank for film.

With the fixer. The same as any acid stop bath: carrying acid forward on the film protects the fixer’s pH; pouring stop bath into fixer drives thiosulfate below the pH at which it is stable. See chemical incompatibilities.

With hard water and with metal. The chelation cuts both ways — helpful against a calcium scum, unhelpful in a metal funnel.

With an indicator dye — a combination the course does not publish. Citric acid will carry a pH-sensitive dye as readily as acetic acid will, and that is how commercial low-odour indicator stop baths are described. The course cannot tell you which dye or at what concentration, because no manufacturer in its corpus discloses either. Part X records the one match it found and refuses to call it an identification.

Variants

SB-1 is what this is based on, and the honest description of the relationship is that SB-1 is the formula and this is a substitution. Where the two disagree — on starting pH, on the sulfur dioxide fraction, on the shape of the exhaustion curve — SB-1’s numbers are published and these are computed.

A three-times-strength citric bath, as the equivalent of SB-1a, is not published here. The arithmetic is obvious and the course has deliberately not done it: SB-1a’s strength is Kodak’s, matched to a developer Kodak names, and tripling a bath the course itself derived would stack one inference on another. If you need it, do the arithmetic yourself, write it in the formula version record as yours, and measure the capacity rather than assuming it.

Commercial citric stop baths are behaviour and disclosed components only, and this page has already said what can and cannot be reported about them.

Your own version of this bath belongs in the formula version record like any other. This one is already the course’s variant; a variant of a variant needs its lineage written down more carefully, not less.

Safety

Level A, which is the second thing the substitution buys and the reason Part X’s practical pages sit at Level A.

Citric acid’s aggregated classification, from 4,373 ECHA reports across 57 notifications, is H319, causes serious eye irritation, in 84.7 per cent and H335, may cause respiratory irritation, in 23 per cent — with 359 of those reports stating that it meets no GHS hazard criterion at all. Set against glacial acetic acid’s H314 and H226 in more than 99 per cent of its reports, that is not a close comparison. Eye protection and gloves nonetheless: 84.7 per cent is not a minority view.

The one hazard that is genuinely worse than the liquid’s is the powder. The safety card records that a dust explosion is possible if the powder or granules are mixed with air, with explosive limits of 0.28 to 2.29 volume per cent, and gives a MAK of 2 mg/m³ for the inhalable fraction. That matters in an industrial silo far more than at a darkroom balance, and the sensible response is the ordinary one: weigh without raising dust, do not pour from a height, and do not weigh near an open flame. Its flash point is 100 °C, so the liquid is not flammable.

What the substitution does not change. The sulfur dioxide route is still there, and immediately after each print arrives it is transiently worse here than in the acetic bath. Ventilation is not optional because the acid is mild. The classification rubric sets what Level A assumes, and the first aid page covers a splash.

Storage

The jar of solid keeps indefinitely if it is kept dry, which is the third thing the substitution buys: nothing is shipped or stored as water, and there is no bottle of corrosive liquid on the shelf. Keep the lid on and the scoop out; citric acid takes up moisture from a damp darkroom and a caked jar no longer weighs what the formula assumes.

The made-up bath has no published life. ILFORD’s seven working days for its own citric working solution is the nearest published figure and belongs to a different product. Kodak’s indefinite keeping for a stoppered bottle of SB-1 shows that an acid is not destroyed by air, so oxidation is not the limit; what the limit is, for a citrate solution, the course cannot source. Date the bottle, inspect it before use, and discard anything cloudy or odd.

Plastic or glass, never metal. Label with the formula, the strength, the date and — because this is a variant — the fact that it is the course’s own, per the labelling SOP.

Incompatibilities

Spent fixer, above everything. Acid meeting thiosulfate decomposes it to sulfur dioxide and colloidal sulfur, and 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. See chemical incompatibilities.

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

Developer, backwards along the tray sequence. One pair of tongs per tray, never interchanged.

Metals, which citric acid attacks and, being a chelator, attacks in a way a simple acid does not. Plastic or glass throughout, including funnels and storage bottles.

Bases and oxidants, which the safety card names as the reactions to avoid. Neutralising the spent bath with sodium bicarbonate before disposal works and foams; add it slowly, in a well-ventilated area, with gloves and goggles.

Waste

A dilute solution of citrate, acetate-free, carrying the sulfite and carbonate it neutralised, at a pH near neutral once it is spent. It carries no silver. Its own labelled container, kept out of the fixer stream both for the reaction above and because spent fixer is a recoverable silver-bearing solution that acid contamination spoils.

Follow the general chemical waste SOP for the procedure and the disposal caveat for the standing limits on what this course can say about it. What is permitted is decided where you live, not here.

Troubleshooting

You cannot tell whether the bath is still working. The honest consequence of having no indicator dye. Use the test Kodak published in 1928 and never improved on: dip a strip of blue litmus paper. Red is acid and working; blue means the bath has gone alkaline and is finished. Or use a meter, which is better and which Part X’s experiment assumes you have.

Prints staining brown after fixing. Developer carried into an alkaline fixer, which is what a spent stop bath allows. Test the stop bath first.

A white scum on prints, in hard water. Look elsewhere. Citrate chelates calcium and should reduce a hard-water scum rather than cause one; the sourced candidates are an over-concentrated bath, insufficient agitation in the first few seconds, or a fixer whose acid reserve has drifted.

A sharp smell at the tray after a fine-grain film developer. Sulfur dioxide from the arriving sulfite, and this bath’s low starting pH makes it transiently worse than the acetic original. Ventilate, cover the tank, and consider a water rinse before the stop.

A cloudy or unpleasant bath after some days on the shelf. No published mechanism, and the mould claim is one the course could not source. Discard it. A litre costs nineteen grams of a food acid.

Experiments

Run the capacity experiment, because this bath exists to be tested. Part X’s stop bath capacity experiment titrates this bath and SB-1 side by side with measured additions of developer, and the whole design turns on whether a bath that starts 0.7 pH units lower exhausts at the same point. If it does, capacity is a count of protons and this substitution is sound. If the citric bath comes in early, this page’s arithmetic is wrong somewhere and the page should say so.

Find the immersion time nobody has published. Develop matched prints, stop them at 2, 5, 10 and 30 seconds with agitation, fix and dry, and compare densities in a mid-tone. Where the density stops changing is your bath’s working time. Record it in the lab notebook as yours, because it is.

Measure the sulfur dioxide claim indirectly. You will not have a gas meter. You do have a nose and a pH meter: record the pH immediately after the first print of a session and again after the tenth, and note when the smell appears. The prediction is that the smell is worst early, when the pH is lowest, and that it fades as the bath climbs — which is the opposite of the intuition that a tiring bath smells worse.

Compare a bath made from the monohydrate weighed as though it were anhydrous. 19.2 g of the monohydrate is 0.0914 mol, nine per cent short. Exhaust it against a correctly mixed litre and see whether nine per cent of the capacity is a difference you can measure. It is a useful calibration of how much your own weighing precision is worth.