Why Stopping Works

Kodak’s SB-1 tells you to rinse prints for five seconds. ILFORD gives ten. Development in the same emulsion takes eight or twelve minutes. Something in those few seconds is roughly a hundred times faster than development, and it is worth knowing exactly what, because the same reasoning tells you how many sheets a litre of the bath will take before it stops doing it.

The reaction is not slowed, it is switched off

Part VIII established the mechanism and this page will not re-derive it: a developing agent reduces silver at a rate that depends on how much of it is present as the ionised form, and the alkali’s whole job is to put it there. Alkalis, buffers and pH works the numbers for hydroquinone, whose first pK_a is 9.88: about two thirds ionised in a paper developer at pH 10.3, about two per cent at pH 8.3.

Take the same curve down to a stop bath’s pH and it stops being a curve worth plotting. At pH 3 the ratio of ionised to un-ionised hydroquinone is 10^3−9.88, which is about one part in ten million. There is no meaningful concentration of the reducing species left, so there is no meaningful rate. That is why the correct verb is stop rather than slow, and why a stop bath does not need to be strong: it needs to move the pH by seven units, and moving pH is cheap in a solution that has no buffer reserve of its own.

The argument survives the gaps Part VIII left open

Part VIII was explicit about what it could not source: no redox potential for any developing agent at a developer’s pH, the semiquinone intermediate contested, and metol’s oxidation product unnamed in any document the course holds. None of that is repaired here, and none of it needs to be, because the stopping argument does not run on a potential. It runs on an ionisation constant, which is measured, tabulated and uncontroversial.

That is worth noticing as a piece of method rather than as a consolation. A missing datum only blocks the arguments that need it. If you wanted to predict how fast hydroquinone reduces silver at pH 10.3 you would need the thermodynamics and the kinetics that Part VIII said were not available. To predict that it does essentially nothing at pH 3 you need only to know that the reactive species is the anion and that its pK_a is 9.88. The same reasoning covers the agents whose pK_a values the course does hold, and it is why an acid stop works on every developer in the formulary rather than having to be matched to one.

What the argument does not settle is the ascorbate case, where the agent is already an acid, or the staining developers, whose oxidised products are the point rather than a by-product. Both are outside this part; both are places where the same question would need asking again.

⚗Two reactions, and which one actually does the work

The reaction everyone writes down is the one with the alkali:

CO₃²⁻ + 2 CH₃COOH → H₂O + CO₂ + 2 CH₃COO⁻

An acetic stop bath meeting the carbonate of a paper developer

The one that does most of the work in a film developer is quieter, because the sulfite is present in far larger quantity than the alkali:

SO₃²⁻ + CH₃COOH → HSO₃⁻ + CH₃COO⁻

The same bath meeting the sulfite, which is where most of D-76's acid demand lives

Both are simple proton transfers and both go essentially to completion in the direction shown, because acetic acid’s pK_a of 4.76 sits below both carbonate’s 10.33 and hydrogensulfite’s 7.19. The buffers page owns the constants; what Part X does with them is count moles.

Getting in: seconds, and why not milliseconds

Part III’s diffusion page gives the rule of thumb for how long a species takes to travel a distance by diffusion alone.

t ≈ x² ÷ 2D

Diffusion time over a distance

t is the time, x the distance and D the diffusion coefficient. Put an emulsion’s order of ten micrometres and a free-solution ion’s 1 × 10⁻⁹ m² s⁻¹ into it and the first acid reaches the bottom of the layer in about 0.05 seconds. Arrival is not what takes ten seconds.

What takes the time is that arriving is not the same as winning. The layer is not empty: it is holding developer, and every carbonate ion and every sulfite ion in it must be protonated before the pH there actually falls. The acid front therefore advances more slowly than a tracer would, roughly in proportion to how much base it has to consume on the way. Even so the arithmetic lands under a second for the emulsion alone, and both manufacturers specify about ten times that. Ten seconds is an engineering margin, not a diffusion time.

The margin exists because of what sits underneath the emulsion, and that is also where the published capacities come from.

Where the developer that spends the stop bath is actually sitting

1. 135 mm film in a spiral — thin emulsion, impermeable base; the load is surface film plus what the spiral traps
2. 2Resin-coated paper — polyethylene both sides; the paper core never wets, so only the surface film travels
3. 3Fibre-base paper — a bare paper core that soaks, holds and then releases developer back into the emulsion
4. 4ILFORD's published capacities — 15 films, 60 RC or 30 FB sheets per litre — the fibre figure is half, and the middle panel is why

Drawn to show where the liquid is, not to scale. The one number to carry away is the factor of two between resin-coated and fibre-base paper, which is a manufacturer's measurement of an absorbency difference rather than anything to do with the image.

Where the ten seconds sits in a film process

Film

Develop, 8–12 minStop, 10 sFix, 3 minWash, 5–10 min

0about 12 minabout 24 min

How much developer actually leaves the tank

This is the quantity everything else divides by, and it is the one the literature is least willing to state. Two manufacturers publish a capacity; neither publishes a carryover volume. But a capacity and a formula together imply one, and the arithmetic runs in both directions.

Kodak’s 1928 primer is the more useful of the two because it states its conditions. The life of the SB-1 acid rinse bath, it says, is decided by the quantity of alkali carried over from the developer, which depends on the quantity of carbonate in the developer, the quantity of developer retained by the print, and the time of draining — and then, with a one- to two-second drain and a typical Elon-hydroquinone developer, approximately seventy-five 8 × 10 prints per gallon. Kodak Limited’s 1949 handbook gives the same bath ninety sheets of the same size per 160 fluid ounces. Convert both and they are the same number: 19.8 sheets per litre.

🧮What a litre of SB-1 holds, and what a litre of developer wants

The acid, from the formula. Kodak Limited’s SB-1 is 17 c.c. of glacial acetic acid in a litre of water. At PubChem’s experimental density of 1.049 g/mL and a glacial acid of about 99.5 per cent, that is 17 × 1.049 × 0.995 = 17.7 g, and at a molar mass of 60.05 g/mol, 0.295 mol of acetic acid per litre. Round it to 0.30.

The demand, from the developer’s formula. D-76 is sodium sulfite 100 g and borax 2.0 g per litre. 100 ÷ 126.05 = 0.793 mol/L of sulfite, each of which takes one proton on its way to hydrogensulfite. 2.0 ÷ 381.4 = 5.24 mmol/L of borax, which dissolves to give two borate ions each, so 0.0105 mol/L of borate, one proton apiece. Total: 0.80 mol of acid demanded per litre of D-76, of which the sulfite is 98.7 per cent and the alkali that gives the developer its name is 1.3 per cent.

A paper developer is the other way round. D-72 stock is sodium carbonate 67.5 g and sodium sulfite 45 g per litre, diluted one part to two for use. That working solution is 0.212 mol/L of carbonate, which takes two protons each, and 0.119 mol/L of sulfite, which takes one: 0.544 mol/L, of which the carbonate is 78 per cent.

So a litre of SB-1 will absorb 0.295 ÷ 0.80 = 370 mL of D-76 stock, or 0.295 ÷ 0.544 = 540 mL of working D-72 — if it is run all the way to neutrality, which is not where anyone stops.

The sheet brings its own buffer with it

There is a second reason ten seconds is a fair specification, and it is the one item on this page that is genuinely about buffer capacity rather than about acid content.

The solution the sheet carries is not merely alkaline. It is strongly buffered, in the sense Part III defines: it holds a large reserve of conjugate pairs, 0.80 mol/L for D-76 and 0.54 for working D-72, and it resists being moved. The stop bath, by contrast, is a weak acid on its own, which is a poor buffer with a large total. So what meets inside the gelatin is a small volume of something stubborn diffusing against a large volume of something plentiful, and the winner is decided by moles, not by pH.

Put numbers on the local case. An 8 × 10 sheet carrying, say, 15 mL of working D-72 has brought about 8 mmol of proton demand into a tray holding 300 mmol. It is comfortably outnumbered — but only in the tray. Inside the layer, for the first fraction of a second, the ratio is the other way round, and the pH there does not fall until enough acid has walked in to consume the carbonate and the sulfite in place. That is the interval the ten seconds is buying, and it is why agitation in the first moments matters: Kodak’s toning manual reports that insufficient agitation of prints, especially during the first few seconds in the stop bath, causes a mottle that does not become visible until the print is toned.

It is also why the same manual warns against an over-concentrated bath, which the manual attributes to evaporation in a tray left more than three days or a tank left more than a month. More acid does not stop development sooner in any way that matters — it was already essentially instant — but it does drive the pH inside the swollen layer further down than it needs to go, which is the condition the next section is about.

Now run the capacity arithmetic backwards. Kodak’s 19.8 prints per litre, against 0.295 mol of acid, is 14.9 mmol of alkali per 8 × 10 print if the bath is spent to the last proton — 27 mL of working D-72. Kodak’s endpoint, though, was a blue litmus paper failing to turn red, which happens somewhere near pH 5, not at pH 7; take that endpoint and only about two thirds of the acetic acid has been consumed, and the same capacity implies 18 mL per print. ILFORD’s thirty fibre sheets per litre, if their bath held a comparable reserve, would imply something nearer 13 mL.

!What the course claims here, and what it does not

Established, from cited documents: the SB-1 formula and its capacity in two Kodak publications; the D-76 and D-72 formulas; ILFOSTOP’s capacities; the molar masses and the density of glacial acetic acid.

Inferred by this course from those numbers: that an 8 × 10 fibre-base print carries somewhere between about 13 and 28 mL of working developer into the stop bath, and that a 135-36 film in a spiral carries around 18 mL — the film figure being ILFORD’s fifteen films per litre divided into the same 0.22 mol of acid, 14.7 mmol per film, divided again by D-76’s demand of 0.80 mol/L. The spread is not noise. It is the drain time and the endpoint criterion, neither of which most published capacities state.

Not claimed: any carryover figure from the literature, because this course found none in a manufacturer or standards document, and will not invent one.

What settles it on your bench in five minutes: weigh a dry sheet or a loaded spiral, put it in the developer, drain it for the count you actually use, and weigh it again. The difference in grams is the carryover in millilitres to better than five per cent. Do it for your paper, your drain and your hands, and every capacity on this page becomes a number about your darkroom.

Capacity, exhaustion, and the difference between them

The course keeps capacity and exhaustion apart, and a stop bath is the clearest case in the darkroom. Capacity is a property the bath had the moment it was mixed: 0.295 mol of acid, and nothing that happens later adds any. Exhaustion is the event at the end of spending it.

What makes the spending hard to watch is that a fresh stop bath is a weak acid on its own, which is a poor buffer, and it becomes a buffer as it works: every neutralisation converts acetic acid into acetate, and the pH begins to be held near the pK_a of 4.76. So the pH falls off a cliff at the start, crawls across the middle, and then climbs steeply at the end. A bath halfway through its reserve does not read halfway.

The manufacturers’ answer is not a pH meter but a dye. ILFOSTOP, ILFORD writes, contains an indicator dye that is pH sensitive and changes colour from yellow to purple as the bath becomes exhausted, and Kodak’s toning manual gives the same advice from the other side: do not use an exhausted or over-concentrated stop bath, replace it frequently, or use an indicator stop bath, which signals when to change the bath.

📷The dye is an instrument, and it has a scale

No manufacturer sheet this course holds names the dye in any commercial product. What is published is the colour change. Bromocresol purple is the dye whose published behaviour matches it: PubChem records it as a pH indicator over the range 5.2 to 6.8, yellow at 5.2 and purple at 6.8, with a pK_a of 6.3.

Two consequences follow if a bath’s dye behaves like that one. The colour does not begin to move until the pH has already climbed past 5, which on the arithmetic above is most of the way through the reserve — so a bath still showing its fresh colour may have very little acid left, and the dye is a warning rather than a fuel gauge. And the change is spread over one and a half pH units, so “it has gone purple” is a decision about hue that different eyes and different safelights will make at different points. Kodak’s low-tech version, blue litmus paper, has the same character and an even coarser scale.

The carbon dioxide question

Darkroom lore says a stop bath that is too strong gives pinholes. The mechanism behind the lore is real, sourced and older than the lore. Kodak’s 1928 primer sets it out under Blisters: when the sodium carbonate of the developer is neutralised by the acid, carbon dioxide gas is evolved, which produces blisters if the gelatin is too soft to withstand the disruptive action of the gas — and it names the conditions, which are an excess of acid, insufficient rinsing, warm weather, and a bath that is not hardening well. On dry film, it says, they appear as tiny crater-like depressions by reflected light. That description is where the word “pinhole” comes from.

Kodak Limited then supplied the control experiment without calling it one. Announcing Kodalk, its metaborate alkali, the 1949 handbook states flatly that films developed in a Kodalk developer will not blister in an acid fixing bath even at high temperatures, because the alkali does not evolve carbon dioxide on acidifying. Change the alkali, keep everything else, and the fault disappears. Part VIII draws the mechanism and this page does not repeat it.

⁇Convention and evidence, kept apart

What the evidence supports: carbonate meeting acid inside a swollen gelatin layer makes carbon dioxide, and gas made inside a soft layer can lift it. Kodak stated the mechanism in 1928 and stated the metaborate control in 1949.

What is convention: the habit of blaming the stop bath. Kodak’s own account puts the acid in the fixing bath, and lists soft gelatin, poor rinsing and warm baths beside it — a strongly acid rinse is one of four conditions, not the cause.

What has changed since: ILFORD’s HYPAM sheet states that a fix hardener is no longer generally recommended because modern camera films are sufficiently hardened at manufacture. The 1928 condition “if the gelatin is too soft” is met far less often now than it was, which is a reason to expect the fault to be rare rather than a reason to say it cannot happen.

What this course cannot tell you: how often it happens with a current film, because it has found no measurement. That is a gap an afternoon and a deliberately over-strong bath could close, and the experiment in this part supplies the arithmetic to design it.

What happens if you leave it out

Three things, in order of how quickly they arrive.

Development continues. Kodak’s 1928 primer prefers an acid rinse to a water rinse for exactly this reason: it arrests development immediately, whereas in a water rinse development continues if the rinsing is unduly prolonged. A print in a tray of water is still developing, more slowly and less evenly, and the effect lands on the sheets that sit longest.

The fixer’s pH drifts. ILFORD’s process-control introduction states the chain plainly: developers are mildly alkaline, fixers are mildly acidic, fixer baths become less efficient as they become neutral and stop working in alkaline conditions, and carrying alkaline developer straight into the fixer affects the fixer’s performance over time. ILFORD RAPID FIXER is published at pH 5.0 to 5.5, and the sheet’s own remedy for a bath that has drifted high — if a stop bath is not used — is a few drops of 50 per cent acetic acid, gradually and with thorough stirring, and not past the published limits. That is topping up an acid reserve the stop bath would have spared.

Then the faults. The 1928 primer names two. A fixing bath that has taken enough carried-over developer throws a white sludge and turns alkaline, and prints fixed in an alkaline bath are likely to stain brown. And dichroic fog — greenish by reflected light, reddish by transmitted — appears when the fixing bath does not contain acid or is old and silver-loaded, and “never occurs in a fresh acid fixing bath, or if the film is rinsed before fixing”.

✕A water rinse is a real option, not a failure — but it is a different one

ILFORD’s own sheet says a water bath can be substituted where a stop cannot be included, and then names the price: an increased risk of processing marks and stains, a water bath that must be completely changed at very frequent intervals, and a fixer whose activity has to be monitored and replenished adequately instead. The next lesson compares that route properly. What it is not is the same process with a step left out: it moves the load from a bath that was designed to take it onto one that was not.

Recap

A stop bath works by moving the pH far enough that the developing agent is no longer in its ionised, reducing form — seven units, not one, which is why the reaction stops rather than slows. The acid reaches the bottom of an emulsion in a fraction of a second; the ten seconds on the sheet is margin, and the margin exists because a fibre-base paper holds developer in its core and gives it back. The bath’s capacity is a fixed number of moles of acid set when it was mixed, and it is spent by the alkali carryover brings in — mostly sulfite in a film developer, mostly carbonate in a paper developer. Kodak’s and ILFORD’s published capacities, run backwards through their own formulas, imply a carryover of roughly 13 to 28 mL for an 8 × 10 fibre print, and the spread in that range is the drain time and the endpoint criterion rather than any disagreement about chemistry. The bath signals its own exhaustion late, through a dye whose transition is wide, so the honest control is a count of work done. Leave the stop out and development continues, the fixer’s acid reserve pays for it instead, and the faults that follow are sludge, stain and dichroic fog.

Next: the acids that can supply those moles — acetic, citric, indicator, hardening and none at all — compared on chemistry, capacity, odour, hazard and what each does to the fixer.

Check your understanding

Question 1. A litre of Kodak SB-1 holds 0.295 mol of acetic acid. A litre of D-76 stock contains 100 g of sodium sulfite and 2.0 g of borax. Which ingredient of the developer will spend most of that acid, and roughly how much D-76 will the litre of stop bath absorb?

• The borax, because it is the alkali; about 30 mL
• The sulfite, because 100 g/L is 0.79 mol/L against the borate pair's 0.021 mol/L; about 370 mL
• The hydroquinone, because it is the acid-reactive ingredient; about 1 L
• They contribute equally, so it cannot be decided from the formula

Show the answer and why

Answer: The sulfite, because 100 g/L is 0.79 mol/L against the borate pair's 0.021 mol/L; about 370 mL

Divide each mass by its molar mass. 100 ÷ 126.05 = 0.793 mol/L of sulfite, each taking one proton to become hydrogensulfite; 2.0 ÷ 381.4 = 5.24 mmol/L of borax, which supplies about 0.0105 mol/L of borate. The sulfite is 98.7 per cent of the demand and the alkali the developer is named for is 1.3 per cent. Total demand 0.80 mol/L, so 0.295 ÷ 0.80 = 0.37 L. The lesson for practice is that the ingredient that decides a stop bath's life in film work is the preservative, not the accelerator — which is why a paper developer, where the carbonate dominates, behaves quite differently.

Question 2. ILFORD publishes ILFOSTOP at 60 sheets of 20.3 × 25.4 cm resin-coated paper per litre and 30 of the same size on fibre base. What does the factor of two measure?

• A difference in emulsion thickness between the two paper types
• A difference in how much developer the base absorbs, since resin-coated paper is sealed with polyethylene and fibre base is not
• A difference in the pH of the two paper developers normally used
• A safety margin ILFORD applies to fibre paper because it is more expensive

Show the answer and why

Answer: A difference in how much developer the base absorbs, since resin-coated paper is sealed with polyethylene and fibre base is not

The emulsion is essentially the same and it is thin either way. What differs is underneath it: resin-coated paper is polyethylene on both faces, so the paper core never wets and only the surface film travels, while a fibre base soaks like blotting paper, holds several times as much and then feeds it back into the emulsion in the next tray. Capacity per litre is acid divided by alkali per sheet, so halving the capacity means doubling the carryover. It also explains why a fibre print tolerates — and needs — a longer time in the stop, and why draining it properly is worth more than any other single habit.

Question 3. A stop bath containing an indicator dye still looks its fresh yellow after a long printing session. What can you conclude?

• The bath is fresh and can be used for another full session
• Very little: if the dye behaves like bromocresol purple it does not begin to move until about pH 5.2, by which point most of the acid reserve has already been spent
• The dye has been washed out of the bath by the prints
• The developer being used contains no alkali

Show the answer and why

Answer: Very little: if the dye behaves like bromocresol purple it does not begin to move until about pH 5.2, by which point most of the acid reserve has already been spent

A weak acid on its own is a poor buffer, so the pH drops steeply at the start of the titration, crawls across the middle while acetate or citrate accumulates, and only climbs steeply at the end. An indicator changes across roughly a unit and a half either side of its own pKa — 5.2 to 6.8 for bromocresol purple, pKa 6.3 — so the colour starts to move near the top of that crawl. Treat it as a warning that the bath is nearly gone, not as a gauge of how much is left, and keep a count of work done as the primary control.

Question 4. Kodak's 1928 primer attributes blisters to carbon dioxide made when the developer's carbonate meets acid, and Kodak Limited's 1949 handbook says a Kodalk developer will not blister even at high temperatures. What follows for a darkroom using a borax film developer and an acetic stop?

• Blisters are impossible, because borax is not a carbonate
• The carbon dioxide route is not available, since borate becomes boric acid rather than a gas — so a blister-like fault in that darkroom needs a different explanation
• Blisters are more likely, because borax is a weaker alkali
• The two statements contradict each other and neither can be used

Show the answer and why

Answer: The carbon dioxide route is not available, since borate becomes boric acid rather than a gas — so a blister-like fault in that darkroom needs a different explanation

Borate takes a proton and becomes boric acid, which stays dissolved; carbonate takes two and the second one leaves as carbon dioxide. Kodak's Kodalk claim is a control experiment in a sentence: change only the alkali and the fault disappears, which identifies the gas as the cause. So in a D-76 darkroom the mechanism the folklore names is simply absent, and marks blamed on the stop bath should be traced to something else — drying, air bells on immersion, or the mottle Kodak's toning manual attributes to an over-concentrated stop and poor agitation in the first few seconds.

Question 5. A printer stops using a stop bath and rinses in a tray of standing water instead. ILFORD's fixer sheet gives a remedy for what happens next. What is it, and what does it tell you?

• Add fresh fixer concentrate, because the fixer is diluted
• Add a few drops of 50 per cent acetic acid, gradually and with stirring, without going below the published pH — which says the fixer's acid reserve is now paying for the carried-over alkali
• Raise the fixing time, because the fixer works more slowly
• Change the wash water more often, because the fault is in washing

Show the answer and why

Answer: Add a few drops of 50 per cent acetic acid, gradually and with stirring, without going below the published pH — which says the fixer's acid reserve is now paying for the carried-over alkali

The sheet gives the instruction under the heading Adjusting fixer pH and prefaces it with "if a stop bath is not used": the fixer has drifted alkaline because carried-over developer has been neutralising its acid, and the fix is to put acid back. That is replenishing the reserve a stop bath would have spared, one bath too late and into a solution where too much acid causes sulfurisation rather than merely exhaustion, which is why the sheet also says not to go below the published 5.0 to 5.5. The whole economics of the stop bath is in that pair of sentences: cheap acid in a cheap bath, or expensive acid in the bath you least want to disturb.

Check my answersClear