Overview: Stopping Development

Ten seconds is the whole of it. A film leaves the developer, meets a tray of dilute acid, and development does not slow down — it ends, before you have finished pouring. Three pages are enough to explain why ten seconds does the job, what those ten seconds cost the bath, and how many films or prints a litre of it will take before it stops working.

The bath with two jobs

A stop bath arrests development by dropping the pH of the emulsion so far that the developing agent is no longer in the form that reduces silver. That is the job everyone names. The second job is the one that decides whether you use one at all: the alkali a wet film or print carries out of the developer would otherwise land in the fixer and push its pH up, and an acid fixer that has drifted alkaline stains, sludges and fogs. ILFORD’s own sheet puts both in one sentence — a stop bath stops development immediately and helps to maintain the activity and prolong the life of the fixer by reducing carryover of alkaline developer into an acidic bath.

Neither job removes anything from the emulsion. A stop bath is not a fixer, it dissolves no silver halide, and nothing in this part will make an image more permanent.

What gets measured

Everything in this part reduces to one quantity: moles of acid against moles of alkali arriving. A litre of Kodak’s SB-1 holds about 0.3 mol of acetic acid. A litre of D-76 demands about 0.8 mol of acid to bring it down to a stop bath’s working pH, almost all of it because of the sulfite rather than the borax. Divide one by the other and a litre of stop bath will absorb a third of a litre of developer — after which the arithmetic is only ever about how much developer a sheet of film or paper actually carries.

So the first lesson does that arithmetic, the second compares the acids that can supply the moles, and the third puts a pH meter in the bath and watches the number climb, addition by addition, until the reserve is gone. It is the cheapest experiment in the course and one of the most transferable: an exhaustion curve measured once is a way of thinking about every bath in the darkroom.

What is on the bench

Two acids, both dilute, and a dye. Acetic acid is corrosive as the concentrate — PubChem’s aggregation of 5,076 ECHA notifications gives H314 in more than 99.9 per cent of them and H226, flammable, in 99.7 — so this course buys it already diluted and says so on every page that touches it. Citric acid is milder: H319 in 84.7 per cent of 4,373 reports, H335 in 23 per cent, and 359 of those reports state that it meets no GHS criterion at all. It is a solid, it does not smell, and it is why ILFOSTOP exists.

Those two figures are the raw substances. The bought product is classified more gently than either of them, and the maker’s own safety data sheet is the authority for it rather than an aggregation: ILFORD classify ILFOSTOP as Eye Irrit. 2 and nothing else — GHS07, signal word Warning, hazard statement H319 alone — and assign it no occupational exposure limit. That is what a dilute citric stop bath is, and it is the strongest argument on this page for choosing one.

The indicator dye is not named by any manufacturer sheet this course holds, and the SDS is the sharpest evidence of that: its composition table names citric acid at 10 to 30 per cent and 2-phenoxyethanol at 1 to 5 per cent, and lists nothing else. A dye present at a working concentration is below the threshold at which a composition table has to declare it. What is published is the colour change, yellow to purple, and the fact that bromocresol purple turns yellow-to-purple across pH 5.2 to 6.8.

⚠The one thing this part does that the safety sheets warn against

Putting a sulfite-rich developer into an acid is the acid-with-sulfite reaction on the incompatibilities page, and it releases sulfur dioxide, which carries one of the tightest workplace limits in the course. Ordinary stop-bath use does it a few millilitres at a time; the experiment in this part does it deliberately, at bench scale, with the vessels covered between additions and the ventilation the classification rubric sets for Level A. The experiment page states the quantities and the controls rather than the reassurance.

What you need first

From Part II, measurement and uncertainty and concentration and dilution. From Part III, acids, bases and pH, buffers and buffer capacity and the pH laboratory, whose meter this part depends on. From Part VIII, development kinetics and alkalis, buffers and pH. Part X applies all of them and re-derives none.

The pages of this part

Part10 of 28Level2 — PractitionerPages3Estimated time3.6 hoursHighest safety levelLevel A

0 / 3 lessons in this part completed

• LessonWhy Stopping Works50 min
• LessonStop Bath Formulations and the Fixer Interaction45 min
• ExperimentExperiment: Measuring Stop Bath CapacityA120 min