Acetic acid (glacial)

A print left in a water rinse goes on developing. A print dropped into an acid rinse stops. That is why acetic acid has a place in a darkroom at all, and the reason the same acid also sits in the fixer is a different argument entirely — one about the difference between how strong an acid is and how much alkali it can absorb.

In photography

The stop bath. Kodak’s 1928 primer is blunt about the choice: an acid rinse, or “short stop”, is strongly recommended instead of a water rinse because it arrests development immediately, whereas in water development continues if the rinsing is prolonged. Its formula SB-1 is water to one litre with 48 mL of 28 per cent acetic acid; Kodak Limited’s 1949 handbook prints the same designation with the concentrate instead — one litre of water and 17 mL of glacial acetic acid, prints rinsed for five seconds — and an SB-1A for Reflex plates at 50 mL of glacial acid per litre. The two SB-1 formulas agree closely: 48 mL of an acid made by diluting three parts glacial with eight parts water is about 13 mL of glacial acid, against Kodak Limited’s 17 mL. A working stop bath is somewhere between one and two per cent v/v of the glacial acid.

⚗The chemistry

The mechanism the primer gives is neutralisation, not poisoning: the acid neutralises the alkali carried over in the film from the developer and thereby arrests development. A developing agent needs its alkali; take the alkali away and the reaction has nowhere to go. What makes acetic acid suitable is that it is weak. The IUPAC compilation records a pK_a of 4.76 at 25 °C and a molality of 0.01, assessed as approximate. Kodak Limited’s 17 mL of glacial acid per litre is about 0.30 mol/L; on the course’s own arithmetic from those two numbers — not a measured value — roughly one molecule in 130 is ionised at any moment and the bath sits near pH 2.6. The other 129 are held in reserve, ready to neutralise the next print’s carry-over.

The acid fixing bath. Here the same weakness solves a harder problem. The primer sets it out exactly. Add a strong acid to hypo and the thiosulfate becomes free thiosulfuric acid, which is unstable and throws sulfur out of solution; sulfite present in the bath drives the same reaction backwards, which is why an acid fixer contains sulfite at all. But alkaline developer is carried into the fixer on every sheet, so a great deal of acid is needed and the bath must still not be strongly acid. The resolution is the distinction the primer draws next: the strength of an acid depends on the fraction of its hydrogen that dissociates, while the quantity of alkali it can neutralise depends on the total hydrogen present, dissociated or not. “Since a large quantity of a weak acid is required, the best acid for the purpose is acetic acid.”

CH₃COOH ⇌ CH₃COO⁻ + H⁺

Capacity and strength are different quantities: nearly all of it stays on the left

Kodak’s stock hardener F-1a shows the quantity involved — 1,700 mL of water at about 52 °C, sodium sulfite 480 g, 28 per cent acetic acid 1,500 mL, powdered potassium alum 480 g, cold water to four litres, dissolved strictly in that order, because if the sulfite is not thoroughly dissolved before the acid arrives a precipitate of sulfur is likely to form. The 1949 tropical bath F-5 and the modern hardener stock F-5a in Kodak’s 2006 toning publication carry the same acid alongside boric acid.

What more and less of it do. Too little acid, and the primer’s diagnosis is a white gelatinous sludge of aluminium sulfite: developer carry-over has neutralised the bath, hardening stops and prints fixed in it stain brown. The primer names the commonest cause bluntly — a formula that calls for glacial acetic acid made up with 28 per cent acid by mistake, which leaves less than a third of the required acid in the bath. Too much acid, and the sulfite can no longer hold the reaction back: the bath sulfurises and goes milky. The fixer lives in a window, and acetic acid is what keeps it there.

Where it is the wrong acid. Ware’s Platinomicon explains why acetic acid is a poor clearing agent for the iron-based processes: iron(III) and acetate form a deep red acetato-oxo-triiron(III) cation, so an acetic bath colours the paper instead of stripping iron out of it.

Properties

A colourless liquid with a sour, vinegar-like odour, miscible with water in all proportions, specific gravity 1.05, boiling at 118 °C. The pure compound is a solid below 16.7 °C (62 °F in the NIOSH pocket guide’s units) — hence “glacial”, and hence a bottle that has set into a white crystalline block in an unheated darkroom in January and needs standing in warm water before it can be measured. NIOSH gives a flash point of 103 °F (39 °C) and classes it a Class II combustible liquid; Chemical Safety Card 0363 gives explosive limits of 6.0 to 17 vol% in air, notes that an explosive vapour–air mixture can form above 39 °C, calls the substance a weak acid and records that it attacks certain plastics and rubber. CAMEO records that dissolution in water generates some heat.

Kodak’s 1928 primer adds a commercial warning still worth having: 28 per cent acetic acid prepared by diluting the glacial acid is not the same product as the “commercial 28% acetic acid” redistilled from the destructive distillation of wood, and vinegar, at 4 to 8 per cent, carries impurities that have no business in a fixing bath.

Handling

The aggregated ECHA notifications on PubChem classify it Danger, with the flame and corrosion pictograms: H314, severe skin burns and eye damage, in more than 99.9 per cent of the 5,076 company reports carrying hazard codes, H226 in 99.7 per cent and H318 in 14.7 per cent. HSE’s EH40 sets a British workplace exposure limit of 10 ppm (25 mg/m³) over eight hours and 20 ppm over fifteen minutes; NIOSH sets the same long-term figure and an IDLH of 50 ppm, with a symptom list running from sore throat through skin burns to dental erosion and chronic bronchitis.

Why Level B. The course rubric caps Level A at a classification “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”, and at “no volatile organic solvents beyond isopropanol in small quantities”. A 1.5 per cent v/v working stop bath would sit inside that; the bottle it is diluted from does not. Two Level B criteria decide it: the handling of a concentrated acid, and a failure mode that is a splash and a burn rather than a spoiled negative. It does not reach Level C — no fume cupboard is the recognised control at darkroom quantities, and the aggregated classification carries no carcinogen, mutagen or reproductive-toxin entry. NIOSH’s symptom list does include skin sensitisation, which is an argument for gloves rather than for a fume cupboard.

⚠Safety

Acid into water, slowly, and never water into acid. Princeton’s environmental health and safety guidance for photography states the rule without qualification, and the physical reason is the one CAMEO gives: dilution generates heat. Poured the wrong way round, that heat is released in the small volume of water sitting on top of a dense acid, which can boil and throw concentrated acid back out of the vessel. Goggles and an apron before the cap comes off the glacial acid; gloves; the window open or the extractor running, because the card records that a harmful air concentration is reached rather quickly on evaporation at 20 °C. Keep it away from the oxidisers — NIOSH names chromic acid, sodium peroxide and nitric acid, and CAMEO records that acetic acid can explode with nitric acid if the mixture is not kept cold — and away from the caustics, from metal vessels, and from any hypo that has no sulfite in it.

Waste

Spent stop bath is acidic and carries developer rather than silver, so it is not part of the recovery stream, and its problem is pH. Kodak’s J-300 guidance for amateur photographers is explicit: KODAK Indicator Stop Bath contains acetic acid, has a pH of 1.0, and should be neutralised with sodium bicarbonate solution before the material goes to the sewer, the bicarbonate added slowly because the mixture foams, in a well-ventilated area, with gloves, goggles and an apron. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, which is the target the neutralisation aims at.

That measured pH of 1.0 is well below the 2.6 the course calculated above for SB-1, so the proprietary bath is considerably more concentrated than the 1928 formula; its composition is not published and the course has not verified it. Chemical Safety Card 0363 records the substance as harmful to aquatic organisms, so a neutralised bath still does not go on a garden. ILFORD’s guidance for domestic users is to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard; failing that, small amounts flushed with plenty of water, unmixed, and never into a septic tank. Check your local regulations, which govern.

History

The primer’s account of where the acid came from is a small industrial history. Fermentation of apple juice gives vinegar at 4 to 8 per cent, full of impurities. The stronger acid was made through acetate of lime, prepared either by neutralising vinegar with chalk or, more commonly, by neutralising with lime the crude acetic acid recovered from the destructive distillation of wood; acid made that way could reach 99.5 per cent and freezes at moderately low temperatures, “and is usually called glacial acetic acid”.

The direction of travel since is away from it. ILFORD’s current sundries sheet describes ILFOSTOP as a low odour citric acid stop bath, and gives the same reason Kodak gave in 1928 for having a stop bath at all: it stops development immediately and neutralises the developer, which helps maintain the activity of the fixer and prolongs its life by reducing carry-over of alkaline developer into an acidic bath. What changed is not the chemistry but the room. Ten parts per million is a low ceiling for a substance you can smell at a fraction of it, and in a small darkroom the smell of acetic acid is the thing most printers remember about it.