Isopropyl alcohol

The course’s safety rubric names exactly one volatile organic solvent as acceptable in a standard home darkroom, and this is it. That is a judgement about scale and about ignition, not a statement that the liquid is mild — its flash point is below room temperature.

In photography

Coagulating an albumen layer. James Reilly’s account of albumen printing gives the clearest photographic use the course holds. Freshly albumenised paper has to be hardened before it can be sensitised, and the traditional method was to cure the sheets for months. The quick method is chemical: briefly immerse the sheets in a 70 per cent solution of isopropyl alcohol. Reilly is precise about why that number and not another. “Pure alcohol is too strong and unevenly coagulates the albumen layer, while too dilute alcohol solutions are not strong enough to coagulate the albumen before it partially dissolves into the water. Experience has shown that a 70% solution is the most effective.” He adds a second rule that shows how carefully the bath has to be matched to the paper: to stop the chloride leaching out of the layer, whatever chloride the albumen contains must also be put into the alcohol — 2 per cent ammonium chloride in the albumen means 2 per cent in the alcohol.

⚗The chemistry

A water-miscible alcohol does one thing to a protein or a gelatin layer, and everything else follows from it: it competes for the water. Abney, writing in 1885, states the property directly — “alcohol and carbolic acid, in large quantities, precipitate gelatine from solution in water” — and gives the mechanism when he describes washing an emulsion by precipitating it with spirits of wine: “the alcohol has a greater affinity for water than has the gelatine, and … in extracting the water the soluble salts are extracted with it”.

Strip the water out of a hydrated protein chain and it can no longer stay unfolded and dispersed; it collapses and aggregates, and the layer becomes insoluble. That is coagulation, and it is why Reilly’s 70 per cent is an optimum rather than a floor. Too little alcohol and the water wins, so the albumen dissolves before it sets; too much and the dehydration is so fast and so local that the layer sets unevenly.

Pulling water out of a finished film. The same competition dries a negative. Wall’s 1912 Dictionary describes the technique under spirit rather than isopropanol — the water in the pores of the gelatine is displaced by strong spirit in successive baths, and the spirit then evaporates in minutes — and the course has found no source in its own collection specifying isopropanol for that job. The chemistry is the same and the practice is common; the page names the gap rather than pretending it has a citation.

Cleaning, and the flame. Bostick & Sullivan’s wet-plate kit sheet gives two everyday uses on one page. Calcium carbonate worked into a plate “with a teaspoon of rubbing alcohol” is the non-scratching abrasive that prepares glass for collodion — the alcohol carries the polish and flashes off, leaving no residue for the silver bath to find. And the kit’s alcohol lamp, used to warm plates, solutions and varnishes, is to be filled with “high-proof grain alcohol or isopropyl alcohol from the drugstore”. That second use is the one to think about twice, and it is discussed under Handling.

Properties

A volatile colourless liquid smelling of rubbing alcohol, boiling at 83 °C and freezing at −90 °C, with a relative density of 0.79 — it floats on water. It is miscible with water in all proportions, which is what makes a 70 per cent bath possible at all, and miscible with most organic solvents; HSDB records it as insoluble in salt solution, which is a warning to anyone adding alcohol to a sensitiser that already carries a heavy salt load.

The numbers that govern its handling are all about vapour. Vapour pressure 4.4 kPa at 20 °C; flash point 11.7 °C, closed cup; auto-ignition 456 °C; explosive limits 2 to 12 vol% in air. CAMEO gives the vapour density as 2.07, heavier than air, so it sinks and pools. Chemical Safety Card 0554 puts the two facts together in one line: “The vapour mixes well with air, explosive mixtures are easily formed.” Evaporation at 20 °C contaminates the air slowly, but spraying or dispersing the liquid contaminates it fast.

Its octanol/water partition coefficient, log P_ow 0.05, says it sits almost equally in water and in fat, which is the physical basis of the dryness and cracking that repeated skin contact causes.

Handling

The aggregated ECHA notifications classify propan-2-ol Danger, with the flame and exclamation-mark pictograms: H225 highly flammable liquid and vapour and H319 serious eye irritation in more than 99.9 per cent of the 8,842 classifying reports, H336 drowsiness or dizziness in 98.7 per cent, and only 3 of 8,845 reports saying it meets no criterion. This is as close to unanimous as an aggregated classification gets. HSE’s EH40 sets a workplace exposure limit of 400 ppm (999 mg/m³) over eight hours and 500 ppm (1250 mg/m³) over fifteen minutes; NIOSH’s recommended limits are the same 400 and 500 ppm, and its IDLH figure is 2000 ppm, which the guide notes is set at 10 per cent of the lower explosive limit — the fire hazard reaches you before the toxicity does.

Why Level A. The course rubric names this substance. Its Level A criterion reads: “No heating of solutions above 50 °C, no open flames, no volatile organic solvents beyond isopropanol in small quantities.” Isopropanol in small quantities is therefore Level A work by name, provided the rest of that sentence holds — and two common uses break it. Warming a bath above 50 °C with alcohol in it is out. The alcohol lamp in the wet-plate kit is an open flame beside a liquid whose flash point is below room temperature, which is Level B work under the rubric’s criterion on procedures whose failure mode is a burn, with the controls that implies: no bottle open near the flame, no bulk on the bench, and a way of smothering a spill fire before it is lit.

⚠Safety

Keep the working quantity small and the bottle closed. Ventilate: an openable window or an extractor, never a sealed cupboard, because the vapour is heavier than air and pools. Goggles for pouring; nitrile gloves; Chemical Safety Card 0554 records dryness and cracking from repeated skin contact and effects on the central nervous system from the vapour, with unconsciousness at high exposure. Store it “fireproof, separated from strong oxidants, cool, well closed”, which is the card’s own wording. CAMEO’s reactivity profile carries the one hazard that catches people out: isopropanol “reacts with air or oxygen to form dangerously unstable peroxides”, so a half-empty bottle that has stood for years is not the same article as a fresh one. Keep it away from oxidisers, acids, halogens and aluminium, and remember that Card 0554 records it attacking some plastics and rubber — check the tubing before you trust it.

Waste

This is the one substance in this family that Kodak’s amateur guidance rules out of the drain by name. J-300 lists what may be sent to the sewer — developers, stop baths, fixers after silver recovery, wash water — and then lists what may not: selenium toners, solvents, and “other materials that are prohibited, i.e., flammable materials”. Isopropanol is both.

The practical routes are evaporation for traces and collection for the rest. Bostick & Sullivan’s sheet shows the first done properly: the collodion bottle is rinsed with 50 to 100 mL of isopropyl alcohol into a shallow glass or ceramic dish placed outdoors, away from buildings and animals, and left for 24 to 48 hours, after which the nitrocellulose crust is wrapped and discarded. Unused liquid is bottled, labelled and taken to a household waste and recycling centre’s chemical cupboard — ILFORD’s route for domestic users in the United Kingdom — and a trade user uses a licensed contractor. Check your local regulations: flammable liquid is among the most tightly controlled household wastes, and the rules differ by jurisdiction.

History

Isopropanol is a twentieth-century arrival in a nineteenth-century job. The alcohol of the wet-plate era was ethanol, in collodion and in varnish, and the alcohol of the gelatine era was methylated spirit, in washing baths and rapid drying. What isopropanol brought was availability: it is sold in every chemist as 70 per cent rubbing alcohol, which is exactly the strength Reilly’s albumen hardening calls for and close to the strength that dehydrates a gelatin layer without shocking it. Reilly himself, writing in 1980 for people reviving a process abandoned in the 1890s, reaches for it in preference to the period method of steaming the albumen, and notes only that the steam route “in effect cooks the albumen”. The modern solvent turned out to fit the historical procedure better than the historical solvent did.