Glutaraldehyde

Formaldehyde has one aldehyde group. Glutaraldehyde has two, one at each end of a five-carbon chain, and that is the whole of its appeal to anyone hardening gelatin: a molecule with two reactive ends can tie two protein chains together, and it boils at a temperature that keeps most of it in the tray rather than in the room. What it does instead is get into the lungs of the people who use it repeatedly, and that is why this page is at Level C.

In photography

What the course could verify, and what it could not. The corpus behind this encyclopaedia yields exactly one photographic recommendation for glutaraldehyde, and it is a footnote. Mike Ware’s chrysotype manual, describing how to harden a gelatin-sized sheet of paper before it is coated with sensitiser, records that the traditional bath was 1 to 2 per cent formaldehyde, that he proposed glyoxal to the alternative-process community instead because it is “much less offensive due to its lower volatility”, and that “recently, an alternative agent, glutaraldehyde OCH(CH₂)₃CHO has been suggested”. He gives a working procedure for glyoxal — 2 per cent for about ten minutes at room temperature, followed by a twenty-minute wash to remove excess hardener, which would otherwise fog the sensitiser — and none for glutaraldehyde. The course states plainly that it could not verify a published photographic formula using glutaraldehyde from any source meeting its standard, and does not print one.

The neighbouring use it is often confused with. Princeton University’s environmental health and safety guidance lists the aldehydes that appear in colour-process prehardeners and stabilisers as succinaldehyde and formaldehyde — not glutaraldehyde. Succinaldehyde is butanedial, one carbon shorter, and Princeton adds that it is similar in toxicity to formaldehyde but is not a strong sensitiser or a carcinogen. If you have read that photographic prehardeners contain glutaraldehyde, that claim is not established by anything in this course’s sources.

Why a dialdehyde at all. PubChem’s ChEBI description classifies glutaraldehyde as a dialdehyde comprising pentane with aldehyde functions at carbons 1 and 5, and gives its roles as a disinfectant, a cross-linking reagent and a fixative. NIOSH supplies the reaction in one clause of a note about incompatibilities: alkaline solutions of glutaraldehyde — what it calls activated glutaraldehyde — react with alcohols, ketones, amines, hydrazines and proteins. Gelatin is a protein, its chains carry amine groups, and a molecule with an aldehyde at each end can reach from one chain to another.

⚗The chemistry

The difference from formaldehyde is geometric and it is also a difference in vapour pressure, which is what decided the substitution.

Formaldehyde is a gas at ordinary conditions: NIOSH gives its vapour pressure as greater than one atmosphere, and photographers only ever handle it dissolved in water, from which CAMEO says it is given up readily. Glutaraldehyde is a liquid; NIOSH’s entry gives a boiling point of 100 °C, a specific gravity of 1.10, a vapour pressure of 17 mmHg and the classification “noncombustible liquid”, figures that describe the commercial aqueous material rather than the neat dialdehyde. Seventeen millimetres of mercury against seven hundred and sixty is the argument Ware makes for a less volatile hardener, and glutaraldehyde and glyoxal are two answers to it.

What the course could not verify from a source meeting its standard is the balanced reaction between either aldehyde and gelatin — which amino acid residues react, in what protonation state, and what bridge results. NIOSH’s “react with proteins” and ChEBI’s “cross-linking reagent” are as far as the sourced chemistry goes, and this page goes no further.

Properties

Form. A colourless liquid with a pungent odour, described by the ILO-WHO International Chemical Safety Card as clear and viscous and by CAMEO as light yellow — the discrepancy is age, since CAMEO also records that it may discolour on exposure to air. Relative molecular mass 100.12. It is bought as a solution: the Haz-Map entry PubChem carries records commercial strengths from 2 to 50 per cent.

Solubility. Miscible with water, and soluble in ethanol, benzene and ether.

Stability, and a paradox. CAMEO’s reactivity alerts for glutaraldehyde are polymerisable and water-reactive, and its air-and-water reaction entry is a single line: “Polymerizes in the presence of water.” It also polymerises on heating. The material is nevertheless sold and used as an aqueous solution, so the practical reading is that this is a slow process and a shelf-life limit rather than an event: a bottle that has thickened, or gone from clear to yellow, no longer holds the concentration on its label. There is no assay for that in a darkroom.

Handling

The aggregated ECHA notifications give Danger and five pictograms — GHS05 corrosive, GHS06 acute toxicity, GHS07 irritant, GHS08 health hazard and GHS09 environmental hazard — as many as any page in this encyclopaedia carries. Four statements are notified by more than 99.9 per cent of 2,123 reports: toxic if swallowed, severe skin burns and eye damage, may cause an allergic skin reaction, and may cause allergy or asthma symptoms or breathing difficulties if inhaled.

The respiratory sensitisation is the governing hazard. HSE’s EH40 sets glutaraldehyde at 0.05 ppm, 0.2 mg m⁻³ over eight hours and the same figure over fifteen minutes — a limit forty times lower than formaldehyde’s in the same table — and attaches the Sen notation. EH40’s paragraph 56 states that this notation is assigned only to substances that may cause occupational asthma, and its paragraphs 53 to 55 explain what that means: once the airways have become hyper-responsive, further exposure “sometimes even in tiny quantities” may cause symptoms; not everyone exposed becomes hyper-responsive; and it is impossible to identify in advance who will. Table 1 of EH40, which is where this limit sits, is not guidance but legally binding under COSHH. NIOSH sets a ceiling of 0.2 ppm (0.8 mg m⁻³) and refers to its Appendix C on aldehydes; it lists no immediately-dangerous-to-life-or-health value, and its respirator recommendation reads “Not available”, which is a gap rather than a reassurance.

NIOSH’s exposure routes are inhalation, absorption, ingestion and contact — all four — and its symptoms run irritation of eyes, skin and respiratory system, dermatitis, skin sensitisation, cough, asthma, nausea and vomiting. CAMEO adds that the chemical readily penetrates skin in harmful amounts.

Why Level C. The course rubric puts at Level C reagents classified as acutely toxic, carcinogenic, mutagenic, reproductive toxins or sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control, and adds “formaldehyde and similar substances in any quantity”. Glutaraldehyde meets the first clause on the sensitisation alone: a respiratory sensitiser notified by more than 99.9 per cent of reports, with a legally binding British exposure limit of 0.05 ppm and an HSE requirement that exposure be reduced as low as is reasonably practicable, cannot be controlled by the Level B measures of splash goggles and work near an open window. The corrosivity and the acute inhalation toxicity are independent reasons. It is not a Level D substance: it has a legitimate modern use under engineered control, and safer alternatives for the same job exist rather than the process itself being abandoned.

⚠Safety

The course teaches glutaraldehyde as chemistry and does not give a home darkroom procedure for it. Where it is used at all it is used under engineered extraction with the container closed, with splash goggles and a face shield, with gloves chosen from permeation data rather than from habit, and with eyewash and quick-drench facilities within reach as NIOSH directs. Keep it away from strong oxidisers and strong bases. The hazard that outlasts the session is sensitisation: HSE states that once a person’s airways are hyper-responsive, tiny further exposures may provoke symptoms, and that no test identifies in advance who is at risk. For hardening a gelatin size, Ware’s published route uses glyoxal and gives a procedure; that is the substitution the alternative-process community actually made.

Waste

Read the environmental statements first. H400, very toxic to aquatic life, is notified by 99.9 per cent of reports, H411 by 16.1 per cent, and the GHS09 pictogram sits on the label for that reason alone. A spent hardening or sizing bath is therefore a stream to be collected rather than diluted, and the dilution argument that sometimes gets made for photographic waste does not apply to a substance classified for long-lasting effects in water.

Collect it in its own closed and labelled container; keep it apart from oxidisers, from strong bases, and from acid wastes; and route it through a licensed hazardous waste service. Florida Atlantic University’s guidance for photographic chemicals treats hardener waste as a stream collected by its environmental health and safety office, and in England and Wales the government publishes a finder for local hazardous waste services. Check your local regulations; they govern.

History

Glutaraldehyde has no long photographic history, and the honest version of its story is a story about formaldehyde. For most of the twentieth century the aldehyde hardener a photographer met was formalin, and the objection to it was never that it failed — Kodak’s 1928 primer records that a 5 per cent solution renders film gelatin insoluble in boiling water in under a minute — but that it left the tray. Kodak’s own primer calls its irritation of the nose and throat “very objectionable” in the same paragraph that describes its power.

The replacement search is recent and it happened in the alternative-process community rather than in industry, because that is where gelatin sizing is still hardened by hand. Ware’s account names the criterion — lower volatility — proposes glyoxal, reports that it was widely and successfully taken up, and notes glutaraldehyde as a further suggestion. Glutaraldehyde’s own occupational record is what kept it a footnote rather than the answer: by the time it was being proposed, HSE had already assigned it a Sen notation and one of the lowest workplace exposure limits in the whole of Table 1. A hardener that stays in the tray is only an improvement if what does leave the tray is less consequential, and for glutaraldehyde that is not established.