Succinaldehyde

Succinaldehyde (butanedial, OHC-CH₂-CH₂-CHO; C₄H₆O₂; CAS 638-37-9), also written succindialdehyde and succinic dialdehyde, is the four-carbon dialdehyde. In photography it is a gelatin hardener: a molecule with an aldehyde group at each end can bridge two protein chains, and a bridge four carbons long turns out to be a good fit for gelatin. Kodak and Fuji have used it in hardening fixers, pre-hardeners and stabilizer baths where the older aldehydes, formaldehyde and glyoxal, were too volatile, too fast, or too toxic.^[1] Outside the darkroom it is better known as the starting material of Robinson's 1917 tropinone synthesis, which is why most people who look it up are chemistry students rather than photographers.

Succindialdehyde

The two names describe one compound. "Succinaldehyde" and "succindialdehyde" both derive from succinic acid, the four-carbon diacid; the dialdehyde is what you get by reducing both acid groups to aldehydes. The systematic name is butanedial, or 1,4-butanedial. Supplier catalogues, older photographic patents and the Kodak process manuals use all three, and the Haist volumes use "succinaldehyde" throughout. The aliases on this page are set so that a search for any of them lands here.

Properties at a glance

Property	Value
Systematic name	Butanedial (1,4-butanedial)
Other names	Succinaldehyde, succindialdehyde, succinic dialdehyde
Formula	C₄H₆O₂ (OHC-CH₂-CH₂-CHO)
Molar mass	86.09 g/mol
CAS number	638-37-9
Appearance	Colourless liquid; commercial material is usually a 20–30 % aqueous solution
Boiling point	About 170 °C, with decomposition
Solubility	Miscible with water; soluble in alcohols
Behaviour in water	Exists largely as the cyclic hydrate 2,5-dihydroxytetrahydrofuran and as oligomers; the free dialdehyde is released on warming or at low pH
Bisulfite adduct	Forms a crystalline bis(sodium bisulfite) adduct, the form used in most photographic formulas
Hazard codes	H302, H315, H317, H319

The pure dialdehyde is awkward to keep. It hydrates, it polymerizes on standing, and it darkens. That is why it is sold in water, why laboratories often make it fresh by hydrolysing 2,5-dimethoxytetrahydrofuran with dilute acid, and why photographic formulas prefer the bisulfite adduct, a stable white solid that releases the aldehyde slowly at the pH of the bath.

Photographic mechanism

Gelatin is a protein. Its chains carry free amino groups, mostly on lysine, and an aldehyde reacts with an amino group to form a carbon–nitrogen bond. A dialdehyde does this twice, once at each end, and so ties two chains together. Enough bridges and the swollen gelatin can no longer take up water freely: it swells less, its melting point rises from about 30 °C to above the temperature of any processing bath, and it stops frilling, reticulating and scratching.^[2]

The length of the bridge matters. Formaldehyde, with one carbon, forms short methylene bridges and reacts fast, but it is volatile, a sensitizer and a carcinogen, and its hardening keeps increasing for days after processing as trapped formaldehyde goes on reacting. Glyoxal, with two carbons, is safer but still quick and can yellow the gelatin. Succinaldehyde's four-carbon chain spans the gap between chains more comfortably, reacts more slowly and more evenly, and is nearly odourless in use. In a replenished machine bath that must behave the same on the first roll and the thousandth, an even, predictable hardening rate is worth more than a fast one.^[1]

Aldehyde hardeners have one more property that alum hardeners lack: they work at neutral and alkaline pH. Potassium alum and chrome alum only harden in an acid bath, which is why they live in acid fixers. An aldehyde can go into an alkaline pre-hardener before the developer, or into a stabilizer after the fixer, wherever the process needs it.

Common photographic uses

• Pre-hardeners for high-temperature colour processes. Colour processes run warm, 38 °C for C-41 and E-6, and early colour films could not survive that unhardened. Kodak's E-4 Ektachrome process of the 1960s opened with a pre-hardener bath built on the succinaldehyde bisulfite adduct, so that the emulsion was tanned before it met the warm developer. Later films were hardened in manufacture and the bath disappeared from the sequence.
• Hardening fixers. Some commercial rapid fixers for machine processing use succinaldehyde in place of alum, because it hardens in a neutral bath and does not lose activity as developer carry-over pushes the pH up. See Kodak Fixer F-5 for the alum approach it replaces.
• Stabilizer and final-rinse baths. The formaldehyde stabilizer at the end of older colour processes did two jobs: it hardened, and it locked the dye couplers against fading. Formaldehyde-free replacements use succinaldehyde or other agents for the hardening part. The modern C-41 and E-6 final rinses on this site are of that later type.
• Hand-coated emulsions. A few drops of a dilute succinaldehyde or glyoxal solution in the finals of a home-made silver gelatin emulsion hardens the coating without the acid pH that chrome alum needs, and without the green tint chrome alum leaves.

Outside the darkroom

The general-chemistry interest in succinaldehyde comes from one reaction. In 1917 Robert Robinson made tropinone, the skeleton of cocaine and atropine, in a single step from succinaldehyde, methylamine and acetonedicarboxylic acid. It was the first synthesis to copy the way a plant builds an alkaloid, and it remains a set piece in organic-chemistry courses. Succinaldehyde is also used, like glutaraldehyde, as a cross-linking agent for proteins in biology and as a precursor for disinfectants. None of those uses involve photographic quantities or photographic purity.

Practical notes

• Home darkrooms rarely need it. Succinaldehyde is a commercial-process reagent. Alum in the fixer, or no hardener at all with modern pre-hardened films, covers everything a small darkroom does. Buy it only if you are reviving a discontinued colour process or hardening hand-coated plates.
• Buy the solution, not the neat liquid. The 25 % aqueous solution keeps for about a year in a full amber bottle; the neat dialdehyde polymerizes within weeks. Sigma-Aldrich, TCI and Alfa Aesar list it; photographic suppliers generally do not.^[3]
• Dose by trial. For a hardening bath, start at 2 to 5 ml of 25 % solution per litre and test with a strip of unhardened film: after processing it should no longer scratch under a fingernail, and it should not have lost density. More aldehyde than that slows development and can raise fog.
• Keep it out of the developer. Aldehydes react with the sulfite in developers, forming bisulfite adducts and depleting the preservative. A pre-hardener sits before the developer with a rinse between them.
• Substitute glyoxal at half the weight if you cannot get succinaldehyde; glyoxal is a C₂ dialdehyde, so a given weight carries more aldehyde groups. Expect faster action and a slight warm tint.
• Storage. Cool, dark, tightly capped, away from amines and strong bases, which polymerize it, and away from oxidizers.

Disposal

Follow the product's Safety Data Sheet and your local hazardous-waste rules; the permitted route depends on the exact formulation and the jurisdiction, and neither smell nor colour tells you a bath is fully reacted. Aldehydes are toxic to aquatic life, so never pour the concentrate to drain. Collect spent hardening and stabilizer baths for hazardous-waste collection where that is required; where a diluted working bath may be drained, adding sodium sulfite first ties up free aldehyde as the bisulfite adduct. Spent fixer that contained succinaldehyde is silver-bearing waste first and aldehyde waste second; see Sodium Thiosulfate for silver recovery.

Related compounds

• Formaldehyde and its solution formalin — the C₁ aldehyde: fastest, cheapest, most hazardous; the hardener that succinaldehyde was brought in to replace.
• Glyoxal — the C₂ dialdehyde: the usual aldehyde hardener in small-darkroom formulas and hand-coating.
• Glutaraldehyde — the C₅ dialdehyde: the standard protein cross-linker in microscopy and the active agent in cold-sterilant solutions; rarely used in photography.
• Potassium alum and chrome alum — mineral hardeners that work by a different mechanism and only in acid.

Alternatives

Most darkroom workers can avoid aldehyde hardeners altogether. Modern films and papers are hardened in manufacture; an acid alum fixer gives what little extra hardening a warm wash needs; and a hand-coated emulsion can be hardened with chrome alum in the finals or simply processed cold and dried slowly. Reach for succinaldehyde when the process itself demands a neutral or alkaline hardener.