Ammonium Dichromate

Ammonium dichromate ((NH₄)₂Cr₂O₇), also known as ammonium bichromate, is the most light-sensitive of the common dichromate sensitizers used in alternative photographic processes.^[1] It is a bright orange crystalline salt, highly soluble in water — roughly three times more soluble than potassium dichromate — which allows very concentrated sensitizer stocks when a process calls for them.

Photographic mechanism

Like all chromated-colloid sensitizers, ammonium dichromate functions by hardening a gelatin or gum layer on UV exposure: the dichromate ion oxidizes and the surrounding colloid undergoes cross-linking, becoming progressively insoluble in proportion to the light received.^[2] The underlying chemistry is identical to that of potassium dichromate and sodium dichromate, but ammonium dichromate is roughly 1.5–2× more UV-sensitive in typical gum formulations — exposure times under sun or UV unit scale accordingly. The hardened layer is slightly less robust than one produced by potassium dichromate, a property that counterintuitively works in the printer's favor: underexposed highlights wash out more cleanly, giving crisper whites.

Common photographic uses

• Gum bichromate printing: The process for which ammonium dichromate is most often preferred today. The combination of higher speed and cleaner highlight washout gives gum prints crisper whites and a more controllable tonal scale than potassium yields. Many contemporary gum printers use it exclusively, sensitizing pigmented gum arabic for contact printing under enlarged negatives.^[3] See also the process overview on photography-fyi.com.
• Carbon transfer printing: Can substitute for potassium dichromate in gelatin carbon tissue, though many workers prefer potassium for its more durable hardening — a property that matters once the tissue is transferred to a final support.
• Carbro printing: Occasionally used in place of potassium dichromate in contemporary revivals of the process, where faster sensitizing is welcome.
• Photo-resist and intaglio processes: Historically the sensitizer of choice for photogravure and photolithographic resist work, where its higher sensitivity speeds production.

Practical notes

Typical gum sensitizer solutions are 5–15% by weight;^[4] because of ammonium dichromate's much higher solubility, saturated stock solutions can reach roughly 35% at room temperature if a formula calls for it. The dry salt is slightly hygroscopic — crystals pick up atmospheric moisture over time and should be stored in a tightly sealed jar with desiccant. Stock solutions photo-fog faster than potassium dichromate stocks, so mix only what will be used within a few weeks and keep solutions in an amber bottle away from light. Sensitized paper should be exposed the same day it is coated; overnight storage causes noticeable dark-reaction fogging that reduces effective contrast.

Disposal

Dichromate solutions cannot be poured down the drain in most jurisdictions. Reduce spent solutions with sodium metabisulfite or ferrous sulfate to convert Cr(VI) to the much less hazardous Cr(III), then dispose as hazardous waste through a licensed collection program. Never combine dichromate solutions with strong acids, bases, or organic solvents before reducing.

Regulatory status

The European Union restricts Cr(VI) compounds under REACH, with authorization required for both purchase and use.^[5] In the United States, OSHA permissible exposure limits for hexavalent chromium in workplace air are 5 µg/m³ (8-hour TWA).^[6] Hexavalent chromium compounds are listed under California Proposition 65.^[7] Many photographic suppliers no longer stock dichromates or sell them only in commercial quantities; small-quantity purchases may require a chemistry-supplier account or a research/educational affiliation. Ammonium dichromate is also subject to additional oxidizer shipping regulations in some jurisdictions because of its decomposition behavior.

Related compounds

Potassium dichromate is the traditional alternative — slower and more stable in solution, still the common choice for carbon transfer. Sodium dichromate is the most soluble of the three and essentially interchangeable with ammonium in most formulas, but is less widely stocked by photographic suppliers. All three share the severe hexavalent chromium toxicity profile described in the safety notes below.

Alternatives

Photographers concerned about chromium exposure should consider:

• Diazidostilbene disulfonic acid (DAS) — a non-chromate sensitizer for gum printing. Slower and more expensive, but with vastly lower toxicity.
• Iron-based alternative processes — cyanotype uses ferric ammonium citrate as a non-toxic iron-based sensitizer. Platinum/palladium printing is another iron-based chromium-free alternative process offering exceptional tonal richness on fine art paper.