Potassium Dichromate

Potassium dichromate (K₂Cr₂O₇, plain-ASCII form K2Cr2O7), also known as potassium bichromate, bichromate of potash, or kalium dichromate (the German chemistry name), is one of the most historically important sensitizers in alternative photographic processes.^[1] It is a bright orange-red crystalline salt, freely soluble in water, that becomes light-sensitive only when combined with a colloid such as gum arabic, gelatin, or fish glue.

Kalium dichromate

"Kalium" is the Latin / German name for potassium (from Arabic al-qalī, "ashes"); "kalium dichromate" is the German-language name for the same compound described on this page, K₂Cr₂O₇ / K2Cr2O7. Continental European chemistry literature and German-language photographic sources almost universally use this form. The compound, formula, properties, photographic behaviour, and regulatory profile are identical to potassium dichromate — a single English-vs-German naming difference, not a chemistry difference.

Properties at a glance

Property	Value
Chemical formula	K₂Cr₂O₇
Plain ASCII formula	K2Cr2O7
Molecular weight	294.18 g/mol
CAS Registry Number	7778-50-9
Appearance	Bright orange-red triclinic crystals
Melting point	398°C (decomposes)
Density	2.676 g/cm³
Solubility (20°C)	12 g / 100 mL water
Solubility (50°C)	~32 g / 100 mL water
Solubility in ethanol	Practically insoluble
Aqueous pH (1% solution)	~4 (slightly acidic)
Stable as dry crystals	Indefinitely (cool, dry, dark storage)
Sensitizing role	Cr(VI) oxidizer; photochemically active in chromated colloid layers

Compare to sister chromates: ammonium dichromate ((NH₄)₂Cr₂O₇) is more soluble (~33 g / 100 mL at 20°C) and faster as a sensitizer; sodium dichromate (Na₂Cr₂O₇·2H₂O) is the most soluble of the three (~180 g / 100 mL at 20°C) and is the form most commonly stocked by industrial chemical suppliers.

Historical timeline of photographic uses

The light sensitivity of chromated organic colloids was discovered three decades before silver halide emulsions came to dominate photography:

• 1839 — Mungo Ponton (Edinburgh) reports that paper soaked in potassium dichromate solution darkens on exposure to sunlight; the first published observation of dichromate photosensitivity, predating Talbot's calotype.
• 1855 — Alphonse Poitevin patents both carbon printing (gelatin + dichromate + carbon pigment) and photolithography (dichromated colloid as a lithographic resist), establishing dichromate as the foundation of two industrial reproduction technologies.
• 1858 — John Pouncy publishes the first practical gum bichromate print process: gum arabic + dichromate + watercolor pigment, contact-printed under a paper negative.^[1]
• 1864 — Joseph Wilson Swan patents the modern carbon transfer process, refining Poitevin's method into a workflow that would dominate fine-art printmaking through the 1920s.
• 1890s-1920s — Pictorialist photographers (Robert Demachy, Edward Steichen, Alfred Stieglitz) embrace gum bichromate for its painterly, hand-touched quality. Multiple-coating gum prints become a signature alternative-process technique.^[3]
• 1919 — Howard Farmer publishes The Bromoil and Transfer Processes, codifying the dichromate-bleach-then-ink technique that would carry pictorialism into the 1930s.
• 1970s — Alternative-process revival movement (Bea Nettles, Eugene Buechel, others) returns gum and carbon to active practice as fine-art workflows.^[4]
• 2000s-present — Gum bichromate, carbon, and carbro remain alive in art-school programs and dedicated workshops despite REACH restrictions on Cr(VI) compounds in the EU.

Photographic mechanism

When a chromated colloid layer is exposed to ultraviolet light (or strong blue-violet visible light), the dichromate ion oxidizes and the surrounding colloid undergoes cross-linking, becoming progressively insoluble in proportion to the exposure received.^[2] This light-driven hardening is the foundation of every chromate-based photographic process. Unlike silver halide emulsions, the chromated colloid does not store a latent image — it must be exposed and processed within hours of sensitizing, since slow dark reactions ("dark fog") progressively harden unexposed areas.

The wavelength sensitivity peaks around 365-400 nm (UVA into deep violet). Tungsten incandescent and most LED light sources are too red-shifted for practical exposure; UV light sources (mercury vapor, dedicated UV LED arrays, or direct sun) are required.

Common photographic uses with concentration ranges

Process	Sensitizer concentration	Notes
Gum bichromate (single coat)	5-13% w/v K₂Cr₂O₇	Higher concentration → higher contrast, shorter scale. Pictorialists often used 5-8% for graded multi-coat prints
Gum bichromate (multi-coat)	3-6% w/v per coat	Each successive layer typically uses lower dichromate to flatten the cumulative contrast
Carbon transfer (carbon tissue sensitizing)	1-4% w/v K₂Cr₂O₇	Lower than gum because gelatin's longer scale needs less aggressive cross-linking
Carbro	4-6% w/v K₂Cr₂O₇	Sensitizes carbon tissue via contact with a bromide silver print
Bromoil (bleach matrix)	1-2% w/v K₂Cr₂O₇ in dilute sulfuric acid	Used as the oxidizing component of the bromoil bleach; concentration matters less here than in sensitization

In all cases, the sensitizer is mixed with the colloid just before coating, or brushed on as a separate layer immediately before exposure. Stock solutions of dichromate alone are stable for weeks; mixed sensitized colloid is stable for hours to a day at most before dark fog becomes visible.

Process-by-process workflow notes

• Gum bichromate printing: The classic dichromate process. Pigmented gum arabic is sensitized with potassium dichromate, contact-printed under a negative, and developed in water — unhardened gum washes away, leaving a pigment image. Used since 1858 and revived by pictorialist photographers; remains popular today for its painterly quality and unlimited tonal/color control via multiple coatings. See also the process overview on photography-fyi.com.
• Carbon transfer printing: Sensitizes a pigmented gelatin tissue for one of the most permanent and tonally rich monochrome processes ever developed.^[3] The print's image consists of pigment held in hardened gelatin — no silver, no dye, no organic colorants, exceptional archival permanence (200+ year tested life).
• Carbro printing: A carbon variant that sensitizes via contact with a bromide silver print rather than direct UV exposure, allowing tricolor color prints from separations.
• Bromoil: Sensitizes a bleached silver gelatin print so that lithographic ink can be applied to the hardened image areas. Falls between alternative-process and traditional silver workflows.
• Photographic bleaches and intensifiers: Acts as the oxidizer in the dichromate bleach used in reversal processing of black-and-white film, and in several historical intensifier formulas (e.g., the Eder-Toth chromium intensifier).

Practical notes

Typical sensitizer solutions are 1–15% by weight depending on process and desired contrast.^[4] Higher concentrations give higher contrast but shorter exposure scales; lower concentrations give longer scales and softer prints. The sensitizer is often combined with the colloid just before coating, though some workers prefer a two-coat approach (colloid first, dichromate brushed on second) to extend working time. Storage of dry crystals is straightforward — kept cool, dry, and dark, the salt is stable indefinitely. Stock solutions, however, slowly photo-fog from ambient UV exposure and should be made fresh weekly to monthly.

When mixing concentrated stock (>10% w/v), the dichromate dissolves more easily in warm water (40-50°C) and crystallizes out on cooling if the concentration is near solubility limit — a refrigerated 15% solution will sometimes crystallize at the bottom of the bottle and need re-warming before use.

Disposal

Dichromate solutions cannot be poured down the drain in most jurisdictions. Reduce spent solutions with a reducing agent (sodium metabisulfite or ferrous sulfate) to convert Cr(VI) to the less hazardous Cr(III), then dispose as a hazardous waste through a licensed collection program. Never combine with strong acids, bases, or organic solvents — Cr(VI) is a strong oxidizer and reactions with organic materials can be vigorous.

A practical reduction recipe: dissolve 30 g sodium metabisulfite per liter of dichromate solution, acidify slightly with sulfuric acid (caution, exothermic), wait until the solution shifts from orange to greenish (indicating Cr(VI) → Cr(III) reduction), then alkalize with sodium bicarbonate to precipitate insoluble Cr(III) hydroxide for filtered disposal.

Regulatory status

The European Union restricts Cr(VI) compounds under REACH, with substantial paperwork and authorization requirements for purchase and use.^[5] In the United States, OSHA permissible exposure limits for hexavalent chromium are 5 µg/m³ (8-hour TWA) in workplace air.^[6] Hexavalent chromium compounds are listed under California Proposition 65 as carcinogens and reproductive toxins.^[7] Many photographic suppliers no longer stock potassium dichromate or sell it only in commercial quantities; small-quantity purchases may require a chemistry-supplier account or a research/educational affiliation.

Comparison: which dichromate to use?

	Potassium (K₂Cr₂O₇)	Ammonium ((NH₄)₂Cr₂O₇)	Sodium (Na₂Cr₂O₇·2H₂O)
Solubility (20°C)	12 g / 100 mL	33 g / 100 mL	180 g / 100 mL
Speed (sensitizing)	Reference baseline	~1.5× faster	Similar to potassium
Contrast	Highest	Slightly lower	Similar to potassium
Stability of dry crystals	Indefinite	Decomposes if heated above 180°C	Hygroscopic — crystals absorb water
Storage of stock solution	Stable; modest dark fog over weeks	Faster dark fog; mix fresh	Stable but hygroscopic crystals make accurate weighing harder
Most common contemporary use	Gum bichromate, bromoil bleach	Modern gum bichromate (favored for higher speed)	Industrial / lab applications; rarely in alt-process
Regulatory profile	Cr(VI) — same restrictions as ammonium and sodium

Most modern alt-process workers prefer ammonium dichromate for gum because of its higher speed; potassium dichromate remains the traditional choice and still gives excellent results, with the practical advantage of more stable crystals (no hygroscopicity, no thermal decomposition concerns).

Alternatives to chromate sensitizers

Photographers concerned about hexavalent chromium exposure have two genuine alternatives outside the dichromate family:

• Diazidostilbene disulfonic acid (DAS) — a non-chromate sensitizer for gum printing. Slower and more expensive than dichromate, but with vastly lower toxicity. Used by a small number of contemporary gum printers who want the gum aesthetic without the chromium exposure.
• Iron-based alternative processes — cyanotype and its variants use ferric ammonium citrate as a non-toxic iron-based sensitizer. The aesthetic is completely different (Prussian blue monochrome rather than pigmented gum) but the health profile is benign. Platinum/palladium printing is another iron-based chromium-free alternative with very different tonal character.