Potassium dichromate

Photography’s second light-sensitive system is not a silver salt. It is an orange-red chromium salt in contact with gelatin, gum arabic or albumen, and for a century it carried carbon printing, gum printing, collotype and photogravure. It is also one of the most severely labelled substances ever in ordinary darkroom use.

In photography

It sensitises a colloid, not a silver salt. Wall’s 1912 dictionary defines it in one clause: a red crystalline substance which has the property of insolubilising many colloids when they are treated with it and then exposed to light. Coat pigmented gelatin, dry it, expose under a negative, and the parts that took light stay put while the rest washes away in warm water. That is carbon printing, gum printing and most of the photomechanical trade.

⚗The chemistry

Light does not act on the dichromate alone. The Encyclopaedia Britannica’s 1911 survey states the constraint plainly: a dichromate solution absorbs only the rays that are effective in altering it, its spectral action beginning in the ultraviolet and reaching no further than the blue-violet, and the change is only possible in the presence of organic matter of some kind, such as gelatin or albumen. The Getty Conservation Institute’s atlas of the carbon process calls what happens the photochemical reduction of the dichromate salt: chromium(VI) takes electrons from the colloid around it and falls to chromium(III), and the reduced chromium binds the colloid so that the exposed layer no longer swells and dissolves. The course has not found in its sources a description of that cross-link at the level of which groups are bridged, and does not assert one.

What more or less of it does runs in opposite directions on the two families of process. In a dichromated colloid, Wall’s account of gum printing has a mixture rich in colour and poor in dichromate tending to hardness and deep shadows, while doubling the dichromate gives greater sensitiveness, a flatter picture and the fainter gradations. Where it is an additive rather than the sensitiser it does the reverse: Reilly’s handbook uses it to raise the contrast of a salted paper, acting mainly on the highlights, and warns that stability and predictability decline as more is added, that the paper must then be handled in yellow light because the dichromate is itself light-sensitive, and that the image colour turns browner. The Getty atlas of the platinotype records the same trick in platinum printing, where a century later a trace of chromium still shows under X-ray fluorescence.

It is also an oxidising bleach. Kodak’s 1928 primer describes the chromium intensifier: the silver image is bleached in a bichromate solution carrying a very little hydrochloric acid — bichromate being an oxidiser of the same type as permanganate or ferricyanide — and then redeveloped, coming back denser. The same power runs the reversal bleach that removes the first, negative image before redevelopment to a positive, and it has not gone: Foma’s sheet for Fomapan R 100, stamped January 2025, still gives bleach FB-2 as potassium dichromate with concentrated sulfuric acid.

Properties

Orange-red crystals, denser than water, odourless, and non-combustible in themselves — but, as CAMEO’s fire entry puts it, liable to decompose and give up oxygen, so they support the combustion of other materials. Kodak’s primer adds that they are stable in air and dissolve easily to a yellow solution. OpenStax’s table of standard potentials lists this element twice over, as the dichromate ion Cr₂O₇²⁻ in acid and as the chromate ion CrO₄²⁻ in base, and the acidity of a bath decides which of the pair it holds. Solubility is moderate and steeply temperature-dependent — 12 g per 100 mL at 20 °C on the ILO-WHO card, against HSDB’s 4.9 g per 100 cm³ at 0 °C and 102 g at 100 °C — and nil in alcohol. The record’s formula carries no water of crystallisation.

The oxidising power is what everything else follows from. OpenStax’s table of standard electrode potentials gives the half-reaction in acid:

Cr₂O₇²⁻ + 14 H₃O⁺ + 6 e⁻ → 2 Cr³⁺ + 21 H₂O

Dichromate reduced to chromium(III) in acid, standard potential +1.232 V

Fourteen hydronium ions on the left is why the intensifier bleach carries hydrochloric acid and the reversal bleach carries sulfuric acid. In alkali the same element sits as chromate and the potential falls to −0.13 V, which is the other half of why the waste treatment below works.

Handling

The aggregated ECHA notifications classify it Danger under six pictograms at once — oxidiser, corrosive, acute toxicity, irritant, health hazard and environmental hazard — with near-unanimity on the worst entries: fatal if inhaled, may cause cancer, may cause genetic defects, may cause allergy or asthma symptoms if inhaled, and causes severe skin burns and eye damage, each in 99.8 per cent of 491 reports. NIOSH treats chromic acid and chromates as a carcinogen at 0.001 mg/m³ as chromium, listing nasal septum perforation, kidney and liver damage, skin ulceration and sensitisation dermatitis; HSE’s EH40 gives chromium(VI) compounds 0.01 mg/m³ over eight hours with carcinogen, sensitiser and biological-monitoring notations — a limit that was reduced in the revision in force from January 2020.

⚠Why Level D, and what that means here

The classification rubric names chromium(VI) compounds explicitly, and it names them at Level C: reagents where a fume cupboard or specialist disposal is the recognised control, in any quantity. This course is written for a home darkroom and cannot supply a Level C facility, so the substance is not handled at all, and the page falls to Level D on the rubric’s other test — that it describes chemistry, workflow in outline, image characteristics and hazards, and states explicitly that it is not a procedure. No quantity, dilution or sequence appears above, and no protective equipment is listed, because none would change the answer in a domestic room. Carbon and gum printing are still practised, well, by people who have the extraction and the waste route, and the rubric’s instruction there is to point to their published accounts rather than reproduce them.

Wall’s 1912 dictionary already knew the skin hazard: bichromate poisoning through soaking the hands, as in the carbon process, especially where there are cuts. What he treated as a nuisance answered by rubber gloves is now H317, skin sensitisation, in 98.6 per cent of notifications — and sensitisation, once acquired, does not go away.

The incompatibilities are those of a strong oxidiser: NIOSH lists combustible and readily oxidisable material — paper, wood, sulfur, aluminium, plastics — and records chromates as corrosive to metals.

Waste

Kodak’s guidance on disposing of small volumes is worth reading for the chemistry as much as the rule. Dichromate solutions must be kept out of a septic tank, because they are hazardous to the bacteria that make it work. To make the waste less troublesome, Kodak has you reduce the dichromate with sulfite or used fixer, then add an alkaline material — used developer among them — to neutralise the acidity and precipitate the chromium; the solid is filtered or settled out and a licensed waste-disposal service takes it. That is the two half-reactions above run deliberately. Under the Environment Agency’s WM3 guidance, code 09 01 05*, bleach solutions and bleach fixer solutions, is an absolute hazardous entry.

One published instruction points the other way, and the course rejects it: Photographers’ Formulary’s cyanotype kit sheet, on the same page where it correctly calls dichromates toxic oxidisers whose spills ulcerate skin, tells the user to wash excess solid dichromate down a drain. A kit sheet is authority for what is in the kit, not for waste law. Check your local regulations; they govern.

History

Mungo Ponton found the effect in 1839 on paper coated with nothing but a dichromate solution, where the only organic partner was the cellulose of the sheet and its gelatin sizing; Edmond Becquerel repeated the work in 1840 and identified the internal size as the active ingredient. Talbot patented the colloid version in 1852, Poitevin made the carbon process from it in 1855, and Swan made that workable in 1864. Wall’s “Bichromate Methods” shows the reach by 1912: gum, carbon, photogravure, collotype, oil and bromoil, and most photographic block and plate making besides.

The retreat was not a chemical discovery. Nothing better at hardening gelatin arrived; what arrived was an understanding of what chromium(VI) does to lungs and skin, and the exposure limits that followed. Carbon and gum survived in equipped hands, photomechanical printing moved into plants built for it, and the amateur darkroom dropped both the chromium intensifier and the dichromate tray cleaner for substitutes that do nearly as much.