Potassium bichromate, also correctly known as potassium dichromate, is an inorganic chemical compound with the formula K2Cr2O7, appearing as bright orange-red crystalline granules or powder that dissolve readily in water to produce a strongly coloured orange solution. In photographic chemistry, potassium bichromate has been used as an active component in certain chemical intensification treatments applied to developed photographic negatives and occasionally prints to increase their density and contrast when the original development has produced an insufficiently dense or contrasty result.
In chromium based intensification processes, potassium bichromate functions as an oxidising bleaching agent that first converts the metallic silver of the developed image into a silver salt compound through an oxidation reaction, bleaching the visible silver image to a pale or colourless state. The bleached image is then redeveloped in a separate developer bath, which reduces the silver salt back to metallic silver while simultaneously depositing additional density from the chromium compounds involved in the reaction, producing a final image of greater density and contrast than the original developed silver image. The degree of intensification achieved can be controlled to some extent by the concentration of the bichromate solution, the duration of the bleaching stage, and the choice of redeveloper used in the subsequent development step.
Potassium bichromate has also found application in several other areas of photographic and alternative process practice beyond chemical intensification. It is a key sensitising agent in the gum bichromate printing process and the related carbon and carbro printing processes, where its property of hardening certain colloids - including gum arabic and gelatin - in proportion to the amount of light they receive is exploited to produce photographic images of distinctive tonal and surface qualities. In the gum bichromate process, a mixture of gum arabic, watercolour pigment, and potassium bichromate is coated onto paper, exposed through a negative to ultraviolet light, and developed in water to wash away the unhardened, unexposed areas while retaining the hardened, exposed areas as a pigmented image.
The handling of potassium bichromate demands careful attention to health and safety, as it is a hexavalent chromium compound classified as a known human carcinogen, a potent skin and respiratory sensitiser, and a strong oxidising agent. Prolonged or repeated skin contact can cause severe allergic dermatitis and chromate ulcers - slow healing skin lesions known as chrome holes - and inhalation of potassium bichromate dust or solution mist can cause serious respiratory sensitisation and damage. The compound is also acutely toxic if ingested and is classified as an environmental hazard due to its toxicity to aquatic organisms. Appropriate protective equipment including chemical resistant gloves, eye protection, and respiratory protection must be used when handling potassium bichromate, and all waste containing hexavalent chromium compounds must be disposed of in strict accordance with hazardous waste regulations rather than through normal drainage channels.